  An experimental study of laser driven electron acceleration in N2 and N2-He
mixed gas-jet target using laser pulses of duration ~60-70 fs is presented.
Generation of relativistic electron beam with quasi-thermal spectra was
observed at a threshold density of ~1.6x1018 cm-3 in case of pure N2, and the
threshold density was found to increase with increasing doping concentration of
He. At an optimum fraction of 50% of He in N2, generation of
quasi-monoenergetic electron beams was observed at a comparatively higher
threshold density of ~2x1018 cm-3, with an average peak energy of ~168 MeV,
average energy spread of ~21%, and average total beam charge of ~220 pC.
Electron acceleration could be attributed to the direct laser acceleration as
well as the hybrid mechanism. Observation of an optimum fraction of He in N2
(in turn threshold plasma density) for comparatively better quality electron
beam generation could be understood in terms of the plasma density dependent
variation in the dephasing rate of electrons with respect to transverse
oscillating laser field. Results are also supported by the 2D PIC simulations
performed using code EPOCH.
