Research and Development Activities on Negative Ion Sources

Spatially resolved diagnostics for optimization of large ion beam sources

Giant negative ion sources for neutral beam injectors deliver huge negative ion currents, thanks to their multi-beamlet configuration. As the single-beamlet optics defines the transmission losses along the beamline, the extraction of a similar current for all beamlets is extremely desirable, in order to facilitate the beam source operation (i.e., around perveance match). This Review investigates the correlation between the vertical profile of beam intensity and the vertical profiles of plasma properties at the extraction region of the source, focusing on the influence of increasing cesium injection. Only by the combined use of all available source diagnostics, described in this Review, can beam features on the scale of the non-uniformities be investigated with a sufficient space resolution. At RF power of 50 kW/driver, with intermediate bias currents and a filter field of 2.4 mT, it is found that the central part of the four vertical beam segments exhibits comparable plasma density and beamlet currents; at the edges of the central segments, both the beam and electron density appear to decrease (probably maintaining fixed electron-to-ion ratio); at the bottom of the source, an increase of cesium injection can compensate for the vertical drifts that cause a much higher presence of electrons and a lower amount of negative ions.

Development of a dual beamlet monitor system for negative ion beam measurements

To evaluate negative ion beam properties, a dual beamlet monitor system has been developed. The dual beamlet monitor system has two diagnostics in one hexagonal box. One diagnostic is a "fast beamlet monitor" for measuring the time evolution of beamlet current profiles with the time resolution of up to 25 MHz. The other diagnostic is a "pepper-pot-type phase space analyzer," which is applied for the evaluation of a phase space structure of the negative ion beamlet. The dual beamlet monitor system is applied to the measurement of the beamlet in the Neutral Beam Test Stand at National Institute for Fusion Science (NIFS-NBTS), in which the beam accelerator is almost identical to those of working beam injectors in the large helical device. It is demonstrated that the overlapping components from the neighboring beamlet can be eliminated, and the phase space structure can be obtained for the single beamlet.

Charged particle flows in the beam extraction region of a negative ion source for NBI

Experiments by a four-pin probe and photodetachment technique were carried out to investigate the charged particle flows in the beam extraction region of a negative hydrogen ion source for neutral beam injector. Electron and positive ion flows were obtained from the polar distribution of the probe saturation current. Negative hydrogen ion flow velocity and temperature were obtained by comparing the recovery times of the photodetachment signals at opposite probe tips. Electron and positive ions flows are dominated by crossed field drift and ambipolar diffusion. Negative hydrogen ion temperature is evaluated to be 0.12 eV.

Electron density measurement of cesium seeded negative ion source by surface wave probe

Electron density measurements of a large-scaled negative ion source were carried out with a surface wave probe. By comparison of the electron densities determined with the surface wave probe and a Langmuir probe, it was confirmed that the surface wave probe is highly available for diagnostic of the electron density in H(-) ion sources. In addition, it was found that the ratio of the electron density to the H(-) ion density dramatically decreases with increase of a bias voltage and the H(-) ions become dominant negative particles at the bias voltage of more than 6 V.