Extraction of volume-produced H− ions from a hybrid magnetic multicusp source

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Abstract
The negative ion density n− is studied using the photodetachment technique in the extraction region of a 5.7‐l hybrid magnetic multicusp source, and is correlated with the extracted negative ion current I−. It is found that biasing positive the plasma electrode (PE) of the extractor affects in a similar way both n− and I−: both attain a maximum for a PE bias of +1.5 V, while the electron density ne and extracted current Ie exhibit a monotonic reduction. At optimum PE bias, n−/ne attains a value of 0.7. Ie is found to be comparable to the electron thermal flux through the extraction slit; this indicates that there is no potential barrier for the negative particles at the plasma border in front of the extraction slit. At optimum PE bias both Ie and I− vary in proportion to n0.8e. Two regimes characterize the variation of I− vs n−; at high density I− varies in proportion to n1− and appears to be larger than the thermal negative ion flux through the extraction slit, calculated under the assumption that the negative ion temperature is close to the neutral molecule temperature. The discrepancy is the largest (a factor of ten) at the lowest gas pressure.