Quasi-monoenergetic electron beams generated from 7 TW laser pulses in N₂ and He gas targets

Quasi-monoenergetic electron beams of energies 12MeV to over 200MeV are generated from both nitrogen and helium gas targets with 7TW laser pulses. Typically nitrogen gas interactions lead to electron bunches in the range of 12 to 50MeV varying from shot to shot. Helium gas leads to higher energy electron bunches from 25 to 100MeV. Occasionally exceptionally high energy bunches of electrons up to 200MeV are observed from nitrogen and helium. Initial full two-dimensional simulations indicate the production of 2030MeV electron energy bunches for the typical interaction conditions as the electrons are injected from wave breaking in the plasma wake behind the laser pulse and injected into the strong electric field gradient propagating with the optical pulse. This is consistent with the experimental observations from the majority of shots. Pulse compression during propagation in the high density plasma does not allow the threshold conditions for the full bubble regime to be reached. However, the electric acceleration field in the wakefield cavity is still sufficient to lead to the formation of a bunch of electrons with an energy peak in the range of 20 to 30MeV. In order to explain the occasional high energy shots most likely a lower density channel created by the laser prepulse may occasionally form a natural low density electron guide channel giving ideal conditions for acceleration over much longer lengths leading to the high energies observed.