Characterization of Electrons and X-rays Produced using Asymmetric Laser Pulses in the Self-Modulated Laser Wakefield Acceleration Regime

The electron injection process into a laser plasma wakefield accelerator can be optimized by modifying the laser pulse parameters. We present an experimental study on the combined effect of the laser pulse duration, frequency chirp, and envelope characteristics on the electron injection process and the associated radiation emission for two different gas types---a 97.5% He and 2.5% N2 mixture and pure He. Our results show that the optimal pulse duration generally produced the highest energy electrons and the most charge. Positively chirped pulses with a fast rising edge sustained electron injection and produced higher charge, but lower energy electrons, compared with negatively chirped pulses with a slow rising edge for similar pulse durations. A similar trend was observed for the X-ray flux. The relationship between the radiant energy and the electron signal followed a power law over a two order magnitude change in the electron density and was not dependent on the pulse duration or sign of the frequency chirp. X-ray spectra showed that ionization injection generally produced more photons than self injection for all pulse durations/chirp and had fewer shot-to-shot fluctuations in the spectra.