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Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

F. Lemery and P. Piot
Phys. Rev. ST Accel. Beams 18, 081301 – Published 3 August 2015
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  • INTRODUCTION
  • SMOOTH SHAPES
  • PHOTOEMISSION OF OPTIMAL SHAPES VIA…
  • FORMATION OF HIGH-ENERGY TAILORED…
  • SUMMARY
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    Collinear high-gradient O(GV/m) beam-driven wakefield methods for charged-particle acceleration could be critical to the realization of compact, cost-efficient, accelerators, e.g., in support of TeV-scale lepton colliders or multiple-user free-electron laser facilities. To make these options viable, the high accelerating fields need to be complemented with large transformer ratios >2, a parameter characterizing the efficiency of the energy transfer between a wakefield-exciting “drive” bunch to an accelerated “witness” bunch. While several potential current distributions have been discussed, their practical realization appears challenging due to their often discontinuous nature. In this paper we propose several alternative continuously differentiable (smooth) current profiles which support enhanced transformer ratios. We especially demonstrate that one of the devised shapes can be implemented in a photo-emission electron source by properly shaping the photocathode-laser pulse. We finally discuss a possible superconducting linear-accelerator concept that could produce shaped drive bunches at high-repetition rates to drive a dielectric-wakefield accelerator with accelerating fields on the order of ∼60  MV/m and a transformer ratio ∼5 consistent with a recently proposed multiuser free-electron laser facility.

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    • Received 22 May 2015

    DOI:https://doi.org/10.1103/PhysRevSTAB.18.081301

    This article is available under the terms of the Creative Commons Attribution 3.0 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

    Authors & Affiliations

    F. Lemery1 and P. Piot1,2

    • 1Northern Illinois Center for Accelerator and Detector Development and Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, USA
    • 2Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA

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    Issue

    Vol. 18, Iss. 8 — August 2015

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    Images

    • Figure 1
      Figure 1

      Example of current profiles described by Eq. (6) (shaded line) with the corresponding induced voltages. The total bunch length is set to Z=5λ (i.e., N=5) and plots (a) and (b), respectively, correspond to the cases q=2 and q=3. The head of the bunch is at kz=0.

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    • Figure 2
      Figure 2

      Example of “quadratic” current profiles given by Eq. (13) (shaded line) with corresponding induced voltage. The parameters are ν=1 and N=5. The head of the bunch is at kz=0.

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    • Figure 3
      Figure 3

      Tradeoff curves between R and E+m for the current profiles listed in Table 1. The “quadratic” and “sin ramps” respectively correspond to the distributions proposed in Secs. 2b and 2a. The Gaussian and ramp distributions are displayed for comparison.

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    • Figure 4
      Figure 4

      Configuration used for the pulse-shaping simulations using a S-band rf gun (a). A temporally shaped laser pulse (b) is optimized to result in a photo-emitted electron-beam with current profile (c) having features similar to the distribution discussed in Sec. 2b. In insets (a) and (b) the tail of the bunch is at t=0.

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    • Figure 5
      Figure 5

      Evolution of the electron-bunch current (a) and longitudinal phase space (b) along the beam line at 20 (red), 60 (green), and 100 cm (blue) from the photocathode surface and (c) comparison of the current profile numerically simulated at s=50  cm (blue symbols) with a fit to Eq. (13) (red line). The head of the bunch is at large values of z.

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    • Figure 6
      Figure 6

      Comparison of nominal (“ideal”) quadratic shape with the shapes achieved when taking into account the photoemission response time (“cathode”), the laser-pulse-shaping finite bandwidth (“shaping”), and both effects (“cathode+shaper”). The ideal laser temporal profile is described by Eq. (19) with α=2 and τ=15  ps. Insets (b) and (c) are zooms of the areas t∈[−15200,−13600]  fs (peak location) and t∈[−16000,−15020]  fs (left edge of the profile), respectively. The head of the laser pulse is at t=0.

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    • Figure 7
      Figure 7

      Comparison of the final electron-bunch current at s=50  cm from the cathode surface for the four cases considered in Fig. 6. The “cathode” and “shaper” respectively correspond to the inclusion of the cathode response time and shaper bandwidth limitation in the initial particle distribution at s=0 while the ideal case is given by Eq. (19) with α=2 and τ=15  ps. The head of the bunch corresponds to z>0.

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    • Figure 8
      Figure 8

      Block diagram of the accelerator configuration explored for the formation of high-energy ramped bunches. The legend is as follows: “QW” stands for quarter-wave, “L0” and “L1” are standard 1.3-GHz cryomodule equipped with 8 TESLA-type SCRF cavities, “L39” is a cryomodule consisting of four 3.9-GHz cavities, and “BC” is a magnetic bunch compressor.

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    • Figure 9
      Figure 9

      Final current distribution (green shaded area) and associated wakefield (blue traces) for the “ideal” (a) and “realistic” (b) cases of compression discussed in the text. The head of the bunch corresponds to z=0.

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    • Figure 10
      Figure 10

      Snapshots of the longitudinal phase spaces and associate current profiles (red traces) upstream of L0 (a) and downstream of L0 (b), L39 (c) and BC (d). Simulations up to L39 are carried with astra whereas a one-dimensional longitudinal-dynamics model is used for BC2. The head of the bunch corresponds to z>0.

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