Spatial distribution of the charged particles and potentials during beam extraction in a negative-ion source
- National Institute for Fusion Science, 322-1 Oroshi Toki-city, Figu 509-5292 (Japan)
- Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522 (Japan)
We report on the characteristics of the electronegative plasma in a large-scale hydrogen negative ion (H{sup -}) source. The measurement has been made with a time-resolved Langmuir probe installed in the beam extraction region. The H{sup -} 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{sup -} density steps down during the beam extraction and the electron density jumps up correspondingly. The time integral of the decreasing H{sup -} charge density agrees well with the electron charge collected with the probe. The agreement of the charges is interpreted to indicate that the H{sup -} 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.
- OSTI ID:
- 22066293
- Journal Information:
- Review of Scientific Instruments, Vol. 83, Issue 2; Conference: ICIS 2011: 14. international conference on ion sources, Giardini-Naxos, Sicily (Italy), 12-16 Sep 2011; Other Information: (c) 2012 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0034-6748
- Country of Publication:
- United States
- Language:
- English
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