Effect of density gradient on direct laser acceleration*

Electron acceleration has drawn significant interest in the plasma physics community during the last decades, motivated by the development of intense laser facilities. Among the many proposed acceleration schemes, direct laser acceleration (DLA) distinguishes itself for providing high charge electron beams ~100s of nC. Recently, a Gaussian plasma density profile was used to accelerate a 140 nC electron beam with energies up to 500 MeV using a moderately-relativistic laser pulse [1]. A varying density profile might have had a significant influence on the DLA process in the experiment. If this proves correct, this will provide an opportunity for optimizing future experiments for obtaining high-energy electron beams with moderate laser intensities.

In this work, we perform full-scale PIC simulations with OSIRIS in Quasi-3D geometry, in parallel with ideal test particle simulations to determine new features in the electron acceleration that arise from introducing a varying density profile. The plasma profile affects the particle dynamics and whether and when the conditions for betatron resonance are met. We explore the gradient steepness, maximum density, interaction length and find the optimal conditions for electron acceleration.

[1] Hussein et. al, New J. Phys. 23 023031 (2021)