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The beam neutralization system for measurement of the spatial and velocity distributions of alpha particles of ITER plasmas was studied. As forward angle detection against the beam injection direction is required for effective neutralization, arrangement of the measurement system using possible ports in ITER configuration is proposed. The count rate of neutralized alpha particles produced by the double charge exchange interaction with energetic He0 beam particles injected is estimated. The ratios of signal to neutron-induced noise are evaluated. When a He0 beam produced by autodetachment from a 1–1.5MeV He− beam of 10mA is injected, the signal to noise ratio becomes greater than 1 at ρ<0.4, even without beam modulation. Usage of a lock-in technique at the frequency of radio-frequency quadrapole accelerator will make measurement at the outer region possible.

An experimental system designed to study systematically interactions of beams with solid surface is being developed. A small magnetic multicusp-type ion source is used to produce both positive and negative ions with a pair of removable filter magnets. The mass-separated extracted ions are then directed onto a solid target, and the angular distributions and energy spectra of the reflected ions are measured with a magnetic momentum analyzer. The performance of the system is tested by investigating the particle reflections from a polycrystalline W surface bombarded by low-energy (1–2 keV) H and O ions.

The density and potential fluctuations were measured in hot-cathode biasing plasma at the Tohoku University Heliac. In the improved mode, high-frequency fluctuations (>100 kHz) appeared in the density signal. On the other hand, lowfrequency fluctuations (<100 kHz) in the density and potential signals were suppressed. The characteristics of high-frequency fluctuation were compared with three kinds of instability, and they were consistent with those of the flute instability driven by the supersonic poloidal rotation. The suppression of low-frequency fluctuations in improved mode is considered the effect of E × B poloidal rotation or its shear. The profile of the anomalous particle flux was estimated by analysing the low-frequency fluctuation signals. The flux decreased in the improved mode in most of the region, although the decrease in flux was small near the rational surface (n/m = 5/3).

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