29th Symposium on Fusion Technology (SOFT 2016)

P4.032 Response of H- ions to extraction field in a negative hydrogen ion source

8 Sep 2016, 14:20

1h 40m

Foyer 2A (2nd floor), 3A (3rd floor) (Prague Congress Centre)

Board: 32

Poster B. Plasma Heating and Current Drive P4 Poster session

Speaker

Shaofei Geng (Department of Fusion Science)

Description

In order to investigated the dynamics of H^-- ions and understand the extraction process inside filament-arc-driven plasmas in a Cs-seeded negative ion source, diagnostic experiments using a directional Langmuir probe combined with photodetachment measurement have been conducted.  Two-dimensional flow pattern of H^-- ions has been obtained as well as the profile of H^-- ion temperature.  The flow result shows that H^-- ions are produced on the surface of the plasma grid, initially launched backward to the beam direction and travel in the beam extraction region.  H^-- ion energy decreases in the travelling and thermalized processes.  During beam extraction, H^-- ion density decreases in the extraction region and the charge density is partially replaced with electrons.  The decrement peak of the H^-- ion density is located at about 18 mm apart from the plasma grid.  Comparing two-dimensional distribution of the H^-- flows before and during beam extraction, a motion map of H^-- ion responding to the extraction field has been obtained.  It was found that the region corresponding to the decrement peak of H^-- ion density acted as an source of extracted H^-- ions.  The results suggest that the surface produced H- ions are not extracted directly from the plasma grid but from the plasma volume in the Cs-seeded negative ion source using filament-arc discharge.