Ion extraction characteristics by an electric field on laser-produced barium plasma

Abstract

Time evolution of a two-dimensional ion-density distribution in a barium plasma produced by simultaneously illuminating a tuned dye laser light and an excimer laser light has been measured by means of laser-induced fluorescence technique to investigate the ion extraction characteristics by an electric field. When the plasma is produced between two parallel plates with a potential difference, the ion behavior is governed by three factors: bulk plasma flow, plasma diffusion, and ion extraction by the electric field. The flow velocity of the bulk plasma is equal to the mean velocity of the atoms emanating from a hot source. The expansion velocity of the plasma boundary into a vacuum agrees well with the ambipolar expansion velocity corresponding to that of an ion acoustic wave, while the diffusion coefficient obtained from the density decay rate is smaller by a factor of about 8 than the ambipolar expansion coefficient for the steady state because of low collision frequency between ions. Enhancement of the ion extraction rate by the electric field is supplied by retrograde motion of the plasma edge which contributes to reduction of the ion extraction time.