Evolution of photoionization two-stream instability in collisional plasma

A kinetic model which takes into account gas jet photoionization by an ultrashort linearly polarized EUV/X-ray free electron laser (FEL) pulse, a self-generated low frequency electric field, and elastic electron scattering on atoms is introduced to study the photoionization two-stream (PITS) instability. The evolution of PITS instability is described in terms of an integral equation for electron density perturbations. This paper presents a generalization of previous theories of the PITS instability in collisionless plasmas. The instability is produced by the anisotropy of photoionized electrons and is strongly affected by electron-atom collisions. Relaxation of the PITS instability is studied for different relations between the FEL pulse duration, the instability growth time, and the electron collision time.