PERLE. Powerful energy recovery linac for experiments. Conceptual design report

A conceptual design is presented of a novel ERL facility for the development and application of the energy recovery technique to linear electron accelerators in the multi-turn, large current and large energy regime. The main characteristics of the powerful energy recovery linac experiment facility (PERLE) are derived from the design of the Large Hadron electron Collider, an electron beam upgrade under study for the LHC, for which it would be the key demonstrator. PERLE is thus projected as a facility to investigate efficient, high current (>10 mA) ERL operation with three re-circulation passages through newly designed SCRF cavities, at 801.58 MHz frequency, and following deceleration over another three re-circulations. In its fully equipped configuration, PERLE provides an electron beam of approximately 1 GeV energy. A physics programme possibly associated with PERLE is sketched, consisting of high precision elastic electron-proton scattering experiments, as well as photo-nuclear reactions of unprecedented intensities with up to 30 MeV photon beam energy as may be obtained using Fabry-Perot cavities. The facility has further applications as a general technology test bed that can investigate and validate novel superconducting magnets (beam induced quench tests) and superconducting RF structures (structure tests with high current beams, beam loading and transients). Besides a chapter on operation aspects, the report contains detailed considerations on the choices for the SCRF structure, optics and lattice design, solutions for arc magnets, source and injector and on further essential components. A suitable configuration derived from the here presented design concept may next be moved forward to a technical design and possibly be built by an international collaboration which is being established.


Abstract
A conceptual design is presented of a novel ERL facility for the development and application of the energy recovery technique to linear electron accelerators in the multi-turn, large current and large energy regime. The main characteristics of the powerful energy recovery linac experiment facility (PERLE) are derived from the design of the Large Hadron electron Collider, an electron beam upgrade under study for the LHC, for which it would be the key demonstrator. PERLE is thus projected as a facility to investigate efficient, high current (> 10 mA) ERL operation with three re-circulation passages through newly designed SCRF cavities, at 801.58 MHz frequency, and following deceleration over another three re-circulations. In its fully equipped configuration, PERLE provides an electron beam of approximately 1 GeV energy. A physics programme possibly associated with PERLE is sketched, consisting of high precision elastic electron-proton scattering experiments, as well as photo-nuclear reactions of unprecedented intensities with up to 30 MeV photon beam energy as may be obtained using Fabry-Perot cavities. The facility has further applications as a general technology test bed that can investigate and validate novel superconducting magnets (beam induced quench tests) and superconducting RF structures (structure tests with high current beams, beam loading and transients). Besides a chapter on operation aspects, the report contains detailed considerations on the choices for the SCRF structure, optics and lattice design, solutions for arc magnets, source and injector and on further essential components. A suitable configuration derived from the here presented design concept may next be moved forward to a technical design and possibly be built by an international collaboration which is being established.  As demonstrated in the conceptual design report [1], the LHeC may be realised by the addition of an intense electron beam to the LHC proton (and ion) beams following the now commencing upgrade of the LHC for increased luminosity. It uses two electron linear accelerators arranged in a racetrack configuration, tangential to the LHC tunnel.
In three-turn operation mode one is able to generate an electron beam of 60 (50) GeV energy for a circumference of U(LHeC)=U(LHC)/n of approximately 9 (5) km length, for n = 3 (5). This configuration would be of immediate use and immense value if the LHC proton energy was doubled, and it has also been considered as the default option for a future electron-hadron operation of the FCC [3]. The value of the Higgs production cross section at the LHeC of O(100) fb sets a luminosity goal of O(10 34 ) cm −2 s −1 which in the linac-ring configuration of the LHeC, at a total power limit of 100 MW, can only 1 Introduction be achieved [3,4,5] by application of the energy-recovery technique recently reviewed in [6,7]. This sets a target for the electron current of PERLE to be of order 10 mA.
The demonstration and optimisation of the LHeC principles and parameters require building a high current, multi-turn ERL facility. Its main parameters shall correspond to the LHeC design, and experience with PERLE's operation would be transferable to the LHeC. The LHeC frequency was chosen to be 801. 58 MHz, which is compliant with the LHC, keeps beam-beam interactions low and further corresponds well to general optimisation considerations including power, surface resistance and cost. That frequency is also a base frequency for the FCC development such that there is a multiple use envisaged of the The design parameters of the facility, its purpose and range of applications distinguish it from a number of further new ERL developments, such as MESA at Mainz [8], BERLin-PRO [9], Cβ [10,11] at Cornell, and the recent ER@CEBAF [12] proposal for a new experiment at the Thomas Jefferson Laboratory. The frequencies of MESA, BERLinPRO and Cβ are 1.3 GHz, while CEBAF operates at 1.5 GHz. MESA is directed primarily to weak interaction measurements. BERLinPRO and Cβ push for very high current developments. The ER@CEBAF intention is for a test at small currents but high energies, of about 6 GeV, in order to study synchrotron radiation effects on the ERL performance [13].
The present paper describes a conceptual design of an LHeC demonstrator and some of its possible applications. PERLE would be of use for the beam based development of SCRF technology, regarding for example the determination of current load limits and the control of higher order modes. It would provide the necessary infrastructure for testing the 3-turn behaviour, stability and reproducibility of the ERL, beam quality measurements in (de)acceleration etc. As is described, the facility would be of use for testing equipment, such as SC magnets and their quench behaviour, under beam conditions. It may also provide a low energy electron test beam for developments of detector technology such as thin Silicon trackers. Various selected and particularly attractive physics applications of PERLE are sketched, comprising, with electron beams, searches for dark photons, weak interaction or proton radius measurements, and, with photon beams, the physics of photonuclear reactions, nuclear structure, particle physics metrology and astrophysics, at photon intensities hugely exceeding that of the ELI facility [14] currently under construction in Southern Europe.
This paper is organised as follows: Section 2 describes the multiple purpose of PERLE, including a possible later application as an injector to the LHeC. Section 3 presents the conceptual design of the facility, its system architecture, optics layout etc. Section 4 characterises the main components, the electron source, injector, SC cavity, cryomodule, magnets, transfers, beam dumps and also the generation of a photon beam through backscattered laser light. Section 5 describes aspects of monitoring and operating such a facility, largely based on experience from CEBAF at the Thomas Jefferson Laboratory. Section 6 provides initial considerations of site requirements, followed by a brief summary in Section 7.

CHAPTER 2
Purpose

SCRF and ERL Tests with PERLE
PERLE is designed to be a multi-purpose and flexible machine that will be able to provide unique test beams in either ERL mode or as a multi-pass re-circulated linac (like CEBAF).
It can also be constructed in a phased approach enabling early operation and logical, minimally invasive upgrades. The high intensity, low emittance beams will be invaluable for many hardware and instrumentation test programs as well as offering the potential for low energy physics experiments, dark matter searches, unique light sources etc. Besides these many advantages, PERLE is also a ground breaking accelerator and SRF demonstration and development facility. The principles of multi-pass acceleration and energy recovery

Cavity module -principle and tests
The cryostat is the less glamorous cousin of the cavity and is often something of an af-

Goals of the ERL design and operation
The purposes of the PERLE ERL demonstrator are to provide flexible test beams for component development, low energy physics experiments, and also to demonstrate and gain operational experience with low-frequency high-current SRF cavities and cryomodules of a type suitable for scale up to a high-energy machine. Since the cavity design, HOM couplers, FPC's etc. will be all new or at least heavily modified, PERLE will serve as a technology test bed that will explore all the parameters needed for a larger machine. There is no other high current ERL test bed in the world that can do this. PERLE will also feature emittance preserving recirculation optics and this will also be an important demonstration that these can be constructed and operated in a flexible user-facility environment. The machine must run with high reliability to provide test beams for experimenters or ultimately provide Compton or FEL radiation to light source users. This demonstration of stability and high reliability will be essential for any future large facility.

Technical Applications
An intense beam facility will offer new opportunities for auxiliary applications.

