Acceleration and scattering of injected electrons in plasma beat wave accelerator experiments*

Abstract

The results from experiments in which a two-frequency CO 2 laser is used to beat-excite large-amplitude, relativistic electron plasma waves in a tunnel-ionized plasma are reported. The plasma wave is diagnosed by injecting a beam of 2 MeV electrons and observing the energy gain and loss of these electrons, as well as the scattering and deflection of the transmitted electrons near 2 MeV. Accelerated electrons up to 30 MeV have been observed. The lifetime of the accelerating structure as seen by small-angle Thomson scattering is about 100 ps, whereas the injected electrons are seen to be scattered or deflected by the plasma for several ns, with diffuse scattering occurring 0.5-1 ns after forming the plasma wave and whole beam deflection occurring at later times. A simple model, which includes laser focusing, ionization, transit time, and relativistic saturation effects, suggests that the wave coherence may be short lived while the wave fields themselves persist for a longer time. This may be the reason for the disparate time scales between the Thomson scattering and the electron scattering diagnostic. The whole beam deflection may be evidence for a Weibel-like instability at later times.

Physics of Phase Space Matching for Staging Plasma and Traditional Accelerator Components Using Longitudinally Tailored Plasma Profiles

“…The first configuration is the so-called injector-accelerator, where a ∼ 100 MeV class electron beam produced by a short, high-density injector stage is further accelerated to ∼ GeV level using a second low-density accelerator stage [11,24,25]. The second example is the external injection scheme where a highquality, relativistic electron bunch is first generated using an RF accelerator and then injected into a PBA [26][27][28][29][30]. The third example concerns the proposed PBA driven light source [31][32][33], where a high-quality electron beam needs to be coupled from the plasma wake to an undulator.…”

Observation of Laser Wakefield Acceleration of Electrons

“…An e ect like the Weibel instability already observed in ref. [19] is a good candidate to explain a long term (ns) deviation of the electrons. It could thus explain this BG.…”

“…A particular emphasis has been given to the separation of the signal from the background (BG) noise in the design of the experimental apparatus [22] and in the analysis of the data. In the case of LBWA experiments, Clayton et al have shown that magnetic and/or transverse electric elds, due to a Weibel-like instability [19], still exist in the plasma a long (a few nanoseconds) time after the excitation of the EPW. Electrons de ected by such elds can scatter on the walls of the vacuum chamber and fake a signal, as is possibly the case in [20] and in the surprising result of [21].…”

Perspectives on high-energy-density physics

“…This led to a wide range of research projects aimed at cleverly inducing the plasma to develop relativistic wakes of nonlinear amplitude, for example, by driving a nonlinear resonance through the beating of two laser beams. [45][46][47][48][49][50] Then in 2002, by means of particle in cell simulations, Pukhov and Meyer-ter-Vehn discovered what is now known as the "bubble regime." In this regime the wake is created directly by a concentration of energy so brief and so intense that it displaces a significant fraction of the electrons as it traverses the plasma.…”