Measurements and modeling of ion and neutral distribution functions in a partially ionized magnetically confined argon plasma

Abstract

The influence of ion-neutral collisions on the ion and neutral distribution functions is studied in low field (<0.15 T)rf heated argon discharges of the H-1 Heliac [S. M. Hamberger, B. D. Blackwell, L. E. Sharp, and D. B. Shenton, Fusion Technol. 17, 123 (1990)], both experimentally and theoretically. The distribution functionmeasurements are based on the Doppler broadening of plasma ion and neutral spectral lines using a novel coherence imaging camera. Measurements show that neutrals are abundant throughout the plasma. The neutral temperature is found to be in the range 1–2 eV, which, when compared with ion temperatures of the order of 10 eV, suggests that neutrals are being heated substantially through ion-neutral collisions.Measurements of the ion distribution function reveal a substantially elevated fraction of low energy particles (whose energy is similar to the neutral temperature), associated with charge exchange and ionization. In order to understand the origin of the high neutral temperature and distorted ion distribution function, the ion and neutral distribution functions are modeled using the Boltzmann equation including collision operators for ion-ion and ion-neutral collisions,ionization, heating, and particle loss. The simulation results compare well with the experimental results.