Magnets, cables, quench tests
Understanding the quench levels of superconducting cables and magnets is important for an efficient design and the safe and optimal operation of an accelerator using superconducting magnets. Quench levels are used as an input to define requirements for controlling beam losses, therefore influencing e.g. beam cleaning and collimation, beam loss monitor positions and thresholds, interlock delays etc..
The quench level defines the maximum amount of energy that can be deposited locally in a superconducting magnet or cable to cause the phase change from superconducting to normal-conducting state. The quench level is a function of the energy deposition distribution and the duration of the impact, the local temperature before the impact, the cooling capacity, and the local magnetic field.
State of the art electro-thermal solvers, which are used to predict the quench levels of superconducting cables and magnets, are mainly based on lab experiments without beam.
To verify their predictions in case of beam impact, quench levels have been extensively studied with beam in the LHC at the end of Run 1 in February 2013. The results for short duration (< 50µs) and steady state (> 5s) energy deposition are in good agreement with predictions based on electro-thermal simulation codes like QP3 [15] and THEA [16].
For intermediate duration energy depositions the electro-thermal models predict a factor 4 lower quench levels than found during the experiment [17], which still needs to be understood.
Currently the LHC is the only accelerator at CERN, where quench tests with beam can be performed for all relevant time scales. Nevertheless, the LHC is not an adequate test  [18,19]. An emittance of 50 µm and a beta-function at extraction of 5 m was used. The bin size was 1 mm 3 .
bed to perform quench tests as: i) only magnets installed in the LHC can be tested, ii) non-trivial beam dynamic studies are required to interpret experimental results and iii) the LHC is a sophisticated accelerator which is ultimately optimized to deliver luminosity to the particle physics experiments. The other facilities at CERN either lack the availability of cryogenics (PS, HiRadMat) or the particle beams (SM18). Furthermore, using the fast extraction from the SPS the HiRadMat facility could only cover the regime of short duration energy deposition. Therefore, a dedicated facility equipped with cryogenics to perform quench tests is required. Combining the peak energy deposition with the quench levels for the LHC main dipoles, as calculated by QP3, the number of primary particles required to reach quench levels for different durations of the energy deposition can be derived. Figure 2.3 summarises the required number of primary particles in case of different particle energies and pulse length durations.

Energy deposition studies
Comparing these numbers to the baseline beam parameters shows that PERLE can pro- by FLUKA [18,19]. An emittance of 50 µm and a beta-function at extraction of 5 m was used. The bin size was 1 mm 3 .  Concerning the layout, PERLE could, for example, be reconfigured keeping only two passages and lowering the accelerating field to 125 MV/linac in order to balance the power between the two of them. Further considerations have to be made:

Technical Applications
• The LHeC requires continuous beam injection, therefore other applications of PERLE would be relegated to the LHeC downtime, thus disrupting its user program; • If PERLE would be located at ground level on the CERN site, a some-hundredmetres tunnel, with a reasonable slope, has to be dug from the location of PERLE to the LHeC tunnel. A kilometre-scale transfer line will probably be needed to transport the beam to the LHeC injection chicane.
It should be noted that with the PERLE accelerating gradient of 15 MV/m, an active length of just 33 m is required to reach the LHeC injection energy even without recirculation. A dedicated linac, placed in a ∼100 m tunnel close to the LHeC injection chicane could be a preferable option. The possibility to reuse PERLE components for this machine could be taken into account. It so seems less preferable, though possible, to consider the genuine PERLE facility when located at CERN as an injector to the LHeC.

Physics with Electron Beam
Elastic ep scattering has been of fundamental importance since, now 60 years ago, it lead to the discovery of a finite radius of the proton of about 1 fm by Hofstadter [20]. This process has a major revival as recent determinations of the proton radius with electrons and muons strongly disagree, see below. With its outstanding luminosity and large energy range, hugely interesting opportunities open up with PERLE measurements of unprecedented precision. These, as sketched below, concern measurements of the scale dependence of the electroweak mixing angle, sin 2 θ , of the electric and magnetic formfactors, G E and G M , of hyperon physics and searches for physics complementing the Standard Model.
New physics may appear in loop corrections or in direct manifestations of new particles, for which dark photons, leading to the reaction e − A → e + e − e − A, are currently a prime example [21,22].

Physics with Electron Beam
Following a brief recollection of the elastic scattering characteristics and the luminosity prospects of PERLE, three interesting physics applications are illustrated subsequently i) the potential for weak interaction measurements using polarised e − p scattering; ii) a discussion of the status and possibilities for new precision measurements of the proton form factors, pion production and iii) the search for light dark matter and new physics.

Elastic ep scattering and luminosity
For a given electron beam of energy, E, scattered off a fixed proton target, the elastic ep cross section depends only on the polar angle θ of the scattered electron. This determines both the negative four-momentum transfer squared, Q 2 , and the energy E of the scattered electron through the relations where M is the proton mass. The cross section, in its Born approximation, is given as the product of four factors, the Rutherford formula, the Mott electron spin modification, a correction, equal to E /E, for the proton recoil and finally a function f (G E , G M , θ ), which characterises the spin and the spatial extension of the proton with α the fine-structure constant. With the convention τ = Q 2 /4M 2 the form factor term is given by To some first approximation, one has G M = µ p G E and G E = 1/(1 + Q 2 /0.71GeV 2 ) 2 , with the anomalous magnetic moment µ p of the proton. The two form factors G E and G M can be separated through a variation of the energy following Rosenbluth. This should be an advantage of PERLE as with its variable energy it may cover a large range from a few hundreds of MeV to almost 1 GeV. The formulae above are sufficient for practical estimates of counting rates, but neglect all the physics which is contained in corrections to Eq.2.2 as arise from electroweak, BSM and higher order QED effects.
The luminosity of a facility like PERLE is obtained as L = ρlN A N e . For a hydrogen target of density ρ = 0.07 g cm −3 and length l = 10 cm one gets L = 4.3 · 10 23 cm −2 N e .
For a source delivering 320 pC of charge and a 25 ns bunch spacing one obtains a current of 12.8 mA corresponding to about 8 · 10 16 e s −1 , or a number of electrons per bunch of N e = 2 · 10 9 . As a consequence the luminosity for elastic ep scattering can be expected to be as high as 3 · 10 40 cm −2 s −1 with a 10 cm proton target.

Parity violation and the Weinberg angle
The unification of the electromagnetic and weak interactions within the SU(2) L xU(1) theory is expressed by the Weinberg angle sin 2 θ W , which has a strong characteristic dependence on the momentum scale ( Q 2 in ep scattering) due to loop corrections [23] to the tree-level expressions, see Measurements with the LHeC (FCC-he), as presented in the LHeC CDR [1], will be based on very large electroweak asymmetry effects and determine the electroweak mixing angle precisely for a range below the Z mass up to high scales of 1 (3) TeV.
With PERLE one can access effects from Z-boson exchange with polarised electron scattering, as well as with charge asymmetry measurements, for Q 2 between about 0.1 and 1 GeV. The intensity of a polarised electron source is probably an order of magnitude higher than that of a positron source. This makes the measurement of a polarised electron scattering asymmetry, A − more likely than that of a charged or combined charge and polarisation asymmetry, B. Both have been discussed in [24]. The polarisation asymmetry can be expressed as where κ = Q 2 G/ √ 22πα determines the size of the asymmetry to be O(10 −4 Q 2 /GeV 2 ).
Here v e and a e are the weak neutral current (NC) couplings of the electron and V and A are new combinations of the form factors G E and G M which also depend on the quark NC couplings as well as the charged current axial vector form factor. Evidently, the asymmetry A − is different from zero through parity violation. With PERLE, it allows to measure the mixing angle in a particularly interesting range of scale, as is illustrated in Fig. 2

.4.
Besides providing a measurement of sin 2 θ W , with ep scattering asymmetries, one accesses also new combinations of quark couplings. Following [24] one sees, for example, that the hadronic axial vector factor A determines a combination of a d + 3.55a u which can be compared with ep scattering at HERA and the LHeC where The measurement accuracy depends on the beam energy and scattering kinematics. This is illustrated in Fig.2.5. Since the asymmetries vanish at small angles while the cross section decreases towards larger angles, an optimum is observed, with striking variations.
One finds for a beam energy E ∼ 1 GeV that asymmetry measurements at θ ∼ 30 − 90 • can be expected to be especially precise. The measurement of the weak mixing angle at small scales is an area of vigorous activity, because of the new level of precision anticipated in a coming generation of tests of its predicted scale dependence, as at Mainz and Jefferson Lab, and because of the relation these measurements have to new physics such as rare Higgs decays and dark Z bosons, see [25] and references therein. The salient potential of the here presented ERL facility consists in its potential large energy coverage and particularly high luminosity which make further studies of the possibility to measure that process with PERLE interesting indeed.

