Imaging laser-wakefield-accelerated electrons using miniature magnetic quadrupole lenses

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Imaging laser-wakefield-accelerated electrons using miniature magnetic quadrupole lenses

R. Weingartner, M. Fuchs, A. Popp, S. Raith, S. Becker, S. Chou, M. Heigoldt, K. Khrennikov, J. Wenz, T. Seggebrock, B. Zeitler, Zs. Major, J. Osterhoff, F. Krausz, S. Karsch, and F. Grüner

Phys. Rev. ST Accel. Beams 14, 052801 – Published 12 May 2011

Abstract

The improvement of the energy spread, beam divergence, and pointing fluctuations are some of the main challenges currently facing the field of laser-wakefield acceleration of electrons. We address these issues by manipulating the electron beams after their generation using miniature magnetic quadrupole lenses with field gradients of $\sim 500 T / m$ . By imaging electron beams the spectral resolution of dipole magnet spectrometers can be significantly increased, resulting in measured energy spreads down to 1.0% rms at 190 MeV. The focusing of different electron energies demonstrates the tunability of the lens system and could be used to filter out off-target energies in order to reduce the energy spread even further. By collimating the beam, the shot-to-shot spatial stability of the beam is improved by a factor of 5 measured at a distance of 1 m from the source. Additionally, by deliberately transversely offsetting a quadrupole lens, the electron beam can be steered in any direction by several mrad. These methods can be implemented while still maintaining the ultrashort bunch duration and low emittance of the beam and, except for undesired electron energies in the energy filter, without any loss of charge. This reliable and compact control of laser-wakefield accelerated electron beams is independent of the accelerator itself, allowing immediate application of currently available beams.

Received 8 September 2010

DOI:https://doi.org/10.1103/PhysRevSTAB.14.052801

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Authors & Affiliations

R. Weingartner, M. Fuchs, A. Popp, S. Raith, S. Becker, S. Chou, M. Heigoldt, K. Khrennikov, J. Wenz, T. Seggebrock, B. Zeitler, Zs. Major, J. Osterhoff, F. Krausz, S. Karsch^*, and F. Grüner^†

Department für Physik, Ludwig-Maximilians Universität, 85748 Garching, Germany
Max-Planck Institut für Quantenoptik, 85748 Garching, Germany

^*stefan.karsch@mpq.mpg.de
^†florian.gruener@physik.uni-muenchen.de; http://www.fel.physik.uni-muenchen.de/

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Issue

Vol. 14, Iss. 5 — May 2011

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Images

Figure 1
Experimental setup and electron beam evolution: (a) A laser pulse is focused into a 15 mm long hydrogen-filled gas cell and accelerates electrons to $\sim 200 MeV$ . The electrons pass through a pair of magnetic quadrupole lenses $\sim 20 cm$ behind the gas cell which can focus, collimate, and steer the electron beam. The first lens (L1) is 17 mm long and the second lens (L2) 15 mm. Both lenses have a measured field gradient of $\sim 500 T / m$ . The spatial characteristics of the beam are observed on a removable scintillating screen (S1) 1.12 m behind the exit of the gas cell. A dipole magnet with field strength $\sim 0.45 T$ disperses the beam which then allows the observation of the energy spectrum by a second scintillating screen (S2) 1.94 m behind the gas cell. (b) Envelope of a 200 MeV electron beam with source size $1 μ m$ and source divergence 1 mrad focused at S2 with lenses as described in (a). (c) Bunch elongation against the rms source beam divergence for focusing at S2 with a PMQ and an EMQ doublet, the bunch has zero initial duration.Reuse & Permissions
Figure 2
Electron beam spatial stability improvement: False color images observed at S1, 1.12 m behind the source (experimental subfigures are each normalized to one). (a) Sum of 20 consecutive electron beams and peak positions of each shot (dots). Shot-to-shot pointing fluctuations as well as the beam divergence lead to FWHM widths in the summed signal of $5.3 \times 5.2 mm$ ( $x axis \times y axis$ ). (b) Sum of 47 consecutive electron beams with a magnetic lens doublet set to collimate 220 MeV electrons, the FWHM widths are reduced to $0.9 \times 1.2 mm$ ( $x axis \times y axis$ ). The inset shows a sum of 30 electron beam spectra taken shortly before. Despite the chromaticity of the magnetic lenses, the beam collimation is still effective even with an FWHM energy spread of 80 MeV ( $\sim 35 %$ ). Results from particle tracking are shown as lineouts (magenta dashed lines) which have already been convoluted with the instrument function of the detection system of S1 in order to compare directly with the experimental lineouts (white solid lines).Reuse & Permissions
Figure 3
Electron beam focusing: False color images of summed electron beam spectra observed at S2 (experimental subfigures are each normalized to one). (a) Sum of 20 consecutive shots without magnetic lenses. (b) Sum of 44 shots, focus at 230 MeV. (c) Sum of 30 shots, focus at 210 MeV. Vertical green lines indicate the intended electron energy to be focused at the screen position. As in Fig. 2b, the summed beam size is reduced for a broad range of electron energies. (d) Simulation with lenses positioned as in (c) for electron beams with energies 150—250 MeV with initial divergence of 1.7 mrad FWHM for three different initial pointing angles ( $θ_{x, y}$ ) leaving the gas cell: on-axis beam $θ_{x, y} = 0$ (blue), $θ_{x} = θ_{y} = 3.6 mrad$ (red), $θ_{x} = - 3.6 mrad$ $θ_{y} = 0$ (green).Reuse & Permissions
Figure 4
Energy resolution improvement: (a) A monoenergetic electron beam entering a dipole magnet spectrometer will have its measured energy spread artificially enlarged by its divergence. This is shown for two cases, monoenergetic electron beams with an initial FWHM divergence of 3.25 mrad (solid blue line) and 4.0 mrad (dashed blue line). These errors can be significantly reduced for any energy that can be focused on the observation plane at S2 by the magnetic lenses. The green (dash-dotted) and red (dotted) curves show the calculated measurement accuracy for two different lens positions. The green shaded area shows the effect of misaligning the second lens by $\pm 5 mm$ . Black squares represent measured energy spreads with the lens system focusing 190 MeV (corresponding to the green dash-dotted curve). Parts (b) and (c) show false color images of electron spectra observed at S2 for this lens setting with FWHM energy spreads of 2.8% (b), and 2.4% (c).Reuse & Permissions
Figure 5
Electron beam steering: False color summed images observed at S1, 1.12 m behind the electron source with symbols marking the peak position of individual shots. The central spot shows the sum of 18 shots (circles). The surrounding four spots (triangles of differing orientation) were observed after offsetting the second lens transversely to the beam propagation direction in four separate positions. This introduces a dipole moment which deflects the electron beam. The lens was moved by $\pm 390 μ m$ and $\pm 530 μ m$ in the $x$ and the $y$ directions and caused corresponding angular deflections of 6.1 and 7.3 mrad in the same direction.Reuse & Permissions

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