Rapid Ionization in Argon Plasmas.

Time histories of argon ion lines in a theta pinch produced plasma were measured. Several spectral lines were identified and two, sometimes mistaken, Ar VII lines were verified. Two plasma conditions of approximately equal electron densities were considered, 1.0 mTorr pure argon and 5% argon seeded in 9.5 mTorr hydrogen. Ion line time history measurements were undertaken in a side-on configuration with grazing incidence and normal incidence VUV spectrometers. It was found that side-on measurements produce more reliable time histories over the full range of ionization states in the theta pinch. The plasmas were well diagnosed by means of a Thomson scattering arrangement and these results were used in a zero dimensional computer simulation of the plasma. Ionization rate coefficients for each case were measured by comparison of the simulation with the time history measurements. The ionization of the argon seeded hydrogen is as predicted by theory and agrees, to the extent that comparison is possible, with other experiments. The pure argon case shows very rapid ionization to Ar IX with ionization rates greater than 10 times a standard semi-empirical formula, the Kunze correction to the Lotz formula. A search has been made to understand the possible mechanisms for this rapid ionization. Non -Maxwellian electrons were ruled out after a number of investigations, Thomson scattering, K_alpha and L_alpha wavelength measurements, and other indicators. Thomson scattering along with other indicators have reduced the probability of hot spots in the plasma. Instabilities are possible but framing camera data does not show any unusual development of the plasma. These and other mechanisms have been considered and will be discussed.