Proton form factors
The proton electromagnetic form factors, G E and G M , which have been studied for many decades, have become the focus of recent research mainly due to the proton radius puzzle, recognised even in the popular press [26]. It is the more than 7σ discrepancy between the determination of the proton radius with electrons (r E = 0.8775(51) fm [27]) and using muon spectroscopy (r M = 0.84087(39) fm). Since its observation in 2010, the discrepancy Spline fit statistical uncertainty stat+systematical uncertainty variation of Coulomb correction Figure 2.6: The fits in [30] for the magnetic form factor G M , divided by the standard dipole, exhibit a maximum-minimum structure at low Q 2 . While the local minimum around 0.2 (GeV/c) 2 is seen in earlier fits, the local maximum around 0.03 (GeV/c) 2 has not been observed before.
has sparked large work efforts on both the experimental and theoretical side, but no widely accepted explanation has yet been found.
On the electron side, both spectroscopy and scattering experiments agree. In the latter, the radius is extracted from the slope of the form factors at Q 2 = 0. Since data can only be taken at finite Q 2 , the form factors have to be extrapolated to 0. Currently, the most precise data set from scattering experiments [28,29,30] has been measured by the A1 collaboration in Mainz at the MAMI accelerator. It contains more than 1400 measured cross sections and reaches closest to the static limit with Q 2 min. ≈ 0.003 (GeV/c) 2 . While there are no structures/changes of curvature expected below this point, it is not possible to rule them out. Such structures would invalidate the extrapolation and may resolve part of the puzzle. This data set also found an interesting structure in G M at low Q 2 , shown in Fig. 2.6. The magnetic form factor, divided by the standard dipole, exhibits two local extrema. While the minimum is found in earlier extractions, the maximum has not been seen and is in fact below the resolution of previous data. This leads to a significantly different magnetic Spline with variable knots fit statistical uncertainty stat + model dependence uncertainty However, the cusp is only visible in the combination of multiple data sets and could be an artefact.
PERLE could provide crucial new high-precision data to study these three phenomena using different experimental approaches: • Possible structures below Q 2 min and their influence on the proton radius could be studied with a single, low beam energy and forward scattering experiment, similar to the PRad experiment [31]. At lower energies and higher beam currents than planned for PRad, an ERL beam with a point-like target (e.g. a gas jet) could provide higher rates and smaller systematic uncertainties. An alternative approach is to exploit initial state radiation, measuring deep into the radiative tail to probe Q 2 -values that are orders of magnitude smaller than directly accessible. This approach is described in more detail e.g. in [32].
• The low-Q 2 structure in G M could be studied in an experimental setup similar to [28]. The interesting region in Q 2 would be covered by performing an angular scan of the cross section and multiple energies up to 300 MeV. Such an experiment would benefit substantially from a point-like target without target walls, which are the main background of [28]. It would produce an electric radius with similar uncertainties, and a magnetic radius with substantially improved precision compared to current results. Additionally, with a polarised beam and target, an asymmetry measurement, sensitive to the ratio G E /G M , could be performed. Such a measurement would help to disentangle G E and G M from the cross-section measurement and would make it possible to study whether the structure is related to imperfect radiative corrections.
• The high-Q 2 structure could be studied with high precision using beam energies of 1 GeV and up, possibly with just one angular scan of the cross section at a fixed energy around 1.3 GeV. Without a good connection to lower beam energies, the precision of the absolute normalisation is not likely to better than a few percent, however the cusp structure is large enough that a good relative normalisation of the data points, e.g. using a detector at forward angles as a luminosity monitor, is enough to extract a meaningful result.

Pion electroproduction
Using virtual photon tagging, it is possible to study confinement-scale QCD. In photoproduction, the photon tagger sets the rate limit and only a small fraction of the tagged photons interact with the target, leading to low data-taking efficiency. At forward angles, the virtual photons are almost real, so that a forward scattering electron tagger can be used as a highly efficient substitute. Because of the high efficiency and high beam currents, it is possible to use pure, thin targets and detect low energy recoil particles which would not escape traditional, thick targets. It is thus possible to measure the reactions γ p → π 0 p, π + n, γn → π 0 n, π − p and γD → π 0 D. Coherent π 0 production in D and 3 He measure relative signs of the γ p → π 0 p,γn → π 0 n amplitudes.
Such an experiment requires beam energies of 300 MeV or more. Depending on the target, beam current and polarization capabilities, different experiments are possible: • With about 1 mA unpolarized beam, a measurement with a thin, windowless, unpolarized gas target, detecting either the π + or the recoiling proton, could be performed. This would allow a test of a nn = a pp and few-body calculations via γD → nnπ + , and also check a np with γD → npπ 0 . It would further be possible to test isospin conservation by testing • At about 100 mA unpolarized beam with a windowless transverse polarized gas target, one could test isospin breaking through a measurement of γN → π 0 N near threshold.

Light dark matter
The search for new physics beyond the Standard Model is a major focus of the nuclear and particle physics community. A simple extension of the SM Lagrangian [34,35] leads to new "dark" Abelian forces with a new dark gauge field A . Among many others, a possible production mechanism is e − p → e − pA (→ e − pe + e − ), i.e. the elastic scattering with a radiated "dark" photon, and the possible subsequent decay of the radiated A into a lepton pair ("visible decay") The DarkLight experiment [36], planned to be run at the Jefferson Lab ERL, aims to search for these visible decays in the region preferred by the muon g-2 results, detecting all four outgoing particles. A variant also looking for invisible decays is planned [37]. The PERLE facility could be an option for a version 2 of the experiment, with increased luminosity.
Alternatively, with high-precision, high-rate detectors measuring just the recoiling proton and electron, it should be possible to mount a competitive search sensible to both visible and invisible decays. More work is needed to study this further.

Speculative ideas
At Q 2 above 1 GeV 2 , determinations of the form factor ratio from unpolarized and polarized measurement do not agree. This has been attributed to two-photon exchange, whose

Physics with Photon Beam
size is directly tested in current experiments [38,39,40]. At lower Q 2 , this effect is believed to be small, but could explain part of the proton ratio discrepancy. A positron source would make it possible to measure the effect directly at small Q 2 , validating theoretical calculations.
The experiments described so far require a fixed target. Colliding beams open additional interesting possibilities. Head-on collisions with a high-momentum proton beam can probably not help with the physics described, however, if it could be arranged that the beam collide almost colinearly, i.e. essentially with the same, not opposite, direction, one would access the fixed-target equivalent of backward scattering at very low Q 2 , accessing the magnetic form factor at unprecedentedly small four-momentum transfer. Similar, a collision of a muon and electron beam in this way would test lepton universality, a further possible explanation for the radius puzzle.

Physics with Photon Beam
This section is meant to briefly sketch the potential for fundamental research with γ-ray beams that the PERLE facility will be capable of producing by laser-Compton backscattering off the intense cw electron beam. The production mechanism and expected γ-ray beam parameters will be described below. Since the scope of this Conceptual Design Report does not allow a comprehensive compilation of all possible research venues, this section includes only a limited selection of research opportunities.
Photonuclear science is currently witnessing a transformation of the field which has started [41] with the advent of intense, energy-tunable, completely polarized, quasi monochromatic γ-ray beams from laser-Compton back-scattering at the High Intensity γ-ray Source (HIγS) [42] at the Duke Free Electron Laser Laboratory (DFELL) at Duke University, Durham, NC, U.S.A., and will continue with the European Extreme Light Infras-

tructure -Nuclear Physics (ELI-NP) which is currently under construction in Magurele,
Romania [14]. ELI-NP is expected to deliver first γ-ray beams in the energy range from 0.5 -19.5 MeV with a band width of 0.5% and a peak-spectral density of 10 4 γs/(eV s cm 2 ) starting in 2017. Photonuclear science at ELI-NP is enjoying a strong international user community of 100 -200 scientists who potentially could later be attracted to the PERLE γ-beam due to its expected superior performance, in particular with respect to intensity, band-width, and the CW time structure.
Photonuclear reactions impact on a variety of research topics in nuclear structure physics, for an advanced γ-ray beam to be established at the PERLE facility, apart from additional commercial or medical applications.

Photonuclear reactions
Gamma-rays with energies up to 30 MeV can induce a variety of photonuclear reactions.
Photoinduced nuclear excitations below the nuclear separation energy will decay by subsequent re-emission of γ-radiation. When this reaction proceeds via a nuclear resonance it is addressed as nuclear resonance fluorescence (NRF). The NRF process may populate an excited low-lying nuclear isomer which may decay by β -decay processes addressed as internal photoactivation.
Photodisintegration reactions become possible when a nucleus is photo-excited above the separation threshold. Then either neutrons or charged nuclear constituents such as protons or even α-particles can be emitted. Photodisintegration reactions that result in a daughter nucleus which is radioactive are called external photoactivation. An extreme mode of photodisintegration is photofission where a nuclear fission process occurs once the nucleus has been activated by the absorption of the γ-ray. The various photonuclear reactions are sketched in Fig. 2.8.

Nuclear structure physics
The field of nuclear structure physics addresses the investigation of the nuclear manybody problem and its understanding in terms of effective nucleon-nucleon interactions that emerge from QCD as the effective interaction between hadrons. Since the electromagnetic interaction is understood quantitatively, photonuclear reactions enable the separation of the photonuclear reaction mechanisms from the nuclear properties and thus nuclearmodelindependent measurements. Due to the clean reaction mechanism of γ-rays with the nucleus, its iso-vector and one-step character, the field of nuclear structure physics has tremendously profited from photonuclear research since the seminal works of Bothe and Gentner in 1937 [43].

Nuclear single-particle structure
The recent understanding of nuclear shell-evolution as a function of nucleon number and the contribution of effective three-body forces [44] to it make the precise measurement of effective single-particle energies in nuclei a research topic of high current interest. Photonuclear reactions offer a unique tool to study E1 and M1 single-particle excitations from the ground state. Of particular interest is the study of the nuclear spin-orbit splitting between a nuclear level with total spin quantum number j > = l + 1/2 and its spin-orbit partner with spin quantum number j < = l − 1/2. These single-particle orbitals are connected by a strong M1 matrix element of the order of 1 nuclear magneton (µ N ) that can be measured precisely by photonuclear reactions, e.g., by the measurement of ground state excitation widths Γ 0 in NRF measurements.
Also the relative assignment of various Nilsson orbitals in deformed nuclei can be clarified with photonuclear reactions. Once sufficiently intense and narrow band-width γ-ray beams will be available at the PERLE facility, it will become possible to study the electromagnetic excitation cross sections of the rotational band-head states of deformed, oddmass isotopes in the rare-earth mass region [45].

