Velocity distributions of multi-ion species in an expanding plasma produced by a 1.05-μm laser

The fast-ion-velocity distributions for polyethylene plasmas produced by spherical laser illumination were observed by a Thomson parabola spectrometer. It has been found that the velocity distributions have undulations, sharp truncations, and broad shelf structure because of multi-ion-species effects. The spherical symmetric plasma expansions including multi-ion-species and two-electron temperature effects have been simulated by a newly developed computer code. The simulation results indicate that protons are accelerated near the front edge of carbon ions, and the maximum velocity is determined predominantly by the ions which run in the region ahead of the carbon-ion front. Consequently, the proton-expansion front has a structure slipped out from the carbon-ion front towards vacuum. Since such protons compensate the accelerating electric field, the maximum velocities of proton and carbon ions become higher and lower, respectively, than in the case of single-ion species. These expansion dynamics reduced by the simulation can explain well the detailed structure of the observed velocity distribution.