Laser photodetachment on a high power, low pressure rf-driven negative hydrogen ion source

Powerful, low pressure negative hydrogen ion sources are a basic component of future neutral beam heating systems for fusion devices. The required high ion currents (> 40 A) are obtained via the surface production process, which requires negative ion densities in the range of n(H-)approximate to 10(17) m(-3) in the plasma region close to the extraction system. For spatially resolved diagnostics of the negative hydrogen ion densities, the laser photodetachment method has been applied to a high power, low pressure, rf-driven ion source (150kW, 0.3 Pa) for the first time. The diagnostic setup and the data evaluation had to cope with the rf field (1 MHz), the high source potential during extraction (-25 kV) and the presence of magnetic fields (< 10 mT). Horizontal profiles of negative ion densities and electron densities along 15 cm with a typical step length of 1 cm and a probe tip of 5 mm length show a broad maximum in the centre of the extraction region. The variation of a bias voltage applied to the plasma grid with respect to the source body yields a correlation between the detachment signals for the negative ion density and the electron density with the extracted ion and electron currents, respectively. The density ratio of negative hydrogen ions to electrons is in the range of n(H-)/ne = 0.3-3, demonstrating that the negative ions are the dominant negatively charged species in these types of ion sources. Absolute negative ion densities are in good agreement with line-of-sight integrated results of cavity ring-down spectroscopy and optical emission spectroscopy.