Plasma State in a Coaxial Accelerator

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Abstract
Spectroscopic measurements of electron temperature and density are reported for plasmas produced in a coaxial accelerator filled with helium to a static pressure of 0.5–1.0 Torr. Luminous front velocities of ∼6 cm/μsec were attained. Since the validity of these measurements depended on the assumption of local thermal equilibrium, the collisional and the radiative processes which determine the population densities of HeIII, and the various bound states of HeII and HeI, are examined. It is concluded that the local thermal equilibrium assumption is in fact a satisfactory approximation for the conditions of this experiment, i.e., the actual temperature deviates by no more than 10–15% from the local thermal equilibrium value. Electron temperatures and densities were found to be ∼4.0 eV and ∼1017 cm−3, respectively, in the region outside the end of the accelerator but before the plasma focus. Density distributions were found to vary greatly in the radial direction and to have strong dependence on polarity effects. For the case of a positive inner electrode, a large fraction of the gas was found to ``snowplow'' at the plasma front.