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Description
In the ITER neutral beam injection (NBI) system, a radiofrequency (RF) negative hydrogen ion source is used to heat the core plasma. However, the current RF ion source has not yet achieved the required beam divergence angle (7 mrad), which remains a major challenge. For the RF negative hydrogen ion source at J-PARC, a large-scale accelerator, the beam distribution has been observed to oscillate at frequencies related to the RF drive under cesium-seeded conditions [1]. Since beam oscillations can increase the beam divergence angle, a more detailed characterization of these oscillations is necessary.
To evaluate the response to RF discharges, time-resolved measurements of the beam phase-space structure were performed using a double-slit emittance meter while simultaneously recording the RF waveform. In the absence of cesium, the phase-space structure exhibited periodic fluctuations, the beam's center of mass rotated, and the emittance oscillated at the RF frequency. These results suggest that the extracted beam is oscillating due to oscillations in the ion source plasma at the beam extraction interface (meniscus). When cesium was added, a distinct additional phase-space structure―not observed in the cesium-free case―was confirmed; this is likely due to the generation of negative ions at the surface.
These measurement results reveal that the beam's phase-space structure exhibits oscillations related to the RF frequency. We plan to conduct similar experiments and analyses using an RF negative hydrogen ion source for NBI.
[1] T.Shibata et al., AIP Conference Proceedings 2373, 050002(2021)
| Email address | ishikawa.takaaki@nifs.ac.jp |
|---|---|
| Visitor's Visa | yes |
| Classification | Plasma and negative ion diagnostics |