Resolving the Transition between Direct Laser Acceleration and Laser Wakefield Acceleration in Near-Critical Plasmas

Irradiation of thin foil targets by low-contrast relativistic laser pulses results in the emission of collimated jets of multi-MeV electrons. This two-decade-old observation forms the most efficient way to convert optical eV photons to MeV particles. We examine the acceleration of electrons due to the combined effect of Laser Wakefield Acceleration (LWFA) and Direct Laser Acceleration (DLA). Our new insights were obtained from Particle-In-Cell (PIC) simulations done in order to explain experimental results. The experiments were performed using the high-contrast 20 TW laser system at Tel-Aviv University. In those experiments an Au plasma is generated from a sub-µm foil. The plasma plume density profile was set using controlled µJ-mJ pre-pulses arriving 0-90 ns prior to the main pulse. The plume’s density profile was characterized by in-situ interferometric measurements. The analysis follows the plasma plume expansion model we developed, which serves as input density to PIC simulations. I will present the insights from those simulations in order to reveal the DLA and LWFA effects on the energetic electrons in different plasma conditions. We can observe highly efficient acceleration of electrons from a specific range of ionization levels.