Self-modulated wakefield and forced laser wakefield acceleration of electrons
Résumé
The interaction of intense laser pulses (power>30 TW) with underdense plasmas has been studied. In the regime where the pulse length is much longer than the plasma period (τl≫2πωp−1), the laser pulse is found to be self-modulated at the plasma frequency by the forward Raman scattering instability. Wavebreaking of the resulting plasma wave results in energetic electrons being accelerated to more than 100 MeV. Reducing the pulse length so that τl ∼ 2πωp−1, but retaining the same power, also leads to wavebreaking. This is a direct result of a combination of laser beam self-focusing, front-edge laser pulse steepening and relativistic lengthening of the plasma wave wavelength, which can result in a forced growth of the wakefield plasma wave, even for initially nonresonant laser pulses (τl ≠ πωp−1). Since, in this forced laser wakefield regime, the interaction of the plasma wave and the bunch of accelerated electrons with the laser pulse is reduced, this can result in higher energy gain (to beyond 200 MeV) and better beam quality.