Supersonic gas injector for fueling and diagnostic applications on the National Spherical Torus Experiment

A prototype pulsed supersonic gas injector (SGI) has been developed for the National Spherical Torus Experiment (NSTX). Experiments in NSTX will explore the compatibility of the supersonic gas jet fueling with the H-mode plasma edge, edge localized mode control, edge magnetohydrodynamic stability, radio frequency heating scenarios, and start-up scenarios with a fast plasma density ramp up. The diagnostic applications include localized impurity gas injections for transport and turbulence experiments and edge helium spectroscopy for edge Te and ne profile measurements. Nozzle and gas injector design considerations are presented and four types of supersonic nozzles are discussed. The prototype SGI operates at room temperature. It is comprised of a small graphite Laval nozzle coupled to a modified commercial piezoelectric valve and mounted on a movable vacuum feedthrough. The critical properties of the SGI jet—low divergence, high density, and sharp boundary gradient, achievable only at M>1, have been demonst...