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Nonlinear equilibria and emittance growth in plasma wakefield accelerators with ion motion

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 Added by James Rosenzweig
 Publication date 2020
  fields Physics
and research's language is English




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The plasma wakefield accelerator may accelerate particles to high energy in a future linear collider with unprecedented acceleration gradients, exceeding the GeV/m range. Beams for this application would have extremely high brightness and, subject to the intense plasma ion-derived focusing, they would achieve densities high enough to induce the plasma ions to collapse into the beam volume. This non-uniform ion density gives rise to strong nonlinear focusing which may lead to deleterious beam emittance growth. The effects of ion collapse and their mitigation has been investigated recently through particle-in-cell simulations, which show that by dynamically matching the beam to the focusing of the collapsed ion distribution, one may avoid serious emittance growth. We extend this work by exploring the near-equilibrium state of the beam-ion system reached after the ions have collapsed, a condition yielding the emittance growth mitigation observed. We show through PIC simulations and analytical theory that in this case a dual electron beam-ion Bennett-type equilibrium distribution is approached. Here, the beam and ion distributions share nearly the same shape, which generates nonlinear transverse electromagnetic fields. We exploit a Bennett-type model to study beam phase space dynamics and emittance growth over time scales much longer than permitted by PIC simulations through use of a 2D symplectic tracking code with Monte Carlo scattering based on Molieres theory of small angle multiple scattering. We find that while phase space diffusion due to parametric excitations of the beam size due to plasma non-uniformity is negligible, scattering from collapsed ions gives rise to manageable emittance growth in the case of a linear collider. The implications of these results on experiments planned at FACET-II are examined.



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89 - C. Hansel , M. Yadav , P. Manwani 2021
A future plasma based linear collider has the potential to reach unprecedented energies and transform our understanding of high energy physics. The extremely dense beams in such a device would cause the plasma ions to fall toward the axis. For more mild ion motion, this introduces a nonlinear perturbation to the focusing fields inside of the bubble. However, for extreme ion motion, the ion distribution collapses to a quasi-equilibrium characterized by a thin filament of extreme density on the axis which generates strong, nonlinear focusing fields. These fields can provoke unacceptable emittance growth that can be reduced through careful beam matching. In this paper, we discuss the rich physics of ion motion, give a brief overview of plans for the E-314 experiment at FACET-II which will experimentally demonstrate ion motion in plasma accelerators, and present results of particle-in-cell simulations of ion motion relevant to the E-314 experiment.
A framework for integrating transfer matrices with particle-in-cell simulations is developed for TeV staging of plasma wakefield accelerators. Using nonlinear transfer matrices in terms up to ninth order in normalized energy spread $sqrt{langledeltagamma^2rangle}$ and deriving a compact expression for the chromatic emittance growth in terms of the nonlinear matrix, plasma wakefield accelerating stages simulated using the three-dimensional particle-in-cell framework OSIRIS 4.0 were combined to model acceleration of an electron beam from 10 GeV to 1 TeV in 85 plasma stages of meter scale-length with long density ramps and connected by simple focusing lenses. In this calculation, we find that for initial relative energy spreads below $10^{-3}$, energy-spread growth below $10^{-5}$ of the energy gain per stage and normalized emittance below mm-mrad, the chromatic emittance growth can be minimal. The technique developed here may be useful for plasma collider design, and potentially could be expanded to encompass non-linear wake structures and include other degrees of freedom such as lepton spin.
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