Enhanced Self-Injection by Optical Field Ionization Heating in a Laser Wakefield Accelerator*

Laser-wakefield acceleration (LWFA) experiments performed using the Hercules laser show a decrease of the self-injection threshold by using circularly polarized (CP) laser pulses compared with linearly polarized (LP) pulses, similar to the threshold lowering in LWFA driven in a warm plasma. In addition to the lower injection threshold, a significantly higher electron beam charge was observed for CP compared to LP over a wide range of parameters. Theoretical analyses and quasi-3D Particle-in-Cell simulations agree with the observed experimental findings and indicate a modified injection path for CP laser pulses, which enables electrons from a much broader range of parameters to be injected.