Laser-driven plasma-cathode electron injector

We discuss the first experimental demonstration of electron acceleration by a laser wakefield over distances greater than a Rayleigh range (or the distance a laser normally propagates in vacuum). A self-modulated laser wakefield plasma wave is shown to have a field gradient that exceeds that of an RF linac by four orders of magnitude (E⩾200 GV/m) and accelerates electrons with over 1-nC of charge per bunch in a beam with space-charge-limited emittance (1 mm-mrad). Above a laser power threshold, a plasma channel, created by the intense ultrashort laser pulse (I~4×10¹⁸ W/cm², λ=1 μm, τ=400 fs), was found to increase the laser propagation distance, decrease the electron beam divergence, and increase the electron energy. The plasma wave, directly measured with coherent Thomson scattering is shown to damp-due to beam loading-in a duration of 1.5 ps or ~100 plasma periods. We also discuss a new concept for controlled laser injection of electrons in order to create monoenergetic femtosecond electron bunches. As in the above experiments it uses the plasma itself as the cathode, but also an additional laser pulse as a trigger. By use of a 2-D particle-in-cell numerical code, it is shown that this technique will produce 1-femtosecond duration electron bunches with energy spread at the percent level