Effects of cold electron emission on the plasma sheath

Journal Article · Mon Feb 01 04:00:00 EST 1993 · Physics of Fluids B; (United States)

DOI:https://doi.org/10.1063/1.860494· OSTI ID:7173428

Schwager, L A; Hsu, W L; Tung, D M ^[1]

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1. Sandia National Laboratories, Livermore, California 94551-0969 (United States)

The use of surfaces that emit cold electrons into a plasma has been shown to reduce the sheath potential drop by at least a factor of 2.6. This effect can be used to mitigate sputtering by causing a reduction of the mean energy of ions striking the emissive surface. The present study has been motivated by the severe sputtering problems that may occur at the divertor plate of a full power fusion reactor. In addition, secondary electron emission is a common occurrence at the bounding surface of a plasma, and so should be accounted for in the plasma analysis. Kinetic theory shows that, for a hydrogen plasma (singly charged, mean ion mass of 2.5 amu) with an electron temperature of [ital T][sub ital e] and ion temperature of 0.1[ital T][sub ital e], emission reduces the sheath potential drop from 3.9[ital T][sub ital e] to 1.5[ital T][sub ital e]. This reduction occurs when the surface emits a cold electron current that is increased to nine times the incident ion current. For comparison with theory, thermionic electron emission has been studied in a hydrogen plasma (composed mostly of H[sub 3][sup +]) powered by inductive coupling from an rf coil. The plasma was in contact with an electrically isolated surface of tungsten impregnated with barium scandate. Plasma potential was measured with a Langmuir probe and with a mass and energy analyzer that sampled the energy distribution of ions passing through an aperture in the emissive surface. As the surface temperature was increased, the surface potential floated up to the plasma space potential so that the ion energy at the surface dropped from 5[ital T][sub ital e] for temperatures below 570 [degree]C to less than 1[ital T][sub ital e] for temperatures above 680 [degree]C.

• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 7173428
• Journal Information: Physics of Fluids B; (United States), Journal Name: Physics of Fluids B; (United States) Vol. 5:2; ISSN 0899-8221; ISSN PFBPEI
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
700350* -- Plasma Production
Heating
Current Drive
& Interactions-- (1992-)
ALKALINE EARTH METAL COMPOUNDS
BARIUM COMPOUNDS
CATIONS
CHARGED PARTICLES
DIVERTORS
ELECTRON EMISSION
ELECTRON TEMPERATURE
EMISSION
HYDROGEN IONS
HYDROGEN IONS 3 PLUS
IONS
MOLECULAR IONS
PLASMA SHEATH
THERMIONIC EMISSION