Spatial distribution of the charged particles and potentials during beam extraction in a negative-ion source

Abstract

We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H(-)) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H(-) density is monitored with a cavity ring-down system to identify the electrons in the negative charges. The electron-saturation current decreases rapidly after starting to seed Cs, and ion-ion plasma is observed in the extraction region. The H(-) density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H(-) charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H(-) density decreasing at the beam extraction is compensated by the electrons diffusing from the driver region. In the plasmas with very low electron density, the pre-sheath of the extraction field penetrates deeply inside the plasmas. That is because the shielding length in those plasmas is longer than that in the usual electron-ion plasmas, and furthermore the electrons are suppressed to diffuse to the extraction region due to the strong magnetic field.

Study of the negative ion extraction mechanism from a double-ion plasma in negative ion sources

“…The result shows that n H − in the upstream region away from the plasma meniscus has been reduced by applying the extraction voltage. This result shows the similar tendency to the result of the experiments [6,7]. At the same time, electorstatic oscillation has been observed in the extraction region.…”

“…However, the extraction mechanisms have not been completely clarified yet. Double-ion plasma layer, which is consisting of H + and H − ions, has been observed in the vicinity of Plasma Grid (PG) of Cs-seeded H − negative ion sources, experimentaly [6]. Recent experiments show that H − density n H − in the upstream region away from the plasma meniscus (H − emitting surface) has been reduced by applying the extraction voltage [6,7].…”

“…Double-ion plasma layer, which is consisting of H + and H − ions, has been observed in the vicinity of Plasma Grid (PG) of Cs-seeded H − negative ion sources, experimentaly [6]. Recent experiments show that H − density n H − in the upstream region away from the plasma meniscus (H − emitting surface) has been reduced by applying the extraction voltage [6,7]. This fact indicates that the transport of H − ions in the double-ion plasma layer even far from the PG are affected by the extraction voltage, although the extraction voltage is shielded by the Debye shield effect.…”

Development of spectrally selective imaging system for negative hydrogen ion source

“…The intensity of H α emission at the far position of z = 18 mm is higher than that at close position of z = 4 owing to the electron contamination. 5 The H α signal intensity decreased 15% by beam extraction near the PG apertures. The electron temperature measured by an electrostatic probe is 2.3 eV at z = 11 mm.…”

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

“…Research oriented to improvements of the negative ion source is necessary. A lot of efforts have been contributed to the improvements of negative hydrogen ion (H − ) sources, in order to increase the performance of negative ion based NBI systems [4][5][6][7][8][9][10][11][12][13] is much higher than the electron density, is generated in the beam extraction region [9]. However, the H − ion rich plasma is affected by the extraction field.…”