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

Abstract

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 − ≈ 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 (150 kW, 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 − /n e = 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.

Effect of fast positive ions incident on caesiated plasma grid of negative ion source

“…It appears that the extracted negative ion current density generated by the fast positive ions is 387 A/m 2 , while that generated by the thermal atoms is 48 A/m 2 , with a total extracted current density of 435 A/m 2 . This compares favourably to the extracted current density reported by Christ-Koch et al, 12 namely, 330 A/m 2 . This agreement supports our main conclusion that fast positive ions dominate the H − ion surface production.…”

Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

“…In the RF source, the optimum bias voltage is near the floating potential of the grid (∼10 V) so that only a small bias current can flow [4,5,24]. Additionally, the effect of the PG bias must be supported by a so-called bias plate near the PG for an effective suppression of the amount of co-extracted electrons, especially for deuterium operation [25].…”

A small radius hydrogen discharge: An effective source of volume produced negative ions

“…However, the rf source in its present design of a large-scale two-chamber source does not ensure efficiency high enough for volume production of the ions, and currently it is considered as a source with surface production of the ions. [2][3][4] The design of the rf sources of negative hydrogen ions is based on the experience accumulated during many years of work on the dc filament sources [5][6][7][8][9][10][11][12][13][14] starting in the 1980s, the time when the idea for the tandem type of the source arouse. The idea is for a design that ensures spatial separation in the source of two regions, respectively, of high-and low electron temperature, and it originates from the conclusion that such a design provides conditions favoring the two-step reaction of the volume production of the negative ions: excitation of the molecules to highly vibrationally excited states that requires high-energetic electrons and production of negative ions via dissociative attachment of electrons to the vibrationally excited molecules that requires low-energetic electrons.…”