Numerical simulation of short laser pulse relativistic self-focusing in underdense plasma

Existing experimental results on relativistic self-focusing are interpreted by means of the particle code WAKE using the ponderomotive approximation to describe the laser–plasma interaction P. Mora and T. M. Antonsen, Jr., Phys. Plasmas 4, 217 (1997). Novel features of the code, such as gas medium ionization and an enhanced paraxial approximation, allow more confidence in data interpretation. Simulations where the pulse power is less or close to the critical value match the experimental data. The transmitted pulse spectrum is shown in this case to shift towards longer wavelengths. The pulse is shown to focus over the vacuum diffraction limit, while the energy is slowly depleted. Simulations of pulses above the critical power match experiment with reduced precision. This can be ascribed to beam filamentation. High energy depletion is expected in this case due to Raman instability.