Collective nuclear structures
Of particular interest is the study of collective nuclear excitation modes with photons.
Prime examples are the Isovector Giant Dipole Resonance (IV-GDR) for a collective E1 excitation or the Scissors Mode of deformed nuclei for a collective M1 excitation mode.
Both are fundamental modes of the nuclear many-body system and have intensely been studied by photonuclear reactions [46]. Due to the limited spectral density and abundant low-energy background at previous bremsstrahlung sources, important questions are still not resolved. What is the quadrupole deformation of the scissors mode? How does the IV-GDR emerge as a function of excitation energy and what is its fine-structure? How does the decay of the components of the IV-GDR depend on their K-quantum number?
What is the nature of the Pygmy Dipole Resonance (PDR) that rides on the low-energy tail of the IV-GDR and dominates the nuclear E1 response near the particle separation threshold? PERLE could contribute to answering these questions. Measurement of the intrinsic E2 matrix element between the scissors mode and the nuclear ground state requires the determination of the absolute monopolar E2 decay width between a state of the scissor mode band and the ground state band, e.g., the J π = 1 + band head of the scissors mode band and the 2 + 1 state of the ground state rotational band in a deformed even-even nucleus. The measurement of the monopolar partial decay width of inter- , requires the measurement of partial decay width , which is routinely done in NRF experiments on the Scissors Mode, and the E2/M1 multipole mixing ratio, δ , of this γ-decay transition. This has not been done so far. Such a measurement will be achievable at the Compton-backscattered γ-beam of the PERLE facility by measuring the azimuthal NRF intensity distribution about the polarization plane of the γ-beam. The measurement will determine the quadrupole collectivity of the scissors mode and will open up a research program on how this collectivity is related to the nuclear shape (prolate, oblate or triaxial,...) and its underlying single-particle structure.
The polarization and high intensity of the new γ-beam will open up another research field on the electric dipole response of nuclei below and above the nuclear separation threshold. Along the lines of research that have been started at the HIγS facility at DFELL, the strength, energy distribution and decay properties of the PDR can be studied with PERLE at much higher sensitivity than before. In particular it will become possible to excite the nucleus at a preselected excitation energy region in the PDR or in the IV-GDR and then to measure the decay γ-ray transitions either to the ground state or to low-energy excited states of interest. It will become possible to search for the PDR of deformed nuclei and to thereby answer the question if the PDR in deformed nuclei exhibits a splitting according to its K-quantum number components, K = 0 or 1. Until now, neither has the PDR been observed in deformed nuclei, nor has it been clarified if the γ-decay of the IV-GDR in deformed nuclei differs between its K = 0 or K = 1 components. A detailed understanding of these phenomena as a function of deformation, neutron excess, or excitation energy above particle separation threshold will become possible.

Nuclear photofission
Nuclear fission represents an extreme case of collective nuclear behavior. It can be trig- A better understanding of the fission processes, in particular of long-lived trans-uranium actinides is of very high interest of the society.

Particle physics metrology
Due to our understanding of the unified electroweak interaction, the electromagnetic reaction processes of photons with nuclei are closely related to nuclear reactions involving the weak interaction [47]. Consequently, photonuclear studies can, at least partly, shed light on weak interactions in materials that are employed in detectors for weak-interaction processes such as detectors for searching for neutrinoless double-beta decay or for neutrino signals from supernovae.

Nuclear matrix elements for 0νβ β -decay
It has recently been demonstrated [48] how photonuclear investigations on the M1 strength distribution of initial and final nuclei in 0νβ β -decay reactions can help to improve the theory for 0νβ β -decay matrix elements. Knowledge of these matrix elements will be mandatory for the determination of the neutrino mass once the 0νβ β -decay rate would have been measured. The M1 decay branching ratio was recently found to be linked to the 0νβ β -decay branching ratio to the low-energy 0 + states of the final nucleus.

Detector response to stellar neutrinos
Supernovae are bright sources for neutrinos. Detectors for the measurement of neutrinos from supernovae are operational or under construction. Due to neutrino oscillations, not all of the neutrinos reaching the detector will be electron-neutrinos ν e but may have oscillated to other possible neutrino-flavors. Non-ν e neutrinos with typical energies of a few MeV may react on the detector material by neutral-current scattering processes, that may be inelastic and are expected to be dominated by Gamow-Teller type matrix elements from the ground state. These are closely related to the matrix elements for M1 excitations. In order to be able to quantitatively interpret the signals from neutral-current neutrino scattering on detector material it is important to precisely know and understand the M1 excitation strength distributions of nuclei present in the detectors searching for stellar neutrinos.

Nuclear astrophyics
Energetic γ-rays belong to the thermal environment in stars. Understanding of nuclei in the variety of stellar conditions requires a detailed knowledge of photonuclear reactions.
Research opportunities for photonuclear reactions in nuclear astrophysics are numerous.
We will mention only two examples.

Stellar capture reactions
Stellar capture reactions, such as (p, γ), (n, γ), or (α, γ) determine the vital "energy production" in stars. For stars slightly heavier than our sun the CNO-cycle dominates, by which 4 protons are converted into an α-particle and released binding energy in a sequence of capture and decay reactions on carbon, nitrogen, and oxygen isotopes. Break-out of the CNO-cycle can occur, when the stable ground state of 16 O will be populated. Of particular interest is the cross section for the 12 C(α, γ) 16 O reaction at energies corresponding to stellar temperatures. This cross section is very small, therefore difficult to measure, and despite of its importance, not known. By the principle of detailed balance in timereversal invariant reactions valuable constraints could be obtained from the inverse reaction 16 O(γ, α) 12 C which could be studied with an intense quasi-monochromatic γ-ray beam. A

Detector Test Beam Use
corresponding research program has started at HIγS but suffers from too low intensity (10 3 γ/(eV s)) and too large energy-spread (1 -3%). The superior properties of PERLEs γ-ray beam will facilitate these measurements.

Nuclear synthesis
One of the most outstanding physics questions is that to origin of the chemical elements in nature. Heavy nuclei beyond iron are produced in the various capture processes in stars, while latest research results indicate that supernova explosions are not capable of producing a sufficient amount of elements heavier than silver [49]. Very heavy elements, such as Thorium or Uranium, undoubtedly require a rapid-neutron capture process (r-process) in a dense and hot environment with a high neutron flux. In order to understand the survival rate of just synthesized heavy nuclei one needs to understand their reactions on the thermal radiation. Thermal γ-rays are capable of inducing photoactivation reactions on seed-nuclei and transforming them in other species. Stellar photonuclear reactions on stable nuclei will become possible to be studied at the PERLE γ-ray beam with unprecedented sensitivity. • detailed effects of electromagnetic calorimeter measurements (very high resolution sampling at normal and low temperature);

Detector Test Beam Use
• novel detector systems concepts, etc.
Detailed tests of detector samples and components for the upcoming High Luminosity LHC, nuclear physics experiments or other colliders to follow could be performed at a PERLE testbeam. The beam energy would be low. A special application then may be to calibrate detectors one would build for the physics with PERLE.
For a test-beam extension of the PERLE scope, the following aspects are important: • the extraction and shaping section has to be foreseen in the design ensuring the space and elements necessary are available; • a beam line enclosure with instrumentation; • suitable shielding, transportation and escape routes have to be taken into account when space requirements for the experimental setup are being discussed; • Interlock system; • Magnet control for momentum selection; • Patch panels with pre-installed cables; • Gas warning systems; • Fast internet connection; • light weight (state of the art) trigger setup; fast and precise. Test beam studies allow education in many respects as in the experimental preparation, trigger setup and evaluation, data acquisition, data taking (shifts, on-call), or software on track reconstruction or alignment. A test beam configuration at PERLE appears attractive to consider indeed.

