Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI

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.

Diagnostics tools and methods for negative ion source plasmas, a review

“…The integration of diagnostics results deepens the understanding how the electrons and H − ions behave in the extraction region plasma. The measured temperature data for neutral hydrogen atoms are higher than those for H − ions [13,91,93]. This H − ion temperature lower than neutral atoms in the plasma suggests the possibility of local H − formation in the extraction region.…”

Deuterium experiment with large-scale negative ion source for large helical device

“…There are technological issues regarding the negative ion source toward higher performance H − beam: higher negative hydrogen-ion beam power; higher H − beam current or beam current density; lower electrostatically co-extracted electron current with H − beam; higher wall-plug efficiency; higher operation stability; and lower consumption of cesium (Cs) seeded to increase surface H − production, etc. These are concerns with ion source physics: processes of H − production on and emission from plasma grid electrode (PG), which is an ion source plasma facing electrode as well as the first electrode in accelerator of the ion source; 16) H − transport in the ion source plasma; 17,18) characteristics on the ion-ion plasma where the electron density n e is quite lower than H − in the vicinity of beam extraction aperture of the PG; 19) Cs recycling in the ion source plasma; [20][21][22] beam extraction mechanism; [23][24][25] beam optics; 26,27) beamlets interaction; 28,29) withstanding high voltage; 30) and isotope effect, 4,6,31) etc.…”