Simulations of Laser Pulse Coupling and Transmission Efficiency in Plasma Channels

Optical guiding of a laser pulse in the laser wake field accelerator (LWFA) via plasma channels can greatly increase the interaction length and, hence, the maximum energy of trapped electrons [1]. Energy efficient coupling of laser pulses from vacuum into plasma channels is very important for optimal LWFA performance. We present 2D particle-in-cell (PIC) simulations of this problem using the VOR-PAL code [2]. Some of the mechanisms considered are enhanced leakage of laser energy transversely through the channel walls, enhanced refraction due to tunneling ionization of neutral gas on the periphery of the gas jet, ionization of neutral gas by transverse wings of the laser pulse and effect of the pulse being off axis of the channel. Using power spectral diagnostics [3], we are able to differentiate between pump depletion and leakage from the channel. The results from our simulations show that for short (≈ λp) plasma ramp, very little leakage is seen and pump depletion is roughly 4 times larger than expected from 1D theory [4]. For narrow channel walls and long ramps, leakage increases significantly.