Demonstration of Collisionless Interactions between Counterstreaming Ions in a Laser-Produced Plasma Experiment

摘要

The width, time development, and ambient pressure dependence of a collisionless interaction front in a plasma have been studied using fast photography, shadowgraphy, and electric potential probes. The object was to investigate the expansion of a laser-produced plasma through an ambient plasma, under conditions where the mean free path for binary momentum transfer collisions was larger than the total expansion radius. Sharp luminous fronts were observed with both framing and streak photography. The change in plasma radius with time was studied as a function of ambient nitrogen pressure. The dynamics of the situation were in good agreement with strong-momentum-coupling models (a radiation-driven detonation wave at early times, and a blast wave at later times). An electric potential in the front was measured, of sufficient magnitude to account for the observed late-time blast wave energies. Shadowgraphy showed that the front consisted of a shell of enhanced density, that the density gradients in the front depended on ambient pressure, and that the front width was of the order of a millimeter. A possible theoretical model for the collective effect necessary to account for the observed collisionless dissipation is the ion-ion two-stream instability in the presence of a magnetic field. (Author)