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As a challenge to measure the extraction surface of the negative ion beam, a laser Thomson scattering system with high spatial resolution was applied to a weakly ionized plasma in a negative ion source for the first time. The first target was to measure electron density around extraction region because shape of the extraction surface can be estimated by electron spatial profile. However, stray light by injected laser, filament and plasma was too intense compared to signal from the low density region. To solve this, a triple grating spectroscopy and a multilayer black screen were introduced, and the laser spot size was minimized to reduce slit sizes of the spectroscopy. As a result, electron density was successfully measured accurately in low density region. From an error estimation, measurement of electron density 3 × 10 16  m -3  with spatial resolution of 0.5 mm at extraction region can be realized with accumulation of signals from multiple laser shots. This system can contribute to the understanding of the mechanism of formation of the extraction surface, which is a long-term issue in the negative ion source.

Application of Thomson scattering system toward direct measurement of extraction surface of the negative ion beam