Observation of Toroidal Flow on LHD

In order to investigate the formation of toroidal flow in helical systems, both NBI driven flow and spontaneous toroidal flow were observed in Large Helical Device (LHD). The toroidal flow driven by NBI is dominant in plasma core while its contribution is small near plasma edge. The spontaneous toroidal flow changes its direction from co to counter when the radial electric field is changed from negative to positive at plasma edge. The direction of the spontaneous toroidal flow due to the radial electric field near plasma edge is observed to be opposite to that in plasma core where the helical ripple is small

Optical Measurement of Cesium Behavior in a Large H− Ion Source for a Neutral Beam Injector

Optical emission in a negative hydrogen ion source for the Large Helical Device Neutral Beam Injector (LHD-NBI) has been measured to investigate the behavior of Cs. Two optical sight lines exist parallel to the plasma grid, in the discharge area and in the magnetic filter area near the plasma grid. In the discharge area, the spectrum intensity from Cs+ ions is considerably increased during 20 s of the beam extraction. This indicates a considerable increase in the Cs+ density inside the plasma due to the impact of back-streaming H+ ions. A strong neutral Cs spectrum is observed in the magnetic filter area, where the electron density is lower than in the discharge area. The rate of increase of neutral Cs is much enhanced after t = 30 s, probably because the Cs adsorbed on the cooled region inside the arc chamber evaporates because its temperature increases during the long pulse discharge

Isotope effects on particle transport in the Compact Helical System

The hydrogen isotope effects of particle transport were studied in the hydrogen and deuterium dominant plasmas of the Compact Helical System (CHS). Longer decay time of electron density after the turning-off of the gas puffing was observed in the deuterium dominant plasma suggesting that the recycling was higher and/or the particle confinement was better in the deuterium dominant plasma. Density modulation experiments showed the quantitative difference of the particle transport coefficients. Density was scanned from 0.8  ×  1019 m−3 to 4  ×  1019 m−3 under the same magnetic field and almost the same heating power. In the low density regime (line averaged density  2.5  ×  1019 m−3) no clear difference was observed. This result indicates that the isotope effects of particle transport exist only in the low density regime. Comparison with neoclassical transport coefficients showed that the difference of particle transport is likely to be due to the difference of turbulence driven anomalous transport. Linear character of the ion scale turbulence was studied. The smaller linear growth rate qualitatively agreed with the reduced particle transport in the deuterium dominant plasma of the low density regime

Global linear gyrokinetic simulation of energetic particle-driven instabilities in the LHD stellarator

Energetic particles are inherent to toroidal fusion systems and can drive instabilities in the Alfvén frequency range, leading to decreased heating efficiency, high heat fluxes on plasma-facing components, and decreased ignition margin. The applicability of global gyrokinetic simulation methods to macroscopic instabilities has now been demonstrated and it is natural to extend these methods to 3D configurations such as stellarators, tokamaks with 3D coils and reversed field pinch helical states. This has been achieved by coupling the GTC global gyrokinetic PIC model to the VMEC equilibrium model, including 3D effects in the field solvers and particle push. This paper demonstrates the application of this new capability to the linearized analysis of Alfvénic instabilities in the LHD stellarator. For normal shear iota profiles, toroidal Alfvén instabilities in the n  =  1 and 2 toroidal mode families are unstable with frequencies in the 75 to 110 kHz range. Also, an LHD case with non-monotonic shear is considered, indicating reductions in growth rate for the same energetic particle drive. Since 3D magnetic fields will be present to some extent in all fusion devices, the extension of gyrokinetic models to 3D configurations is an important step for the simulation of future fusion systems

Difference of co-extracted electron current and beam acceleration in a negative ion source with hydrogen-isotope ions

Improvement of the performance on a hydrogen/deuterium negative ion source for a nuclear fusion device is reported. In particular, the suppression of the co-extracted electron current, Ie, is an important issue to ensure the stable beam acceleration. Improvement of the Ie has been confirmed by optimizing the magnetic field of the electron deflection magnet in the extraction grid. Two other new methods for reduction of the Ie were validated. The first was an electron fence whose rods were set between the rows of apertures on a plasma grid. The electron and negative ion current ratio, approximately Ie/Iacc, was greatly improved from 0.7 to 0.25 in deuterium. The second was an outer iron yoke which enhanced the magnetic flux density 19% inside the arc discharge chamber. The Ie/Iacc using the outer yoke decreased by 0.1 compared with using a normal magnetic filter in a deuterium operation. These attempts have improved the total deuterium injection beam power of 8.4 MW by three negative ion based NBIs

Fast-Ion-Diagnostics for CHS Experiment

Fast-ion-diagnostics have played an important role in investigating issues related to fast ion orbits and fast-ion-driven MHD instabilities in CHS experiments. The fast-ion diagnostics employed in CHS are reviewed and experimentally obtained knowledge is summarized

Validation of the distribution of stripping loss neutrals in the accelerator of the negative ion source

The difference of the stripping loss between hydrogen and deuterium is examined using two approaches. The first is the measurement of the optical beam emission. The wavelength of beam emission spectrum reflects the energy distribution of beam particles by the Doppler effect. The low-energy stripping peak is observed in the energy band corresponding to the extraction voltage, and also a moderate shoulder is distributed in the lower energy region. Secondly, the spatial and the energy distribution in the accelerator is estimated by the attenuation calculation using the vacuum pressure distribution in the accelerator. Stripping neutrals are concentrated in the low energy region, and a peak is formed at 9 keV in the energy distribution due to stripping neutrals inside the extraction grid aperture. The total stripping loss inside the accelerator is 16% for hydrogen and 24% for deuterium. The calculated Doppler-shifted spectra for hydrogen and deuterium clearly show the peak with the moderate shoulder on the redshift side, which is consistent with the measured results

Extension of high power deuterium operation of negative ion based neutral beam injector in the large helical device

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 Ie/Iacc(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