Work function measurements during plasma exposition at conditions relevant in negative ion sources for the ITER neutral beam injection

Cesium seeded sources for surface generated negative hydrogen ions are major components of neutral beam injection systems in future large-scale fusion experiments such as ITER. The stability and delivered current density depend highly on the work function during vacuum and plasma phases of the ion s...

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Bibliographic Details
Published in:Review of scientific instruments 2011-02, Vol.82 (2), p.023506-023506-7
Main Authors: Gutser, R., Wimmer, C., Fantz, U.
Format: Article
Language:English
Subjects:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ANIONS
BEAM INJECTION
CESIUM
CURRENT DENSITY
EVAPORATION
HYDROGEN IONS
ION SOURCES
ITER TOKAMAK
MOLYBDENUM
PLASMA
STABILITY
WORK FUNCTIONS
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Summary:Cesium seeded sources for surface generated negative hydrogen ions are major components of neutral beam injection systems in future large-scale fusion experiments such as ITER. The stability and delivered current density depend highly on the work function during vacuum and plasma phases of the ion source. One of the most important quantities that affect the source performance is the work function. A modified photocurrent method was developed to measure the temporal behavior of the work function during and after cesium evaporation. The investigation of cesium exposed Mo and MoLa samples under ITER negative hydrogen ion based neutral beam injection relevant surface and plasma conditions showed the influence of impurities which result in a fast degradation when the plasma exposure or the cesium flux onto the sample is stopped. A minimum work function close to that of bulk cesium was obtained under the influence of the plasma exposition, while a significantly higher work function was observed under ITER-like vacuum conditions.
ISSN:0034-6748
1089-7623
DOI:10.1063/1.3541790