Electron assisted glow discharges for conditioning fusion tokamak devices

Journal Article · Tue May 01 00:00:00 EDT 1990 · Journal of Vacuum Science and Technology, A (Vacuum, Surfaces and Films); (USA)

DOI:https://doi.org/10.1116/1.576587· OSTI ID:6800064

Schaubel, K M; Jackson, G L ^[1]

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1. General Atomics, San Diego, CA (USA)

Glow discharge conditioning of tokamaks with graphite plasma-facing surfaces has been used to reduce impurities and obtain density control of the plasma discharge. However, a major operational disadvantage of glow conditioning is the high pressure required to initiate the glow discharge, e.g., {approx}70 mTorr for helium in DIII-D, which requires isolating auxiliary components that can not tolerate the high pressure. An electron-gun-assisted glow discharge can lower breakdown pressure, possibly eliminating the necessity of isolating these auxiliary systems during glow discharge conditioning and allowing glow discharge operation at lower pressures. An electron-assisted glow discharge experiment has been carried out in a small vacuum vessel to evaluate whether such a system can be employed in the DIII-D tokamak. With an electron gun to produce an initial source of electrons, the pressure at which a helium glow discharge can be initiated has been decreased by two orders of magnitude. The glow was produced in a 0.40 m{sup 3} Inconel test chamber (3.5 m{sup 2} surface area) and was pumped with a 330 l/s turbomolecular pump. The electron gun consists of a tungsten filament and grid assembly. An electron current of up to 18 mA with energies up to 1.6 kV has been used. The pressure is measured with a capacitance manometer, the gas composition with a residual gas analyzer and the electron temperature, density, and plasma floating potential with a Langmuir probe. With the addition of an electron current of 10 mA, the initiation pressure was reduced from 165 to 1.6 mTorr. The lowest sustaining pressure dropped from 3.0 mTorr in the absence of electron assist to 1.5 mTorr with 10 mA of electron current.

• OSTI ID: 6800064
• Journal Information: Journal of Vacuum Science and Technology, A (Vacuum, Surfaces and Films); (USA), Vol. 8:3; ISSN 0734-2101
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
TOKAMAK DEVICES
GLOW DISCHARGES
ELECTRON GUNS
ELECTRON TEMPERATURE
LOW PRESSURE
PLASMA DIAGNOSTICS
TURBOMOLECULAR PUMPS
CLOSED PLASMA DEVICES
ELECTRIC DISCHARGES
EQUIPMENT
LABORATORY EQUIPMENT
PUMPS
THERMONUCLEAR DEVICES
VACUUM PUMPS
700102* - Fusion Energy- Plasma Research- Diagnostics