Energy and rf cavity phase symmetry enforcement in multiturn energy recovery linac models

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Energy and rf cavity phase symmetry enforcement in multiturn energy recovery linac models

R. Koscica, N. Banerjee, G. H. Hoffstaetter, W. Lou, and G. Premawardhana

Phys. Rev. Accel. Beams 22, 091602 – Published 5 September 2019

Abstract

In a multipass energy recovery linac (ERL), each cavity must regain all energy expended from beam acceleration during beam deceleration, and the beam should achieve specific energy targets during each loop that returns it to the linac. For full energy recovery, and for every returning beam to meet loop energy requirements, we must specify and maintain the phase and voltage of cavity fields in addition to selecting adequate flight times. These parameters are found with a full scale numerical optimization program. If we impose symmetry in time and energy during acceleration and deceleration, fewer parameters are needed, simplifying the optimization. As an example, we present symmetric models of the Cornell BNL ERL Test Accelerator (CBETA) with solutions that satisfy the optimization targets of loop energy and zero cavity loading. An identical cavity design and nearly uniform linac layout make CBETA a potential candidate for symmetric operation.

Received 3 April 2019

DOI:https://doi.org/10.1103/PhysRevAccelBeams.22.091602

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Beam code development & simulation techniques

Energy recovery linacs Timing & synchronization

Accelerators & Beams

Authors & Affiliations

R. Koscica ^*, N. Banerjee, G. H. Hoffstaetter, W. Lou, and G. Premawardhana

CLASSE, Cornell University, Ithaca, New York 14853, USA

^*Corresponding author. ros.koscica@gmail.com

Article Text

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Issue

Vol. 22, Iss. 9 — September 2019

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Images

Figure 1
Layout of the CBETA ERL [3]. A 6 MeV beam is generated in the injector, accelerates through a linac with six 7-cell cavities, circulates clockwise through the return loop, and recirculates to cover a total of 4 accelerating and 4 decelerating linac passes. The particle path concludes at the beam stop (top right). Splitter and recombiner regions at either end of the linac independently control the flight times of beams with the target energies of the 4 loops.
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Figure 2
Cavity encounter sequence for a 2-pass ERL with $N = 2$ physical cavities, denoted C1 and C2, and cavity phase inputs $ϕ_{in, m n}$ and outputs $ϕ_{out, m n}$ associated with corresponding input/output times $t$ . The total time spent in the ERL, $t_{total}$ , is represented by the length of the horizontal line. This schematic can be extended to general multipass ERLs by inserting additional cavities or return loops on both sides of the dotted symmetry axis.
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Figure 3
Performance of TL, UR, FT, and FT7 cavity models with respect to a RK reference cavity at an incident beam energy of 6 MeV (solid lines) or 12 MeV (dashed lines) and maximum energy gain of 6 MeV. For cavities with length shorter than 7 cells (TL and FT), drift pipes have been added to either side to compensate for the length difference between the models.
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Figure 4
Time evolution of 1000-particle Gaussian beam in longitudinal phase space: an initial Gaussian (both plots, overlaid), the beam after complete acceleration (left), and at beam stop (right). The beam is in an FT model ERL with $M = 8$ , $N = 6$ , and the ERL is set with ideal particle solutions (Table 2). The ideal particle is defined at coordinates $(z, δ) = (0, 0)$ . Vertical and horizontal lines represent one standard deviation of the stated output from the mean $z$ or $δ$ coordinate.
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Figure 5
Tilted 1000-particle beam in longitudinal phase space for the FT model, shown after full acceleration (left) and deceleration (right). The input distribution is a tilted Gaussian: each particle’s original Gaussian $δ$ coordinate is offset according to the matrix slope from Eq. (35) and Table 5; due to the relatively large initial beam size, the edges of the final beam do extend beyond the linear regime.
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Figure 6
Tilted 1000-particle beam in the UR model after full acceleration (left), and at beam stop (right), where the beam tilt is determined by scanning through injector phases until the one that yields the most symmetric input/output $σ_{δ}$ is found. Note that the center of the final distribution after the ERL is a mirror image of that before entering the accelerator. This symmetry also determines that the slope of the distribution is zero at its center after half the ERL has been traversed.
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