Design and Parameters
The PERLE facility aims at a maximum of 1 GeV energy recovery demonstration of a recirculating SC linear accelerator. The test facility should serve as a test bed to gain quantitative and qualitative understanding of the electron beam recovery process. The accelerator development purposes of this test facility, as introduced above, are first, confirming the feasibility of the LHeC ERL design by demonstrating stable intense electron beams with the intended parameters (current, bunch spacing, bunch length); secondly, testing novel accelerator components such as a (polarized) DC electron gun, SC RF cavities, cryomodule design and feedback diagnostics; finally, experimental studies of the lattice dependence of stability criteria. The realisation of this facility will allow addressing several physics challenges such as maintaining high beam brightness through preservation of the six dimensional emittance, managing the phase space during acceleration and energy recovery, stable acceleration and deceleration of high current beams in CW mode operation. The facility design must also allow addressing other performance aspects such as longitudinal phase space manipulations, effects of coherent synchrotron radiation (CSR) and longitudinal space charge, halo and beam loss and microbunching instability. These issues could have sizeable impacts on machine performance in the region of the design parameter space. Thus a design emerges of a system that, in principle, needs to be flexible in supporting multiple operating points and indeed, provides a reasonable validation of the LHeC accelerator baseline. There are up to three passes. There will be a pre-acceleration unit following the source to enter the ERL with relativistic electrons (>5 MeV). Each beam recirculates up to three times through both linacs to boost the energy to 900 MeV. To enable operation in the energy recovery mode, after acceleration the beam is phase shifted by 180 • and then sent back through the recirculating linac at a decelerating RF phase. During deceleration the energy stored in the beam is reconverted to RF energy and the final beam, at its original energy, is directed to a beam dump. The set of main parameters incorporated into the ERL prototype injector is shown in   (Fig. 3.2). A subsequent upgrade could be the installation of two additional arcs on each side to raise the beam energy up to 450 MeV (Fig. 3.3). This configuration accommodates for available space for implementation of feed-back, phase-space manipulations, and beam diagnostic instrumentation, giving the possibility of a full validation testing with energy recovery. In phase 3, as shown above (Fig. 3.1), four additional cavities in each linac will be added to permit energy recovery recirculation tests at full energy. The facility, in this final configuration, could represent, in principle, a smaller clone of the final LHeC project and could serve as a model for a pre-accelerator/injector to the final 60 GeV machine, see 2.3.

Transport Optics
Appropriate recirculation optics are of fundamental concern in a multi-pass machine to preserve beam quality. The design comprises three different regions, the linac optics, the recirculation optics and the merger optics.
A concise representation of multi-pass ERL linac optics for all six passes, with constraints imposed on Twiss functions by sharing the same return arcs by the accelerating       (Fig. 3.7).

Layout and Magnet Inventory
The path of each pass is chosen to be precisely an integer number of RF wavelengths, except for the highest energy pass whose length is shifted by half an RF wavelength to recover the energy through deceleration. The total beam path for a full 3 pass accelerating cycle is around 300 m. This leads to an approximate footprint of 43 m × 16 m of the ERL itself. Accurate values are presented in Table 3.2.
Diverse plausible optics layouts have been studied. A possible option would consist of arcs with identical configurations in order to have compact magnets stacked on top of each other.
A preliminary inventory of the magnets of the LHeC Test Facility lists: • 40 bending magnets (vertical field); • 36 bending magnets (horizontal field) in the spreaders / combiners; • 114 quadrupole magnets; • 6 magnets in the injection / extraction parts; • a few magnets for path length adjustment. The waves indicate the RF electromagnetic oscillations.
Turn number Total pathlength

Bunch Recombination Pattern
The bunch spacing at the injector, dump and delivered is 25 ns, as shown in Fig. 3 Special care has been taken to select a pattern that maximises the distance between the lowest energy bunches inside the RF structure: the ones at the first and the last turn, as shown in Fig. 3.9 and summarised in Table 3.3. This comes from the fact that, with a nearly constant β function, the kicks from HOMs are more disruptive at lower rigidities, thus, if two low energy bunches follow each other, the BBU threshold current can be reduced.

End-to-end Beam Dynamics Simulations
Tracking simulations have been performed initially with the tracking code elegant [52], to investigate single-bunch effects as the coherent synchrotron radiation (CSR) and the impact of multipolar field components, and later with PLACET2 [53], to verify the recombination pattern and asses the BBU threshold current.  Comparing the longitudinal phase space at injector and at dump (see Fig. 3.12) one can note that the bunch length is well preserved, proving the isochronicity of the whole lattice.

Single-bunch end-to-end
A small energy chirp is present at the dump, which shall be removed with a fine tuning of   the arc lengths. Figure 3.12 (right) shows the longitudinal phase space at 900 MeV. While the curvature induced by the RF can be seen, the total energy spread remains extremely contained (below 0.01 %).

Multi-bunch tracking and BBU
PLACET2 is capable of tracking many bunches simultaneously in the lattice preserving their time sequence everywhere in the machine. This allowed to verify the bunch recombination pattern and assess multi-bunch effects in a realistic operational scenario. A Gaussian spread has been introduced in the frequencies of the cavity HOMs assuming a detuning factor of 1 × 10 −4 . It has been verified that for the final design stage, including a total of 16 cavities, different detuning seeds lead to similar results.
The plots in Fig. 3.13 show the amplitudes of the HOMs in one of the cavities as many bunches pass by. One can see that when the bunch charge is increased from 1.6 nC to 1.9 nC a mode starts to build up in the vertical plane leading to an instability. Note that this bunch charge is more than 5 times the one foreseen for operation.

Source and Injector
The   More robust photocathodes based on Sb are less sensitive to vacuum conditions and to back ion bombardment. Pioneering experiments at Boeing [66], and the University of Twente [67], at Brookhaven Laboratory [68], TJNAF [69], and Cornell University [58] demonstrated the possibility to obtain a reasonable Q.E. for Sb-based photocathodes at a level of 5-10% and, most importantly, their ability to deliver a high current for a substantial period of time.

Photocathode gun
The main decisive parameter of a DC photocathode gun is its operational voltage. It defines the energy of electrons at the exit of the gun and the 'rigidity of the beam'. This operational voltage also dictates the electric field on the photocathode which defines maximum emission density and, as a result, the beam emittance which may be estimated as ε n = qkT 2πε 0 E c mc 2 .  In order to get preliminary estimates required on the drive laser system to deliver beam with parameters required for PERLE, the performance is calculated of a 350 kV gun with a JLAB-DL electrode system operating with Cs3Sb photocathode (Fig. 4.2). Simulations have shown that an optimal beam emittance of 2 πmm-mrad can be obtained with illumination of the photocathode with a laser pulse with hat top spatial distribution with a diameter of 3 mm and a flat top laser pulse with a length of 80 ps. The RMS bunch length at 1 m from the photocathode is 8.5 mm (36 ps) which only slightly depends on the laser pulse length.

Buncher and booster
Once emerged from the gun, the electron beam begins to elongate due to the space charge repulsion. To longitudinally compress the bunch to the required 3 mm a compensation energy chirp should be introduced which is typically done with an RF buncher. In order to provide linear energy modulation the frequency of the buncher should be selected to have

Cavity Design
PERLE will be a low to medium energy facility in several stages from 150-450-900 MeV for both technology validation and a versatile test bench for high average current applications. This section will outline some key aspects of the linac cavity design and its opti-mization. Table 4.3 lists the cavity configurations for the three phases of the ERL facility.

Choice of operating frequency
The choice of frequency and gradient is important for any project and depends on a range of factors. It is definitely not a one-size-fits all situation. For large projects, the total cost is dominated by a few competing items such as RF power, cryogenics, structure costs (e.g. modules) and conventional facilities (tunnel, surface buildings, penetrations, etc.).
Each of these has a frequency and gradient dependence and depends on the choice of underlying technology assumed. In general the overall cost optimum is a balance between linear costs (such as structure and tunnel) which increase as the gradient is lowered and the machine gets longer, and quadratic terms such as RF power and cryogenic capacity, which increase as the gradient is increased but result in a shorter machine. The result is a rather broad cost minimum allowing some flexibility in the choice of frequency and gradient to accommodate other factors. There are various cost models in use or under development but in general the optimum frequency for this type of machine is somewhere between a few hundred MHz and one GHz. Below this range the structures become very expensive and above this range RF power costs increase. As has been extensively studied in the conceptual design of the LHeC the frequency needs to be significantly below a GHz also for avoiding adverse effects due to beam breakup instability [1]. For compatibility with the LHC, a harmonic of 200 MHz is highly desirable. A frequency of 801.58 MHz is a convenient harmonic 1 that is close to the estimated cost optimum and also compatible with other systems currently in use or under development at CERN [72,73,74]. The optimum gradient range is also quite wide, ranging from around 10 to 20 MV/m depending on assumptions about the temperature and Q 0 that can be reliably expected. In general for a large machine the lowest reasonable gradient should be adopted to maximise reliability and minimise the chances of field emission. However, for a small machine like PERLE, at least in the first phase, the cost optimum may favour a higher gradient. 1 Note that 801.58 MHz is the 20 th harmonic of the bunch repetition frequency, and, since 20 is not an integer multiple of 3, the bunches of the three re-circulations cannot be equally spaced; this is discussed in more detail in Section 3.4 above.

Design considerations
The maximum accelerating gradient is primarily limited by the CW power dissipated on the cavity walls. Due to the quadratic dependence, a medium accelerating gradient with the lowest surface resistance (high Q 0 ) at moderate to high gradients is required. The number of cells per cavity is a compromise between a reasonable "real estate gradient" while reducing the probability of trapped modes.
The salient feature of an energy recovery linac, at least in CW operation, is the continuous transfer of stored energy from the cavity to the accelerated beam and simultaneously the transfer of (almost equal) energy from the decelerated beam back into the cavity. To first order, the power fed into the cavity through the fundamental power coupler (FPC) from the power source is equal to the power losses in the cavity walls, which can be extremely small. Another formulation of this feature is that the net beam loading at the fundamental frequency is zero in spite of a large beam current. As a consequence, the excitation of HOMs, notably at frequencies where accelerated and decelerated beam currents are not in anti-phase, will be dominating the design.

