Beam Loading by Distributed Injection of Electrons in a Plasma Wakefield Accelerator

Abstract

We show through experiments and supporting simulations that propagation of a highly relativistic and dense electron bunch through a plasma can lead to distributed injection of electrons, which depletes the accelerating field; i.e. beam loads the wake. The source of the injected electrons is ionization of the second electron of rubidium (Rb II) within the wake. This injection of excess charge is large enough to severely beam load the wake, and thereby reduce the transformer ratio T . The reduction of the average T with increasing beam loading is quantified for the first time by measuring the ratio of peak energy gain and loss of electrons while changing the beam emittance. Simulations show that beam loading by RbII electrons contributes to the reduction of the peak accelerating field from its weakly loaded value of 43 GV/m to strongly loaded value of 26 GV/m.The FACET facility [1] at the SLAC National Accelerator Laboratory is presently being used for basic research on the Plasma Wakefield Accelerator (PWFA). In this concept, a short, dense electron bunch [2] is used to generate a high gradient accelerating field in plasma that accelerates a second trailing beam to high energies [3]. Two critical physics issues that affect the energy gain and the emittance of the accelerated beam are beam head erosion [4] and ion motion [5]. The severity of both effects can be mitigated by using a plasma containing a heavier ion species such as rubidium (Rb), rather than the previously used lithium [6,7]. In this Letter, we show that the maximum accelerating gradient that a wake can support is determined not by the plasma density as expected [8], but by the onset of electron injection due to further ionization of rubidium ions. This injected charge extracts energy from the wake in a process known as beam loading and thereby reduces the maximum energy gained by the electrons. Other heavier ions that are potential candidates [9] for mitigating the aforementioned issues may have a similar limitation on the maximum achievable acceleration gradient.The onset of electron injection in the wake leads to a reduction in the important figure of merit for a PWFA known as the transformer ratio T = E + /E − [10]. Here, E − and E + are the peak decelerating and accelerating fields that act upon the beam electrons, respectively. In this experiment, injected electrons are generated via the further ionization of Rb + by the beam electrons' self field or induced wakefield. These "Rb II electrons", observed as excess charge exiting the plasma, are born within the wake [11][12][13][14] as Rb + is ionized to Rb 2+ at the tightly pinched regions of the initially mismatched drive bunch as it executes envelope oscillations [15] in the plasma. Since different slices of the beam lose energy at a different rate due to the wakefield, betatron oscillations of these beam slices quickly become out of phase resulting in continuous injection of the Rb II electrons. This unwanted beam loading results in a reduced average transformer ratio T . Simulations confirm t...

Limitation on the accelerating gradient of a wakefield excited by an ultrarelativistic electron beam in rubidium plasma

“…Note that if the range of pyro is extended by adding data from other data sets such that the overall change in pyro is dominated by beam current and the range of ΔW − is extended, the same correlation between ΔQ and ΔW − as described earlier is clearly observed [38].…”

“…4. (b) Reproduced from [38]; energy loss is shown as a function of excess charge for the same data as in (a). (c) The electron beam energy spectra for the data shown in (a) and (b).…”

Plasma expansion dynamics in the presence of a relativistic electron beam

“…Initial conditions correspond to the study of the propagation of a highly relativistic and underdense electron bunch through a plasma. Injected electrons are generated via the further ionization of Rb + which gives Rb +2 (Vafaei-Najafabadi et al, 2014). Initial parameters correspond to β = 10 2 , δ = 2.10 −3 , N i0 = 1, for an initial beam density less than the plasma density (n i ∼ 10 17 cm −3 ).…”

“…A stream of charged particles can be produced by a short, dense electron bunch, which generates a high-gradient accelerating field in plasma that accelerates a second trailing beam to high energies (Vafaei-Najafabadi et al, 2014). Because plasmas tolerate as high as 1 GV/cm electric field, which is at least 3 orders higher than radio frequency cavities (Li et al, 2013a), a beam of particles can be accelerated to 200 MeV over a distance of millimeter (Bingham et al, 2004).…”

Optical plasma torch electron bunch generation in plasma wakefield accelerators

“…In recent years the ability to excite and control suitable plasma waves driven either by laser (laser wakefield acceleration, LWFA) [1][2][3][4][5][6][7][8] or electron beams (plasma wakefield acceleration, PWFA) [9][10][11][12][13] has increased substantially. In both cases, injection of electron beams into the proper phase of the plasma wave is of paramount importance to obtain high-quality witness bunches from the plasma.…”