Gas fueling studies in the PDX tokamak: II

Measurements of the gas fueling characteristics of the PDX tokamak have been extended in parameter range. An earlier study presented the gas fueling efficiency for H₂ and D₂ for the standard PDX divertor configurations with a large conductance between the divertor and main plasma chambers. This study presents the observed variations in H₂, D₂ and He neutral pressure and ionization light emission for divertor configurations with a restricted conductance between the divertor and main plasma chambers. The restricted conductance improved the divertor/main-chamber compression ratio by more than an order of magnitude. For the same plasma density, gas fueling from the divertor chamber was twice as efficient as fueling from the main chamber. At the highest plasma densities that were investigated, n_e ≃ 4 × 10¹³ cm^-3, a decrease in the plasma temperature in the divertor was indicated by a decrease in the ionization light at the divertor throat for D₂ and He fueled discharges. These observations are consistent with a Monte-Carlo model of neutral gas transport in the divertor.