  An experimental study on 55fs laser driven plasma accelerator using mixed
gas-jet target with varying plasma density is used to identify the role of
different acceleration mechanisms, viz. Direct Laser Acceleration (DLA) and
wakefield. At lower electron density electron acceleration could be attributed
mainly to DLA with ionization induced injection. With increase in density,
increasing role of wakefield was observed leading to hybrid regime, and at
densities higher than self-injection threshold, observed experimentally for He
target contribution of DLA and wakefield was found to be comparable. Dominant
DLA mechanism was also observed in case of pure nitrogen target. 2D PIC
simulations performed using the EPOCH code corroborate the above scenario, and
also showed generation of surface waves, considered as a potential mechanism of
pre-acceleration to DLA.
