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

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

Simultaneous Measurements of Proton Ratio and Beam Divergence of Positive-ion-based Neutral Beam in the Large Helical Device

A spectroscopy system was installed on the diagnostic neutral beam injector in LHD. The Hα lines spectrum emitted by full, half and one-third energy component are clearly observed, and the proton ratio and the beam divergence were estimated by the line intensity and the line width, respectively. The proton ratio of 85?90 % is achieved in high arc power discharge. The beam divergence of them shows their minimum with the same perveance. It was experimentally confirmed that the spectroscopy system is useful for the monitor of the proton ratio and the divergence of the beam

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

Calibrations of Fast Ion Flux Measurement Using a Hybrid Directional Probe

A hybrid directional probe method both “thermal and Langmuir probe” was applied for fast ion measure- ments in the compact helical system. In order to obtain absolute values of fast ion density and power density, a calibration of the probe was performed using neutral hydrogen beam and a mixture beam of hydrogen and proton, of which beam current and energy were controlled. The conversion factor from temperature increase of the probe head to local power density and secondary electron emission yield was obtained. The density of fast ions was obtained by directional thermal probe (DTP) method inside the last closed flux surface, and the density ratio was nFastIon/nBulkPlasma = 2.7 × 10?3 at r/a = 0.9. The observation of the directional Langmuir probe (DLP) method is consistent with the DTP results

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

Visualization of H? Dynamics in Extraction Region of Negative-Ion Source by Hα Imaging Spectroscopy

We developed a new imaging spectroscopy diagnostic tool for Hα emission and installed it on a negative hydrogen ion (H?) source to investigate the H? dynamics in the extraction region. During beam extraction, the Hα emission dropped; the same drop also appeared in the H? density (as measured by cavity ring-down spectroscopy). The reduction in the Hα emission results from the reduction in the excited hydrogen population caused by mutual neutralization processes between H+ and H? ions, which in turn are due to a decrease in the H? density. We find a reduction structure in Hα that is observed inside the plasma farther than 20 mm from the plasma grid (PG) surface. The result indicates that H? ions produced at the PG surface accumulate in the extraction region, so we conclude that they flow toward the PG apertures

LHDにおけるNBI用水素負イオン源とビーム生成の最新の研究成果

The state of hydrogen plasma in the extraction region in a hydrogen negative-ion (H−) source for NBI has been investigated. We clearly observe an improvement of H− density owing to the surface production effect with Cs seeding. H− ions are widely distributed in the extraction region which is obtained by movable cavity ring down (CRD). We confirm a negative ion rich plasma with a few electrons in the extraction region, which state is important for reduction of electron contamination in extracting beam. An extraction area is reached 30mm from the PG surface, which is measured by a 2D imaging diagnostic for Hα emission. We find the insensitive area for H− extraction at the PG surface between the apertures. Negative ions produced at the surface are considered tohave been supplied in the extraction region. The flow velocity of H− ions is obtained by a four-pin Langmuir probe using a photodetachment technique with an Nd:YAG laser. H− ion flows from the plasma grid surface, and its direction drastically changes at 20mm from the production surface. This flow behavior is considered to be an important characteristic for improving H− density in the extraction region