The nonlinear theory of propagation of an intense laser beam in a partially stripped magnetized plasma channel is investigated. An external magnetic field is considered, along the direction of propagation of the laser pulse. A three-dimensional envelope equation for the evolution of the laser field is derived. Using the source dependent expansion technique, an expression for the laser spot evolution is derived. The effect of the external magnetic field on the transverse oscillation of the laser spot is analyzed analytically and numerically. A formulation for the amplitude modulation instability is derived and the results are compared against the fully stripped plasma case. The excitation condition and the growth rate of the modulation instability are discussed. It is observed that increased positive chirp in the laser frequency enhances the modulation instability significantly. The group velocity dispersion is also calculated and the results are compared with those for the case of a fully stripped magnetized plasma channel. The presence of an external magnetic field and that of bound electrons have a significant effect on the dephasing length, as compared to that for a fully stripped plasma. The separability of the plasma frequency and laser frequency (ωp and ω0 respectively) is considered, such that it satisfies the criterion ωp/ω0 < 1.
Filamented plasmas in laser ablation of solids