Excitation and Ionization Rates of Neon Ions in a Stellarator Discharge

Abstract

The intensities of various resonance multiplets of neon ions, from Ne ii to Ne viii, emitted by an ohmically heated hydrogen discharge of the C stellarator, with about 3% neon added, were measured with a calibrated grazing-incidence monochromator. The electron densities (N_e∼10¹³ cm⁻³) were determined by means of 4-mm microwave interferometers, and the electron temperature (T_e∼10–50 eV from the electrical conductivity of the plasma. The neon-ion concentrations at the different states of ionization were calculated from the light intensities by means of an assumed energy dependence for the excitation cross sections, the magnitude of the cross sections being adjusted to make the sum of the calculated ion concentrations agree with the known total neon concentration. Atomic oscillator strengths deduced from the excitation-rate coefficients thus obtained, were found to be in reasonable agreement with various calculations. Ionization-rate coefficients were deduced from the time behavior of the ion concentrations and the known particle-confinement time in the discharge. As compared with expected values, the measured rate coefficients appear to be increasingly too large with successive states of ionization. The most likely reason for the discrepancies is a possible deviation from Maxwellian electron-energy distribution in the discharge, to which these rate coefficients would be particularly sensitive.