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Electric Discharge Vacuum Gauges
Published in Igor Bello, Vacuum and Ultravacuum, 2017
A radio frequency discharge 6, as illustrated in Figure 23.4, can be formed by supplying radio frequency power into a low-pressure gas in a tube 5 via a capacitive coupling (2 and 4 ring electrodes) or, alternatively, inductive coupling. The tube with a diameter of about 1 cm is made of a dielectric material, hard borosilicate glass. The shape and configuration of external ring electrodes is important to eliminate hysteresis of measured parameters. The proper geometrical configuration inhibits the effects of hysteresis and permits formation of only a single plasmoid in the tube chamber. The plasmoid is a glowing plasmatic formation with distinct plasma boundary induced in a gas environment by an electromagnetic field. In a discharge, one or more plasmoids may exist. At variable pressure, these plasmoids can suddenly join to form a single plasmatic formation, and vice versa, leading to sudden changes in optical and electrical parameters.
Supersonic Gas Injector for Plasma Fueling in the National Spherical Torus Experiment
Published in Fusion Science and Technology, 2019
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
In the tokamak, supersonic gas jet operation principles are based on diverse physics fields: gas dynamics, compressible fluid mechanics, neutral gas transport, and magnetized plasma physics. An expansively cooled supersonic gas jet is obtained by expanding room temperature or cooled gas from a high-pressure reservoir through a nozzle into vacuum. In the tokamak, the gas jet interacts with low-density plasmas. It penetrates through the plasma scrape-off layer (SOL) perpendicular to the magnetic field lines, ionizes in the separatrix region, and creates a localized high-pressure plasma region. This plasmoid region expands along field lines, locally cooling and fueling the edge plasma. The radial propagation of the high-pressure gas jet through the edge plasma is determined to the first order by the fluid force balance, mainly by the relative magnitude of the plasma (magnetic and kinetic) pressure and the gas jet impact pressure. The high-pressure gas jet undergoes molecular and atomic (charge exchange, ionization) reactions as it propagates through the SOL plasma, retaining a neutral core shielded by an ionizing layer. The gas jet density plays a critical role in the penetration mechanism, as has been demonstrated by analytic and numerical modeling.10,21,22 However, a deep penetration may be inhibited by a high-density ionizing plasmoid that rapidly develops in front of the gas jet and blocks the jet from further penetration.