Yasuaki Haba

The shape of a plasma meniscus is a key factor to determine the beam focusing. The physics model of the meniscus formation for hydrogen negative ion sources has not been established yet. A backward trajectory calculation based on experimental observation is performed in order to derive the particle information at the meniscus. It is observed that the negative ion density is spatially nonuniform in the direction parallel to the magnets for suppression of co-extracted electrons. A nonuniformity of the negative ion density in the vicinity of the meniscus is taken into account in the forward trajectory calculation. It reveals that the nonuniform negative ion distribution leads to degradation of the beam focusing and the beam splitting in phase space. The importance of the spatial distribution of negative ions on meniscus modelling is discussed with a comparison to uniform extraction model.

Well focused negative ion beams are required for neutral beam injection systems for heating and current drive in magnetically confined fusion plasma experiments. The control of a single negative ion beamlet divergence is a significant challenge with the use of a cesium-seeded negative ion source, where negative ions are mainly produced by conversion of hydrogen or deuterium atoms on a cesiated surface of a plasma grid. The single negative ion beamlet was found to be made by three-Gaussian components in our previous work. The origins of such multiple components are considered to be related to dynamics in the ion source and extraction processes of negative ions. This work has demonstrated a measurement of the abundance ratio of the three components (41%, 40%, and 19%) based on a full picture of their transverse velocity distributions, which is a powerful technique to investigate the origins of individual components and will contribute to improve the divergence of negative ion beamlet.

Beam focusing is one of the most important elements for the stable and safe operation of high power negative ion beams, such as neutral beam injection into magnetically confined fusion plasmas. In order to investigate impacts of the source plasma fluctuation on beam focusing, a simultaneous measurement of the source plasma fluctuation and the beam current profile has been carried out in the research-and-development negative ion source at the National Institute for Fusion Science. The responses of beam width and of the beam centre deviation are observed for the first time, indicating the importance of the source plasma stability for the negative ion beam focusing.

Simultaneous measurement of negative ion source plasma and extracted beam is carried out in order to clarify a key plasma parameter governing the meniscus formation in negative ion sources for fusion. The plasma discharge is performed with various discharge powers at different bias voltages in order to vary the plasma parameters. It is shown that the beam width changes along the same curve with respect to the negative ion density at any bias voltage while it varies along different curves with other plasma parameters depending on the bias voltage. This implies that the mechanism of meniscus formation in negative ion sources could be described along the similar manner as positive ion sources.

Negative ion beam focusing is a key element for advanced applications of negative ion beams such as accelerators for particle physics, compact accelerators for medical fields, and plasma experiments for nuclear fusion because complicated magnetic fields exist both inside of the source plasma and the grid system. In order to understand the beam focusing, phase space structure measurements for a single beamlet have been performed with a research-and-development negative ion source at the National Institute for Fusion Science. A complicated phase space structure is observed in the direction parallel to the filter magnetic field in the vicinity of the plasma grid, while a single-Gaussian beamlet structure is observed in the direction perpendicular to the filter field. Detailed analyses for the phase space structure of the single beamlet reveal that the complicated structure can be identified as a combination of three beam components with different beam axes. The shifts of each axis are also observed to depend on the ratio of the acceleration voltage for the extraction voltage, which may significantly degrade the beamlet focusing.

Density distributions of negative hydrogen (H⁻) ions and negative deuterium (D⁻) ions were measured with the laser photodetachment method in the extraction region of the negative ion source. The distribution of H⁻ ion density peaks at the center of the ion source, while that of the D⁻ ion shows a flatter profile in the direction parallel to the plasma grid. The positive ion densities of hydrogen and deuterium estimated from the positive saturation current indicate similar profiles with different amounts close to the grid. The difference in the H⁻ ion and D⁻ ion distributions can be explained by the difference in the negative ion yield and the survival probability of the ions due to the isotope effect.

Second deuterium operation of the negative ion based neutral beam injector was performed in 2018 in the large helical device. The electron and ion current ratio improves to I_e/I_acc(D) = 0.31 using the short extraction gap distance of 7 mm between the plasma grid (PG) and the extraction grid (EG). The strength of the magnetic field by the electron deflection magnet installed in the EG increases by 17% at the PG ingress surface, which effectively reduces the electron component in the negative ion rich plasma in the vicinity of PG apertures. The reduction of the electron current made it possible to operate at a high power arc discharge and beam extraction. Then, the deuterium negative ion current increases to 55.4 A with the averaged current density of 233 A/m². The thermal load on the EG using 7 mm gap distance is 0.6 times smaller than the thermal load using a 8 mm gap caused by the reduction of coextracted electron current. The injection beam power increases to 2.9 MW in the beam line BL3, and the total beam injection power increases to 7 MW by three beam lines in the second deuterium campaign.

The consumption rate of cesium (Cs) for negative hydrogen (H^-) ion source increases when the source operation gas is changed from hydrogen to deuterium. There was observed a clear indication that a deuterium discharge erodes Cs atoms on the plasma grid (PG) surface to increase work function and the co-extracted electron current. We have proposed a model, that the enhanced sputtering yield of Cs from the PG due to deuterium ions, which carries more kinetic energy to Cs adsorbed on the surface directly or indirectly, is the main reason for this fast dissipation of Cs. Introduction of helium (He) into discharge can verify the enhanced sputtering effect due to the lager mass ions in the source discharge. Comparing the effect due to seeded Cs before and after the He injection into discharge through Cs OES signals as well as the H^- density measured with cavity ring-down method, the sputtering/evaporation enhancement due to He is estimated. Neutral atoms and positive ions in the He discharge should cause enhanced sputtering like deuterium, while the system does not generate neutron under the induction of acceleration voltage to diagnose the extracted negative ion beam. Plasma parameters of the H2 and He plasmas are investigated by diagnostics tools installed on NIFS-RNIS (National Institute for Fusion Science, Research and development Negative Ion Source) together with H^- density measurement by cavity ring down. Enhanced consumption rate of Cs is compared with proposed sputtering yield data to predict the rate for deuterium operation of negative ion sources.

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.