Electron acceleration in a plasma wave above a laser-irradiated grating

A plasma with a modulated density of ñ/n=8% was produced by illuminating a grating with a ruby laser at an intensity of 1010 W/cm2. A CO2 laser of intensity 3×1011 W/cm2 then irradiated this modulated plasma and generated plasma waves. The phase speeds of the waves are vph = ±ω0/kg, where kg is the wave number of the grating and ω0 is the frequency of the CO2 laser. Electrons were injected at an energy of 92 keV in one of the plasma waves. In order for the phase speed of the wave to synchronize with the accelerating electrons, a grating with constantly increasing line spacing (chirped) was used. Some electrons accelerated by 70 keV in 70 μm (gradient of 1 GeV/m) were detected, but the number of accelerated electrons was three orders of magnitude less than the expected value. This discrepency is the result of a large electric field perpendicular to the surface of the grating, which deflects the electrons onto the grating. This detrimental field is produced when fast electrons are emitted by the plasma, and leave it positively charged. Some techniques to remedy this difficulty are proposed.