Energy loss of a high charge bunched electron beam in plasma: Simulations, scaling, and accelerating wakefields

Abstract

The energy loss and gain of a beam in the nonlinear, ''blowout'' regime of the plasma wakefield accelerator, which features ultrahigh accelerating fields, linear transverse focusing forces, and nonlinear plasma motion, has been asserted, through previous observations in simulations, to scale linearly with beam charge. Additionally, from a recent analysis by Barov et al., it has been concluded that for an infinitesimally short beam, the energy loss is indeed predicted to scale linearly with beam charge for arbitrarily large beam charge. This scaling is predicted to hold despite the onset of a relativistic, nonlinear response by the plasma, when the number of beam particles occupying a cubic plasma skin depth exceeds that of plasma electrons within the same volume. This paper is intended to explore the deviations from linear energy loss using 2D particle-in-cell simulations that arise in the case of experimentally relevant finite length beams. The peak accelerating field in the plasma wave excited behind the finite-length beam is also examined, with the artifact of wave spiking adding to the apparent persistence of linear scaling of the peak field amplitude into the nonlinear regime. At large enough normalized charge, the linear scaling of both decelerating and accelerating fields collapses, with serious consequences for plasma wave excitation efficiency. Using the results of parametric particle-in-cell studies, the implications of these results for observing severe deviations from linear scaling in present and planned experiments are discussed.

Nonlinear plasma waves driven by short ultrarelativistic electron bunches

“…As we will see below, the pointlike driver with Q 1 kicks plasma electrons at relatively small distances from the driver r ∼ k −1 p . Yet, the size of the plasma bubble observed in simulations [17] is much larger than k −1 p . This discrepancy and underlying physics mechanism of the bubble formation are explained in the present paper.…”

Observation of acceleration and deceleration in gigaelectron-volt-per-metre gradient dielectric wakefield accelerators

“…Since the wakefields are excited in a dielectric material by ultra-relativistic particles the modes excited are classified as guided Cerenkov 20 radiation. The peak E z associated with such a coherent Cerenkov excitation process is estimated 20 21 as E z ∝ eN b / σ 2 z , where e is the electron charge, N b the number of beam electrons, and σ z the root mean squared (r.m.s.) beam length.…”

Plasma Sources and Diagnostics

“…There is, therefore, a density dependence in the ideal plasma length. This length also depends on the characteristics of the wakefield, which are determined by the radial and longitudinal bunch dimensions as well its charge [16], and so the optimum length is often found by simulation, e.g. Ref.…”