Hydrodynamics of a non-uniform super-Gaussian ion plasma produced by pulsed lasers in an electrostatic field

摘要

The two-dimensional hydrodynamics of ion extraction from a non-uniform quasi-neutral plasma, in an electrostatic field, has been simulated numerically. Experimentally, tunable pulsed lasers with a spatially Gaussian profile produce a non-uniform plasma through stepwise photo-excitation and photo-ionization or multi-photo-ionization processes. For such a non-uniform plasma, the ions were assumed to be initially super-Gaussian with a finite gradient density profile at the boundaries. Poisson's equation was solved simultaneously with the equations of mass and momentum, assuming the Maxwell-Boltzmann distribution for electrons. In these calculations the dynamics of the electric potential, ion density and velocity, and ion current density were assessed and the extraction time was estimated. Three separate regions of the plasma, i.e. the ion sheath, the transition region (pre-sheath) and the quasi-neutral plasma were considered in these calculations. The time history of the pre-sheath front as well as the ion current density across the cathode was also examined. The results show that while the extraction time remains constant for both the super-Gaussian and the uniform plasma, the pre-sheath velocity, ion current density and the ion density are significantly different.