Supersonic gas jet fueling efficiency studies in NSTX.$^*$

Electron and carbon inventory analysis is used to infer the fueling efficiency (FE) of a pulsed high-pressure supersonic D$_2$ jet, produced by a low field side~supersonic gas injector (GI) at a flow rate $3-9 \times10^{21}$ s$^{-1}$ at distance 5-15 cm from the plasma. In ohmic and 2-6 MW NBI-heated L-~and~H-mode plasmas, the FE of the Mach 4 jet is found to be in the range 0.1-0.4, higher than FE of a conventional GI. During supersonic GI pulses, the pedestal density increases by 5-40~\% suggesting that particles are deposited mainly in the pedestal region. A ``single particle'' model of lower-end pressure supersonic GI fueling is developed using the DEGAS 2 neutral transport code. Details of high-pressure jet interaction with background plasma are not included in the model. The modeling suggests that adding a directed velocity does not guarantee a FE improvement. While the supersonic GI does focus the molecules towards the core, there is a reduction in the number of dissociation product atoms that provide much of the transport for the conventional puff, resulting in comparable FE's of a supersonic and a conventional GI's. $^*$Supported by U.S. DOE under Contracts W-7405-Eng-48 and DE-AC02-76CH03073.