Ion behavior in photoionization plasma originating from one-directional atomic vapor flow under external electric field

After plasmas were instantly produced between parallel plate electrodes by illuminating pulsed laser light onto a one-directional atomic vapor stream, their ion extraction characteristics were clarified by measuring the time evolution of the ion density distribution. The flow velocity of the bulk plasma agrees with the mean velocity of atoms at the evaporation temperature for collisionless emanation. However, when the collisional effect plays an important part, the plasma is accelerated due to energy transfer to the linear motion by relaxation of metastable atoms, in addition to the adiabatic expansion. The expansion velocity of the plasma edge into a vacuum is given by the ambipolar expansion velocity corresponding to that of an ion acoustic wave. Ion extraction by the external electric field proceeds in three distinct stages. First, an ion sheath is formed as a result of electron repulsion by a potential penetration. Then, only the plasma edge in the lower potential side recedes rapidly from its initial position without any appreciable decrease of the ion density, and lastly, retrogression of the plasma edge and the density decay evolve simultaneously, accompanying the plasma diffusion. (author)