Initial design choices
The choice of five cells per cavity is retained from technical arguments derived in Ref. [74].
The standard parameterisation for elliptical cavities is used [75]. Fig. 4.3 shows the en-  This design will be referred to as version 2. An alternative "low-loss" like design was also considered; it is described below in Sect. 4.2.3.
Relevant RF parameters for the mid-cell and five-cell geometries are listed in Table 4.3 and compared to the initial scaled version.

Impedance spectra
The

Loss factors and HOM power
The very small bunch length can excite frequencies well up to 50 GHz or above. This is characterised by the longitudinal loss factor k || . Fig. 4.6 shows the frequency dependence of the integrated loss factors for the initial two versions of the cavity.
In addition to HOM damping, the induced HOM power from the short bunches is of the   However, resonant excitation of a HOM can easily lead to powers in the 1 − 2 kW range (assuming R/Q = 50 Ω and Q ext = 10 4 ). Therefore, the couplers will have to be designed to handle this power and impose the condition of HOM impedance to not exceed 500 kΩ for the longitudinal modes. For transverse modes, single and multi-bunch simulations have to be carried out to determine the acceptable damping levels. The effect of the transition sections using tapers and bellows is already discussed in Ref. [74].

External Q and power requirements
Considering the steady state condition of recirculating beams and energy recovery only, the beam loading can be assumed to be small. Then the input RF power required to maintain the cavity voltage is directly proportional to the peak detuning, see Fig. 4.7.
A realistic Q ext ∼ 10 7 with a corresponding power of 50 kW will allow for sufficient margin during transients. At these power levels and frequency range, standard UHF television IOTs become an attractive and robust option.

Cavity optimisation
The cavity cell shape should be carefully optimised to balance accelerating mode efficiency with HOM damping needs (loaded Q's) and HOM power extraction (HOM frequencies relative to the high current lines in the beam spectrum), as well as mechanical and cleaning considerations. Shapes such as the JLab ERL high-current profile [77] and BNL3 ERL [78] cavity are good examples. Starting from these so-called "Low-Loss" shapes, which feature cavity shapes with a steep wall angle down to 0 • , led to the cavity optimisation described here. The low-loss type profile (vertical wall) and contoured irises produce moderate surface magnetic and electric field enhancements normalised to the accelerating gradient; the vertical walls also are the main difference compared to the initial designs with larger inner diameter describe above. This is a one -die design, meaning all the cell cups are produced from the same profile with the end cells simply being trimmed shorter to tune for field flatness.
Extracting HOM power from the cavities to room temperature absorbers must be considered in the cryomodule design (see below). Very effective HOM damping can be achieved by absorbers on the beamline either side of the cavity, providing the beam pipe is sufficiently enlarged to allow the dangerous HOMs to propagate. These, however, consume valuable space and the absorbers must be thermally isolated from the cold beamline components. The JLab waveguide damping scheme [77] avoids this by taking the HOM power out sideways to warm loads but is probably overkill for the LHeC requirements. As already indicated above, loop-coupled HOM dampers, possibly similar to the LHC type mounted on the ends of the cavity close to the end cell, will be sufficient. An example of the implementation of these couplers is described in detail in Sect. 4.3 below. Many other configurations are of course possible. For this type of coupler, the HOM power is removed via a cable to a warm termination. This also allows easy monitoring of the HOM signals for diagnostic purposes.  Table 4.4, comparing it to a subset of the shapes investigated in this study with iris diameters varying from 115 to 160 mm and limited to solutions with equal iris and tube diameters.
Normalised to λ , the beam tube and iris diameter of the selected solution are slightly larger than the TESLA or CEBAF upgrade (LL) shapes, but smaller than the original CE-BAF (OC) or JLab high-current (HC). This allows good cell-to-cell coupling for HOM damping and reduced sensitivity to fabrication errors, while preserving high shunt impedance for the operating mode for good efficiency. The outer part of the cell profile is tuned to keep   Fig. 4.4).

Cavity Design
harmful HOMs far away from beam harmonics.

Cryo Module
PERLE comprises up to four cryo modules each containing four 802 MHz five-cell cavities. A convenient concept for these may be developed by adapting the four-cavity SNS high beta cryo module designed by JLab [79], to accommodate 5-cell β =1 cavities, as is shown in Fig. 4.11. Since the cavities are almost the same length as the original 805 MHz β = 0.81 6-cells, no major changes to the module would be required. This design uses a single, large volume helium vessel for each cavity, Fig. 4.12, with the vessels connected by a two-phase pipe to allow gas and liquid to pass freely along the module. No separate gas return or two-phase pipes are needed. At the ends of the module this header is connected to supply and return end cans that contain the bayonet connections, valves, reliefs, etc.,  correctors, BPM's etc. Each helium vessel has an end-mounted, Saclay-type tuner [80] and there are bellows between the cavities that minimise mechanical cross talk during tuner operation. On the other end of each cavity, there is a coaxial fundamental power coupler [81] developed from the Tristan design at KEK. The cavities are suspended from a warm space-frame by low conductivity rods. The couplers are at longitudinal fixed points in the support scheme so only have to accommodate radial motion during cool down. This is achieved with an external warm bellow in the top hat connection. There are no cold bellows or indeed any bellows in the RF section of the coupler. For SNS, the cold part of the outer conductor is trace cooled with counter-flowing helium gas to minimise the heat load to 2 K. This gas flow is controlled by a separate dedicated valve. This active cooling may not be required for LHeC. The module could also be adapted to use an LHC type or other proven coupler.
The helium vessel may be titanium like the SNS modules or stainless steel like the CE-BAF 12 GeV upgrade modules. For Titanium, a NbTi transition piece is used adjacent to the end irises to connect the helium vessel to the cavity and titanium bellows are used. For stainless steel, a Nb to stainless brazed joint can be used and the vessel bellows and piping can all be stainless steel. Care must be taken to avoid introducing permeable or magnetic material close to the cavity. Fig. 4.12 shows a concept with provision for three such couplers mounted symmetrically on the end group to share the damping duties without in-  For this type of coupler the HOM power is removed via a cable to a warm termination, or taken outside the module where it can be monitored for diagnostic purposes.
The measured static loads at 2 K of the SNS type cryo-module were typically less than the 28 W budget, and shield static load was less than the 200 W budget at ∼ 50 K (inlet 40 K, outlet up to 80 K). For LHeC the dynamic loads of the CW cavities will be much higher than the pulsed SNS cavities. For standard Nb material at 2 K dynamic heat loads of 30 − 40 W per cavity at 18.7 MV/m with Q 0 ∼ 2 · 10 10 may be expected. Thus the maximum dynamic load per module may approach 160 W, with total 2 K load less than The SNS cryo-module is therefore a convenient model for PERLE and could be adapted with minimal changes to host the new 802 MHz 5-cell β = 1 cavities. A new concept [82] using many of the design features of this module, as well as attractive features of other JLab designs, is being developed for the JLab Electron Ion Collider [83]. Features of that module might also be considered for an eventual LHeC production cryo-module. A simple cavity design has been developed that is a favourable balance between good HOM properties and good operating efficiency. Further refinement and optimisation of these concepts is expected in the near future.

Arc Magnets
The inventory of the main magnets for PERLE lists: for arcs 3 to 6. The same cross-section could be used for both, though they would differ in terms of length and curvature radius. In both cases a curved construction is assumed, with possibly machined yokes. A tentative cross-section is shown in Fig. 4.16. An H type yoke is proposed, rather narrow in the vertical direction, to minimize the vertical distance between the arcs. The dimensions could be further reduced -in particular horizontallyafter an iteration on the required field quality. The coils will need to be designed as part of an overall optimization, including the power converters. The shaded area in Fig. 4.16 refers to 6-7 A/mm 2 of current density at the maximum field of 1.31 T of arc 6.
While the dipole strenghts simply scale across the arcs, this is not the case for the quadrupoles, as each arc has a different optics. Table 4.5 summarizes the maximum and minimum integrated gradients as well as pole tip fields for the quadrupoles. This is based on the two lengths -200 and 300 mm -currently specified in the lattice, as in Figs. 4.18 to 4.23. This results in a quite wide range of integrated gradients and pole tip fields. Moreover, some quadrupoles are rather weak. This prompts an iteration with the optics, which needed to be refined after a full design of the bending magnets including the edge effects.
The possibility of making families, grouping by gradient or length or both, would need to be considered. Two preliminary cross-sections are shown in Fig. 4.17. Since the aperture is the same throughout the arcs, an option could be to keep the same iron design, though to have only 2 instead of 4 coils for the weaker quadrupoles. The impact of this asymmetry on the field uniformity is rather minor, about 2 · 10 −4 at 2/3 radius on the skew octupole in 2D.
As for the main bending units, the coils could be water cooled (for compactness) and they will need to be designed as part of the overall optimization, including the power converters, the magnet manufacturing cost and the operational scenarios, considering for example different baseline optics. The shaded area in Fig. 4 Table 4.5: Summary of integrated gradients and pole tip fields of quadrupoles, in T.
A further analysis will address in detail the magnets in the spreaders and combiners regions. Furthermore, a set of vertical / horizontal dipole correctors will most likely need to be added. According to their strength and field uniformity tolerances, these correctors could be combined with some of the quadrupoles in a hybrid design. Path length adjustments, mainly from seasonal contraction and expansion effects, amounting to an expected O(1) cm correction, may be addressed via dog legs in the arcs. Finally, multiple aperture magnets could be analyzed as part of an overall cost optimization, though much could depend on the staged construction of the facility.

