Recombination of Ions and Electrons in a Highly Ionized Hydrogen Plasma

A hydrogenic plasma with an electron density near 5×{}10¹⁵ cm^-3 and a temperature between 1 and 3 eV is produced by a powerful transient discharge in a tube of 14-cm diam and 86-cm length. Spectroscopic techniques are used to follow the subsequent decay of both electron density and temperature for about 300 {μ}sec. A strong uniform axial magnetic field suppresses rapid radial particle losses, but does not appreciably reduce the radial energy transport. The axial transport apparently results merely in the slow growth of relatively thin dense boundary layers at the tube ends. In the interior the ionization is shown to decay primarily by volume recombination, and the rate coefficient agrees well with that calculated by Bates, Kingston, and McWhirter for plasmas that are virtually opaque to the Lyman-line radiation. Evidently, the plasma starts out essentially fully ionized and seems to remain close to local thermal (Saha) equilibrium for 120 {μ}sec. The inferred energy-loss rate, on the other hand, initially exceeds that expected for kinetic transport by perhaps an order of magnitude, indicating that early in the afterglow radiative transfer may be significant.