Electron distributions generated by tunnelling ionization during gas breakdown by high intensity laser radiation

The electron distribution resulting from the breakdown of gas during irradi ation by ultra-short high intensity laser pulses is considered using the qu asi-static tunnel approximation. A general expression for arbitrary ellipti c polarization is derived together with a number of approximations. The rol e of the pulse shape in modifying the distribution by determining the actua l field and its variation during the ionization of a particular stage is co nsidered in some detail. The ionization process is found to be sequential i n most practical situations. Relaxation of the distribution in both anisotr opy and energy due to collisions is investigated. The application of Monte Carlo methods for simulating these problems is described. A brief discussio n of the consequences of the work for x-ray laser design is given.