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Supersonic Gas Injector for Plasma Fueling in the National Spherical Torus Experiment

V. A. Soukhanovskii, W. R. Blanchard, J. K. Dong, R. Kaita, H. W. Kugel, J. E. Menard, T. J. Provost, R. Raman, A. L. Roquemore, P. Sichta

Fusion Science and Technology / Volume 75 / Number 1 / January 2019 / Pages 1-17

Technical Paper / dx.doi.org/10.1080/15361055.2018.1502034

Received:May 18, 2018
Accepted:July 16, 2018
Published:December 8, 2018

A supersonic gas injector (SGI) has been developed for fueling and diagnostic applications on the National Spherical Torus Experiment (NSTX). It is comprised of a graphite converging-diverging Laval nozzle and a commercial piezoelectric gas valve mounted on a movable probe at a low-field-side midplane port location. Also mounted on the probe is a diagnostic package: a Langmuir probe, two thermocouples, and five pick-up coils for measuring toroidal, radial, vertical magnetic field components and magnetic fluctuations at the location of the SGI tip. The SGI flow rate is up to 33.25 Pa m3/ (1.75 × 1022 euterium particles/s), comparable to conventional NSTX gas injectors. The nozzle operates in a pulsed regime at room temperature and a reservoir gas pressure up to 665 kPa (5000 Torr). The deuterium jet Mach number of about 4 and the divergence half-angle of 5 to 25 deg have been measured in laboratory experiments simulating the NSTX environment. Reliable operation of the SGI and all mounted diagnostics at distances 0.01 to 0.20 m from the plasma separatrix has been demonstrated in NSTX experiments. The SGI has been used for fueling of ohmic and 2- to 4-MW neutral beam injection–heated L- and H-mode plasmas. Fueling efficiency in the range 0.1 to 0.3 has been obtained from the plasma electron inventory analysis. The SGI-fueling–based plasma discharge scenarios enabling better density control have been developed.

 
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