- Conference date: 3–7 September 2012
- Location: Jyväskylä, Finland
Negative hydrogen ion sources for fusion are based on the surface conversion process and operate with both cesium seeded hydrogen and deuterium plasmas. Although the required extracted negative ion densities are achieved, the source performance is limited at present by the amount of co-extracted electrons, which is much higher in deuterium than in hydrogen for up to now unknown reasons. Systematic Langmuir probe measurements combined with emission spectroscopy have been carried out in the ITER prototype source. They show very similar electron densities and temperatures in the plasma generation region, the “driver”, but the degree of dissociation is clearly higher in deuterium than in hydrogen. The filter field in the expansion region generates a plasma drift which is more pronounced in hydrogen. Consequently, for deuterium, more electrons are present in front of the plasma grid. Close to the extraction, a recombining plasma evolves where the Balmer line emission is higher in deuterium due the relevance of the dissociative recombination process and thus more molecular ions. Throughout the source the plasma potential is higher in deuterium. It is concluded that the transport across the filter field plays a crucial role in the differences obtained for particle densities and coextracted electrons in hydrogen and deuterium operation.
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