Effect of chirp on self-modulation and laser wakefield electron acceleration in the regime of quasimonoenergetic electron beam generation

Abstract

Strongly self-modulated regime of laser wakefield electron acceleration has been experimentally studied using positively/negatively chirped Ti:sapphire laser pulses with duration $\ensuremath{\ge}45\text{ }\text{ }\mathrm{fs}$, and intensity $\ensuremath{\le}1.2\ifmmode\times\else\texttimes\fi{}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ interacting with a plasma having density $>5\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$. The total charge of the high energy electrons produced from self-injection and acceleration in the wakefield was found to depend strongly and asymmetrically on the magnitude and sign of the chirp, which was also corroborated by the forward Raman scattering signals. Generation of low divergence ($\ensuremath{\le}10\text{ }\text{ }\mathrm{mrad}$), quasimonoenergetic electron beam was observed with a maximum energy of $\ensuremath{\sim}28\text{ }\text{ }\mathrm{MeV}$ for a positively chirped laser pulse of duration $\ensuremath{\sim}70\text{ }\text{ }\mathrm{fs}$. The observations are explained considering faster rise time associated with a positive chirped laser pulse which generates intense seed for stronger modulation of the laser pulse and consequently stronger wakefield excitation leading to higher electron beam charge and energy.