Search

In the Large Helical Device plasma discharges, the size of an externally imposed island with mode number ( $n/m\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/1$) decreases substantially when the plasma is collisionless ( ${\ensuremath{\nu}}^{*}l\ensuremath{\sim}1$) and the beta is finite ( $g\ensuremath{\sim}0.1%$) at the island location. For the collisional plasmas with finite beta, on the other hand, the size of the island increases. However, there is a threshold in terms of the vacuum island size below which the island enlargement is not seen.

Large-scaled hydrogen negative ion source development is reviewed for a negative ion based neutral beam injector (NBI) in the large helical device (LHD) fusion machine. The target performance of the ion source is characterized by a high current of 30–40 A with a relatively low energy of 120–180 keV. A series of negative ion source development is conducted with a one-dimensionally reduced size of ion sources which still have a large beam area of 25 cm×26 cm or 50 cm with multi apertures. We employed a cesium-seeded volume production source with an external magnetic filter for the source development. Improvement of the arc plasma confinement is effective to produce a high-current negative ion of 16 A with a current density of 31 mA/cm2 at a low operational gas pressure below 0.4 Pa. Suppression of the accelerated electrons is achieved both by strengthening the magnetic field at the extraction grid and by shaping the inside of the extraction grid aperture to shield the secondary electrons against the acceler...

The experimental results from the Large Helical Device (LHD) heliotron / torsatron of the first two years are reviewed. The world's largest superconducting helical coils have been driven up to 2.9 Tesla on the axis which is close to the designed value (3 T) The obtained plasma performances are better than those predicted by the database from the medium-size helical devices. These improvements are attributed mainly to the optimization of a magnetic field configuration which can be controlled by shifting the magnetic axis inward than that of standard case. This configuration improves particle orbits oftrapped high energy ions resulting in success of ICRF heating in LHD. Efforts have also been made on steady state plasma operation, and long pulse discharges more than one minute have been achieved both by ICRF and NBI. It should be noted that the feature of no current-disruption in helical plasma makes the discharges easy.

In parallel to the conceptual design of the R-tokamak, research and development efforts on the systems and the components have been carried out to obtain scopes to realize the device. Because we intended to introduce DT fuels into the device, the items of research and development works are quite different to that of non-burning machines. Alminium alloy developments to avoid activations by 14 MeV neutron irradiations, tritium safe handling and vacuum system with tritium are examples of this category. Beside specific requirement directly caused by DT introduction, highly reliable operation of the machine is strongly requested, because it is the necessary condition to perform experiments safely. However, these efforts are still in an early stage of investigation and many efforts are needed to prepare a rigid data base tokamak operation with reacting plasma.

A large bucket-type plasma source has been developed for a neutral beam injector. An ion current density of 0.25 A/cm2 in a hydrogen plasma was achieved with a density uniformity of better than ∼10% over an area of 15×60 cm2 at about 100 kW of arc power in pulsed operation for 1s. Plasma density profiles and the discharge voltage-current characteristics, etc., were studied by varying the geometrical parameters of a tungsten filament i.e., the disposition, number, diameter, and shape. The profile was affected by the filament disposition and the gas pressure. A measure of the gap length between filaments to generate a dense uniform (< 10%) plasma was roughly 14-22 cm. An experimental scaling relation of the arc power is discussed; the arc power for the same plasma density appears to increase with an increase in the total loss-area in various sources.