Determination of Neutral-Atom Density in a Highly Ionized Decaying Hydrogen Plasma

A photoabsorption technique in the vacuum uv region is used to measure the density of atomic hydrogen in a decaying, highly ionized hydrogen plasma as a function of time and position. The plasma is formed in a metal cylinder with a strong axial magnetic field by a powerful pulsed cross-field discharge that takes the form of an axially propagating ionization front. It is found that during the discharge a fraction of the gas is driven to the boundaries forming a layer of increased neutral density which later relaxes back into the plasma by diffusion. In the interior, the plasma is initially highly ionized and the atom density builds up as the ion density decreases. The rate is consistent with predictions based on the collisional-radiative recombination model.