Measurements of the beatwave dynamics in time and space

We report on continuing experiments on the plasma beat wave accelerator, which uses an intense two-frequency CO₂ laser pulse to resonantly drive a large amplitude, relativistically-propagating electron plasma wave suitable for electron acceleration. Previously, energy gains of a factor 15 (from 2 MeV to 30 MeV) have been obtained. Also, collective scattering of a probe laser beam has allowed us to measure n˜(ω,k) and n˜(ω _m,t) where m=1,2 are the fundamental and harmonic of the large amplitude plasma wave. This powerful Thomson scattering technique has now been extended to measure n˜(z,t), where z is the coordinate along the CO₂ propagation direction. Nonlinear dynamics such as relativistic detuning and ponderomotive effects can complicate the longitudinal amplitude profile and coherence. Experimental results show a plasma wave with a peak amplitude of approximately 35%, with a FWHM of 100 ps, and extending for 1 cm in space. These wave parameters are consistent with the observed energy gains of accelerated electrons