Dumps and Transfers
The

Setup dumps
During the commissioning period of PERLE, and in general during the beam setup, it is important to be able to dump the beam at the different energies. The easiest solution is to keep switched off the first horizontal dipole of the arc corresponding to the energy of interest and let the beam go straight towards the dump (Fig. 4.27). This dipole has to have a C-shape to allow the installation of a Y chamber for the recirculating and the extracted  materials, the setup should be performed with a reduced intensity. In Table 4 to a constant power deposition at the beam dump of 64 kW for the different energies of PERLE and assuming an initial current of 12.8 mA.
The dump system will consist of three superimposed blocks of graphite with a radius of 20 cm and a maximum length of 1.2 m (for the 950 MeV beam) to absorb also the secondary showers. Additional shielding has to be envisaged and a total occupancy of 2 m×3 m has to be considered around the dump blocks.

Emergency dumps
Up to now only DC magnets have been considered. In the eventuality that the setup dumps have to be also used as emergency dumps, fast kickers have to be included in the lattice.
The CW operation mode and the 25 ns bunch spacing require a rise time t m = 23 ns to allow for some jitter. A system impedance Z of 25 Ω is assumed, and a rather conservative

Test facility
The in Tab. 4.7 are used for the dump design. It is assumed that the beam setup is done with a reduced intensity, the full intensity beams will then be dumped on the samples. For further analysis, more detailed optics studies have to be performed, the dynamic range of the magnets and potential RP issues to be evaluated.

Optical system
Depending on the electron-beam time-structure, various optical systems capable to produce high gamma-ray fluxes are nowadays available. On the one hand, for bunch trains of low repetition rate, non-linear [84] or passive [85] optical recirculators may be used (e.g. ELI-NP-GS [86]: trains of 32 bunches separated by 16 ns at a repetition rate of 100 Hz).
The related laser system has to provide the maximum pulse intensity allowed by the foreseen spectral density (e.g. ELI-NP-GS: 400 mJ at 100 Hz for green 515 nm light, 14 µm transverse spot size of the intensity profile and 3 ps longitudinal pulse width). On the other hand, for CW electron bunches of repetition rate 10 MHz, Fabry-Perot cavities [87] (i.e. optical resonators) may be used [88,89,90,91]. This is the technical solution envisaged for the PERLE photon beam facility.
Fabry-Perot cavities consist of a sequence of high reflectivity mirrors (see Fig. 4.30).
When the laser beam frequency satisfies resonance conditions (see [92] for pulsed beams), the power is enhanced at most by a factor G = F/π inside the cavity (in practice laser/cavity spatio-temporal mode mismatches can reduce this factor by several dozens of percent).
The cavity finesse F depends on mirror losses and reflection coefficients. However, the higher the cavity enhancement factor the narrower the optical resonance ∆ν/ν = λ /(LF), where ν = c/λ is the laser frequency and L the cavity optical round-trip length. Dedicated laser cavity feedback is needed to preserve the resonance conditions [93,92]. Experimentally, a cavity with F ≈ 28000 (G ≈ 9000) for picosecond pulses and with L = 4 m was demonstrated by some of us in [94]. for storing 10 ps pulses of average power above 600 kW is presently under development at LAL by some of us for the Compton X-ray machine ThomX [98]. This is a similar optical cavity that is needed for the PERLE photon beam facility. Besides, a CW laser beam of 700 kW will also be stored in the VIRGO interferometer in a near future [99]. There is thus a global effort to achieve stable and routinely operating cavities in high average power regime. One should also mention that developments on long L ≈ 30 m monolithic and high finesse cavity are also on-going [100].
Mode properties (wave front profile, polarization) of optical cavities solely depend on their geometries. Specific optical designs must then be supplied to fulfill the requirements of Compton experiments [101,102]. Following the arguments of Ref. [102], one must consider planar four-mirror cavities made of at least two concave reflective surfaces for the ERL SCRF photon beam facility (see Fig. 4.30). The distance between the two planar mirrors (M 1 and M 2 ) can be adjusted to lock the cavity round-trip frequency to the accelerator radio-frequency while the distance between the two concave mirrors (M 3 and M 4 ) can be varied to tune the laser beam spot size at the IP. This geometry has been successfully tested at the ATF [91]. Eventually, with a careful design of the high reflectivity mirror coating, the mode polarization of a planar four-mirror cavity can be freely tuned.
The laser source is of prior importance for high finesse cavities. One must start from a low phase noise mode-locked oscillator and then amplify the signal using the chirped pulse amplification technique [103]. The laser amplifier system is also of prior importance

Cavity design
There is freedom in choosing the cavity geometry. Here a trade-off is proposed between a small laser-electron crossing angle, small enough laser beam spot size at the IP while polarization instabilities [101]. As for the mechanical mirror mounts, motion actuators and vacuum vessel, we propose to adopt the technical solutions tested successfully over years at ATF/KEK [91], [98]. It is noticeable that these elements were recommissioned without any difficulty after the 2011 earthquake, and the design can thus be considered as robust.

Machine Commissioning
Machine commissioning has combined goals of validating system design architecture and defining a recoverable system operating point. For an ERL, this requires demonstration of the control of phenomena of concern -such as beam break-up (BBU) and the microbunching instability (µBI) -while generating settings for hardware components. Following pre-commissioning 'hot' checkout of accelerator components and commissioning of hardware subsystems, beam operations commence with threading of low power beam so as to establish a beam orbit and correct it to specified tolerances. This requires orbit correction systems based on beam position monitors and steerers (typically every quarter-betatron wavelength); unique to a multipass ERL with common transport of multiple beams in a single beam line is the requirement that the system correct perturbations locally so that the multiple passes respond identically and the orbits not diverge unacceptably from turn to turn. Similarly, a baseline for longitudinal beam control must be established, by synchronising the beam to the RF using recirculator arcs as spectrometers for precision measurements of energy gain. Any path length adjustments needed to set RF phases and insure energy recovery per the design longitudinal match are thus determined. With a 6D phase space reference orbit thus defined, the beam and lattice behaviour is tuned and validated.
Lattice performance is measured, tuned, and certified using differential orbit/lattice transfer function measurements; these, too, will require pass-to-pass discrimination amongst beams in common transport. Both transverse and longitudinal measurements (using phase transfer function diagnostics [113]) are necessary for a full analysis of lattice behaviour.

Machine Operation: Monitoring and Maintaining Machine Health
Corrections must be applied to 'rematch the lattice' and bring both transverse (betatron motion/focusing) and longitudinal (timing/momentum compaction) motion into compliance with design (or to establish an alternative working point). Certification of lattice performance allows analysis, tuning, and validation of beam parameters, and matching of the beam to the lattice. This requires measurements of both betatron (emittance, beam envelope functions) and longitudinal (bunch length/energy spread/emittance, phase/energy correlation) properties. Disentangling the properties of multiple beams in common transport may prove challenging and require use of beyond-state-of-the-art techniques. If beam properties differ excessively from specification, 'matching' of the beam to the lattice is performed using appropriate correction algorithms. As with orbit correction, perturbations will likely require local correction so as to avoid excessive pass-to-pass divergence of beam properties. Given a validated working point, beam power scaling is performed, with currents increased from tune-up levels to full power CW. Transient control and beam stabilization (see below) will be initially investigated and demonstrated during commissioning; they remain a persistent activity through the operational lifetime of the machine, and are therefore discussed below.

