Guiding and stability of short laser pulses in partially stripped ionizing plasmas

Abstract

Laser pulse propagation can be strongly influenced by nonlinearity effects (relativistic and/or atomic electrons), ionization processes, and finite pulse length effects. In this paper these processes are included in the analysis of the propagation and stability of intense laser pulses in plasmas. An envelope equation, which includes ionization and nonlinear effects, is derived and the spot size is found to be unstable to an ionization–modulation instability. Introducing a quasiparaxial approximation to the wave equation, a pair of coupled envelope-power equations including finite pulse length effects, as well as nonlinearities, is derived and analyzed. In addition, short laser pulses propagating in plasma channels are found to undergo an envelope modulation that is always damped in the front and initially grows in the back of the pulse. Finite pulse length effects are also shown to modify nonlinear focusing processes. Finally, it is shown that, in a partially stripped plasma, the bound electrons can significantly alter the stability of laser pulses. In the presence of both free and bound electrons, an atomic modulation instability develops that can have a growth rate substantially higher than either the conventional relativistic modulational instability or the forward Raman instability. The filamentation instability is shown to be enhanced by bound electrons while the backward Raman instability is unaffected. An example of laser wakefield acceleration to electron energies greater than 2.5 GeV in a plasma channel is described.