Dynamics of plasma bunches in slowly varying external fields

We use the quasineutrality approximation and the method of moments to analy ze a system of kinetic equations that describes the expansion into vacuum o f a plasma bunch generally containing several species of charged particles. For a two-component collisionless plasma in slowly varying external potent ial fields, we obtain a complete description of the dynamics of the matrice s of centered second moments of the particle velocity distribution function s. We construct a new class of self-similar solutions of the kinetic equati ons in which the moments of the distribution functions act as parameters. T hese solutions are found to be valid for any mass and energy ratios of the constituent particles and generally describe the dynamics of a plasma bunch that is asymmetric in space. For a symmetric bunch we also find an analyti cal solution corresponding to the presence of eddy electric currents in the plasma, while for an asymmetric bunch we find that interparticle collision s, which give rise to anisotropy in the process of expansion of plasma into vacuum, play an important role. The method developed in the paper is used to study the acceleration and compression of a plasma bunch in time-depende nt magnetic fields with a mirror configuration. (C) 1999 American Institute of Physics. [S1063-7761(99)01009-4].