ACCELERATION AND SCATTERING OF INJECTED ELECTRONS IN PLASMA BEAT-WAVEACCELERATOR EXPERIMENTS

The results from experiments in which a two-frequency CO2 laser is use d to beat-excite large-amplitude, relativistic electron plasma waves i n a tunnel-ionized plasma are reported. The plasma wave is diagnosed b y injecting a beam of 2 MeV electrons and observing the energy pin and loss of these electrons, as well as the scattering and deflection of the transmitted electrons near 2 MeV. Accelerated electrons up to 30 M eV have been observed. The lifetime of the accelerating structure as s een by small-angle Thomson scattering is about 100 ps, whereas the inj ected electrons are seen to be scattered or deflected by the plasma fo r 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 coherenc e may be short lived while the wave fields themselves persist for a lo nger time. This may be the reason for the disparate time scales betwee n the Thomson scattering and the electron scattering diagnostic. The w hole beam deflection may be evidence for a Weibel-like instability at later times.