Maintaining Machine Health
Routine machine operations entail numerous monitoring and correction functions intended to provide beam stability for users and to control and preserve machine performance at a specific set point. These include timing and energy control, which is needed to provide synchronism, for example, at an interaction point, and to maintain the stability of delivered beam properties. This may require a high resolution timing system (if user timing is critical), and will require continuous measurement of energy and energy stability and control mechanisms for energy stability (see the following discussion of stabilisation).
Similarly, user requirements may demand measurement and precise control of the orbit of the delivered beam. This can be provided by appropriate enhancements to -and utilisation of a subset of -the beam orbit correction system provided for orbit control during commissioning. Both transverse and longitudinal controls of this type are needed as the machine is used to explore beam dynamics, instability control, and beam quality preservation. Machine performance is susceptible to degradation as system parameters change due to thermal effects and hardware parametric drift. Beam and lattice properties, control parameters, magnets, and RF variables are all susceptible to such effects; control algorithms providing appropriate monitoring of, and intervention/correction so as to restore RF gradients/phases, beam orbits, lattice focusing, and beam properties are required. These may be established as intermittent machine performance checks and retuning procedures, or, alternatively, be considered as 'low speed feedback' systems in which critical beam and machine parameters are monitored and corrected. These provisions are also used for recovering machine configurations/working points after trips and system shutdowns. Halo control is critical to the operation of high power ERLs. Halo sources include field emission in SRF systems, cathode-driven sources (such as light scattered onto active areas and surface defects) that can change with ageing, beam/residual gas interactions, beam/wake interactions, and beam dynamical effects during beam formation and handling. All can lead to significant radiation background and potentially unacceptable levels of beam loss.
Methods/hardware for monitoring and independent tuning of large amplitude components of multiple beams in common transport are therefore necessary to avoid activation and damage to system components. These can include collimation and/or nonlinear matching using, for example, higher order multipoles (sextupoles, octupoles, etc), and require the use of large dynamic range diagnostics [114]. Transient control (maintaining machine and beam health through RF trips, other fast shutdowns, and/or inevitable hardware problems) is needed for all phases of machine operation and is discussed below.

System Stabilisation
ERLs are non-equilibrium systems subject to drift, jitter, and instability in any of numerous system variables on any of several time scales. They are typically under-constrained, with the number of noise-subjected control parameters much larger than the output observables of relevance to users. Specific strategies for system stabilisation are therefore needed. User requirements must be established from the outset of the system design process, and provision for hardware, software, and procedural control made so as to achieve adequate stability.   [115]. Energy control is coupled to synchronism and timing Orbit stability also varies over time and can be subject to jitter. Though orbit stabilisation techniques are well established, the presence of multiple beams in common transport places constraints on both the diagnostics on which the controls are based and on the feedback methods to be used so as to insure that beam-and pass-specific results are achieved.
Given the presence of both high beam brightness and high beam power, the possible need for instability control (BBU, wake effects, etc) must be considered, and the system design should provide opportunity for fast feedback if necessary. Similarly, stability of beam properties is not assured, and means of continuous monitoring/adjusting delivered beam quality (e.g. energy spread, bunch length, spot size/divergence, bunch, etc.) should be provided as necessary.

Transient Control and Machine Protection
ERLs are subject to numerous transient effects, two classes of which are of particular operational importance: the impact of RF transients (beam off/on transients, variable beam loading during current ramps, and RF trips), and machine protection fast shutdowns. RF transients due to variations in beam loading [116] are manageable with appropriate RF drive design. Care in choice of Q ext is of importance, as is planning for the type and operational range of the longitudinal match; implementation of incomplete energy can result in greater transient control requirements than encountered in systems with complete energy recovery. The RF drive system (control loops, feed-forward/back) must be configured to manage transients as experienced under different machine operating conditions and operating points; RF power and cavity tuning should be monitored during routine operation to insure that stability is maintained. Dramatic transients (particularly in beam loading) will occur during machine-protection-system (MPS) driven fast shutdowns. As ERL beam powers are very high, loss tolerances are tight and large losses must be prevented. Critical to machine safety, the MPS continually monitors the accelerator for beam loss and rapidly shuts off the beam if unsafe loss levels are observed [117]. The machine control system monitors and records the interlock sequence precipitating the fast shutdown so as to characterise the source of the transient event and provide guidance on correction of the fault.

Site Considerations
The interest in PERLE, sketched in the present report, is threefold, regarding its technology development potential, its physics and applied user programme and its importance for demonstrating and studying the technology choice of the LHeC. At present there is no decision as to where PERLE may be placed. An initial study, of also general interest, considered the possibility of hosting PERLE at CERN. This is sketched below. It was subsequently studied to possibly build this facility at LAL Orsay, may be at reduced beam energy for keeping its dimension fit to the available infrastructure and halls. This is also mentioned below. Recently, an idea has also been considered of building a low energy, lower current version of PERLE at Darmstadt in Germany.

Introduction
As mentioned in the lattice section, the genuine footprint of the PERLE facility at its maximum energy of about 1 GeV occupies a rectangle of 42 × 14 m 2 . This area should be enclosed by shielding at a sufficient distance to allow passage and maintenance operations.
We estimate the required passage and half thickness of the accelerator component to 2 m.
A concrete shielding of 50 cm thickness is assumed here to stop photons and neutrons produced by halo electrons. Detailed simulations of the radiation generated by the impinging electron will be necessary at a later stage. An increase of the shielding required could be alleviated by the use of denser materials like lead. Access conditions and the geographical location of the site may also influence the final choice of shielding. In addition to this central area, space needs to be allocated for the auxiliary systems like: • Power converters for magnets, septa and kickers; • RF power. Assuming IOTs or solid state amplifiers as close as possible to the SRF modules to minimize RF losses; • Water cooling.The dimensioning of this system greatly depends on the operational modes; • Cryogenics. The use of a dewars for storing liquid helium at 4.5 could avoid the cost of a liquefier. However it will limit flexibility of operation in non-recovery mode and needs to be studied further; • Source; • Dump. A design of the dump exists with a minimum length of 50 m (reference) but a more compact version could be used by limit the current or repetition rate when working on non recovery mode; As a rough estimate one would like to double the area of the accelerator itself to accommodate all services. It is worth noting that some services like RF power generation or power supplies may be placed on a different level than the accelerator itself, while the source or the dump may not. We do not consider here the use of the interior part of the ring as the escape routes would be compromised. It may however be used to house a low energy dump which itself needs to be shielded and which will have restricted access.

CERN
For an initial study, we have been considering existing buildings around the CERN site 1 .
The building needs to be equipped with a crane, water and electricity services. The availability of cryogenic fluids would be an interesting option and provide considerable savings.
The installation of electrical power and demineralised water seems to be less costly. The total area of the installation would be then of the order of 1500 m 2 with an incompressible 1 With the appearance of the Orsay option, detailed below, CERN as a site has not been considered further.

LAL Orsay
Prior to the publication of this report it has been realised that LAL Orsay would be very well prepared to house PERLE, preferentially at up to 450 MeV energy which required an inner area of about 20 × 7 m 2 . The building that would host this version of PERLE is a former experimental hall, the Super ACO hall, which is equipped with cranes and electricity. The ground of the building is made of concrete slabs with variable ground resistance.
Nevertheless, more than the half of the hall area has a sufficient resistance to allow the installation PERLE. A complete study will be performed to confirm this fact. Being next to the tunnel of the old Orsay linac and close to the "Igloo", where new accelerators are being installed currently, the building is partially shielded and water-cooling circuits could be shared with the other machines. The location is illustrated on Fig. 6  PERLE is foreseen to demonstrate and gain operational experience with low-frequency high-current SCRF cavities and cryomodules of a type suitable for scale up to a highenergy machine. Since the cavity design, HOM couplers, FPC's etc. will be all new or at least heavily modified, PERLE will serve as a technology test bed that will explore all the parameters needed for a larger machine. There is no other high current ERL test bed in the world that can do this. PERLE will feature emittance preserving recirculation optics and this will also be an important demonstration that these can be constructed and operated in a flexible user-facility environment. The machine, when transformed from a test to a user facility, must run with high reliability to provide test beams for experimenters or ultimately provide Compton or FEL radiation to light source users. This demonstration of stability and high reliability will be essential for any future large facility.
As an example for technical impact, the present study has demonstrated the use of the electron beam to perform quench tests on SC components and magnets. The facility may be used for low energy test beam measurements and it may serve as a base to design or build the injector of the LHeC. An exiting physics programme has been detailed from operating PERLE as a gamma ray facility with a very high flux, at least two orders of magnitude above expected upgrades of existing facilities, and superior spectral density. A path is shown to discoveries using up to 30 MeV photons and for a variety of novel, unique and precise measurements on photonuclear reactions, nuclear structure as well as to important measurements for neutrino and nuclear astrophysics.
A thorough simulation study is presented of the system architecture, the transport optics and start-to-end beam dynamics. The paper presents initial design concepts of the main components for PERLE, applicable also to its possible lower energy version. These comprise descriptions of the source and injector, the 802 MHz cavity, under design and construction by us, of a cryomodule and HOM design considerations. Further, the inventory and novel designs are presented of the arc magnets. A section is devoted to rather detailed considerations for the dumps and transfers.
For CW electron bunches of larger than 10 MHz repetition rate, Fabry-Perot optical resonators are suitable to provide a high quality photon beam and are presented in this paper as a preferred reliable solution.
A final chapter is devoted to the monitoring and operations tasks including the commissioning, system stabilisation and protection aspects. Considerations have also been presented for the site and its infrastructure. These naturally will be updated once a site is finally chosen which most likely will be at the campus of the Linear Accelerator Laboratory at Orsay (Paris).