Plasma and Fusion Research

Volume 11, 2405037 (2016)

Regular Articles

Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI
Shaofei GENG1), Katsuyoshi TSUMORI1,2), Haruhisa NAKANO1,2), Masashi KISAKI2), Katsunori IKEDA2), Masaki OSAKABE1,2), Ken-ichi NAGAOKA1,2), Yasuhiko TAKEIRI1,2), Masayuki SHIBUYA2) and Osamu KANEKO1,2)
1)
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi, Toki, Gifu 509-5292 Japan
2)
National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292 Japan
(Received 29 November 2015 / Accepted 24 February 2016 / Published 15 April 2016)

Abstract

Experimental measurements with Langmuir probe and laser photodetachment in beam extraction region of a cesium-seeded negative ion source for NBI has been conducted in order to investigate the response of charged particles to applied external field. The profiles of probe saturation current and H− ion density in the direction normal to the plasma grid (PG) surface were measured by scanning the tip position. By comparing the results before and during beam extraction, the charged particle responses due to the beam extraction have been analyzed. During beam extraction, probe saturation current increases since H− ions are extracted and electrons flow into the extracting region for charge neutrality. The maximum increment of the probe saturation current due to electron flow appears in the range of 15 - 25 mm apart from the PG surface. Meanwhile, the maximum decrement of the H− ion density is at around 18 mm from the PG. In the region far from the PG, probe saturation current increment is low as well as the decrement of the H− ion density. The extrapolations of the profiles suggest that the depth of the influence on the plasma by beam extraction is 42 mm from the plasma grid surface.

Keywords

negative hydrogen ion source, beam extraction, plasma response, Langmuir probe, photodetachment

DOI: 10.1585/pfr.11.2405037

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