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Spatial distribution of the charged particles and potentials during beam extraction in a negative-ion source
K Tsumori1, H Nakano, M Kisaki
1National Institute for Fusion Science, 322-1 Oroshi Toki-city, Figu 509-5292, Japan. tsumori@nifs.ac.jp
The Review of Scientific Instruments
|March 3, 2012
Summary
This study characterizes hydrogen negative ion (H(-)) sources, finding that electron density increases when H(-) ions are extracted. This indicates electrons diffuse from the driver region to compensate for extracted negative charges.
Area of Science:
- Plasma Physics
- Atomic and Molecular Physics
Background:
- Electronegative plasmas are crucial for generating negative ions.
- Understanding plasma characteristics in large-scale sources is vital for applications.
Purpose of the Study:
- To investigate the behavior of electronegative plasma in a hydrogen negative ion (H(-)) source.
- To analyze the relationship between H(-) density and electron density during beam extraction.
Main Methods:
- Utilized a time-resolved Langmuir probe in the beam extraction region.
- Employed a cavity ring-down system to monitor H(-) density.
- Observed plasma characteristics with and without cesium (Cs) seeding.
Main Results:
- Cesium seeding led to an ion-ion plasma and a decrease in electron density.
- H(-) density decreased during beam extraction, while electron density increased correspondingly.
- Charge balance was observed between decreasing H(-) and increasing electron densities.
Conclusions:
- Electron diffusion from the driver region compensates for extracted H(-) charge.
- Low electron density in plasmas results in deep pre-sheath penetration due to longer shielding lengths.
- Strong magnetic fields suppress electron diffusion to the extraction region.
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