NUMERICAL-SIMULATION OF SHORT LASER-PULSE RELATIVISTIC SELF-FOCUSING IN UNDERDENSE PLASMA

Existing experimental results on relativistic self-focusing are interp reted by means of the particle code WAKE using the ponderomotive appro ximation to describe the laser-plasma interaction [P. Mora and T. M. A ntonsen, 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 experimen tal data. The transmitted pulse spectrum is shown in this case to shif t towards longer wavelengths. The pulse is shown to focus over the vac uum diffraction limit, while the energy is slowly depleted. Simulation s of pulses above the critical power match experiment with reduced pre cision. This can be ascribed to beam filamentation. High energy deplet ion is expected in this case due to Raman instability. (C) 1998 Americ an Institute of Physics. [S1079-664X(48)01809-6]