M. Kisaki

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.

We report on the characteristics of the beam-generated plasma in the multibeamlet case of a hydrogen negative ion beam at NIFS. The plasma potential, and the energy of secondary particles in the drift region of an ion beam, offer an insight into the mechanisms that allow beam transport in low pressure gasses. The first measurements reported here were made by means of a four-gridded retarding field energy analyzer, combined with the measurement of the drain current at the beam dump, and with infrared beamlet monitoring technique. The retarding field analyzer measures the energy distributions of particles emitted radially from the beam-generated plasma [1]. The beam is dumped onto a graphite calorimeter, which is electrically insulated, so that it can be electrically biased and the collected current can be measured. The thermal image of the calorimeter is acquired during the beam pulse, thus offering a quantitative estimation of the single beamlet optics and multibeamlet focusing. Finally, the influence of the neutral gas density is studied by puffing hydrogen gas in this drift region. The measured ion and electron energy distribution functions are strongly affected by the electric bias of the calorimeter. For a 47kV, 0.3A beam, an electron temperature of 3eV at the plasma edge was found. The energy distribution of the ions for a pressure of about 0.03Pa is presented, from which the plasma potential can be inferred.

Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m². We have clearly found the lower limit of electron current ratio depending on the arc discharge power. Certainly, the extracted electron current ratio increased in deuterium operation. However, we have had no trouble on the beam line components.

The properties of cesium (Cs) recycling due to back-streaming ions have been investigated using an optical emission spec-troscopy in the hydrogen negative ion (H⁻) source with a slot-type grounded grid (GG). The slot-type GG performed well to enhance the beam performance, and to reduce the thermal loading on GG by high transparency. We clearly observed increase of Cs optical emission intensity during beam extraction owing to the increase of the Cs ions sputtered from the back plate of the source due to the back-streaming positive hydrogen ions. Increase of Cs is closely related with the extracted H⁻ current, but it does not depend on the beam energy. Recycling Cs from the back plate is deeply relevant to the perveance condition of the H⁻ beam, and is minimized at the optimum perveance. Strong Cs recycling from the back plate owing to the back-streaming ions is promoted in the high perveance condition with large divergence, which is consistent with the numerical calculation for the distribution of back streaming ions. This high Cs recycling condition, however, is not suitable for safety beam operation with high energy beam. The output of beam power is saturated by the space charge limitation, and divergent beam is trapped in the grounded grid (GG), which may cause damage on the slot-type GG surface by high thermal loading as large as in the aperture-type GG.

To improve the performance of negative-ion based neutral beam injection on the Large Helical Device, the accelerator was modified on the basis of numerical investigations. A field limiting ring was installed on the upper side of a grounded grid (GG) support and a multi-slot GG was adopted instead of a multi-aperture GG. As a result, the voltage holding capability is improved and the heat load on the GG decreases by 40%. In addition, the arc efficiency is improved significantly only by replacing the GG.

The electron density was measured at various bias voltages to understand how the plasma grid bias affects the electron near the plasma grid in large-scaled negative ion sources. It was found that the response of the electron to the bias voltage changes depending on negative ion production processes. The electron density remarkably decreases with increasing the bias voltage in the pure-volume plasma. On the other hand, the electron density depends on the bias voltage weakly in the Cs-seeded plasma. In addition, it was observed that the response of the co-extracted electron current to the bias voltage has similar trend to that of the electron density.

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.