Study of ion cyclotron range of frequencies heating characteristics in deuterium plasma in the Large Helical Device

The characteristics of ion cyclotron range of frequencies (ICRF) minority ion heating with a hydrogen minority and deuterium majority plasma were studied by ICRF modulation injection experiments in the Large Helical Device (LHD). In recent experiments with deuterium plasma, no significant increase in the neutron emission rate due to ICRF second harmonic deuteron heating was observed. Therefore, in this study, the neutron emission rate was used to refer to the information regarding the thermal ion component. Like the results of the observations of the heating efficiencies at various minority proton ratios, the experimental results showed good agreement with the simple model simulation of ICRF wave absorption. During these experiments, the accelerated minority hydrogen ions were observed by neutral particle analyzers. The counting rates of the energetic particles were higher in the lines of sight passing through the helical ripple than across the magnetic axis, and the counting rate decreased as the minority hydrogen ion ratio increased. The dependence of the minority hydrogen ion ratio on the density of the energetic ions was consistent with the experimentally observed heating efficiencies and simulations. The heating efficiency of ICRF minority ion heating could be well explained by simple model simulation in the LHD deuterium experiment

First experiments on plasma production using field-aligned ICRF fast wave antennas in the large helical device

The results of the first experimental series to produce a plasma using the ion cyclotron range of frequency (ICRF) in the large helical device (LHD) within the minority scenario developed at Uragan-2M (U-2M) are presented. The motivation of this study is to provide plasma creation in conditions when an electron cyclotron resonance heating start-up is not possible, and in this way widen the operational frame of helical machines. The major constraint of the experiments is the low RF power to reduce the possibility of arcing. No dangerous voltage increase at the radio-frequency (RF) system elements and no arcing has been detected. As a result, a low plasma density is obtained and the antenna-plasma coupling is not optimal. However, such plasmas are sufficient to be used as targets for further neutral beam injection (NBI) heating. This will open possibilities to explore new regimes of operation at LHD and Wendelstein 7-X (W7-X) stellarator. The successful RF plasma production in LHD in this experimental series stimulates the planning of further studies of ICRF plasma production aimed at increasing plasma density and temperature within the ICRF minority scenario as well as investigating the plasma prolongation by NBI heating

Upgrade of ICRF Antennas by Utilizing Impedance Transformers in LHD

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of plasma in the Large Helical Device (LHD), two types of ICRF antennas are used. One is the Field-Aligned-Impedance-Transforming (FAIT) antenna. It has an In-Vessel Impedance Transformer (IVIT) in the vacuum region of the antenna and shows the possibility of high-power injection despite the short antenna head. To enhance the performance more, an Ex-Vessel Impedance Transformer (EVIT) was attached outside the LHD vacuum vessel. As a result, the injectable power increased. The other is the Handshake form (HAS) antenna. Plasma can be efficiently heated by adjusting the phase difference between currents in straps. However, the injectable power from the HAS antenna was originally small. Therefore, later an EVIT was attached to it. Moreover, the transmission line in the vacuum region was remodeled to form an IVIT. By utilizing these impedance transformers, the performance of the HAS antenna was drastically improved

Investigation of Capability of Current Control by Electron Cyclotron Waves in the Quasiaxisymmetric Stellarator CFQS

The capability of plasma current control by the second harmonic electron cyclotron current drive in the quasiaxisymmetric stellarator CFQS is investigated. We used the ray-tracing code TRAVIS to evaluate the electron cyclotron (EC) wave power deposition and driven current. In the standard magnetic field configuration of CFQS, the poloidal distribution of the magnetic field is nearly axisymmetric, i.e., equivalent at all toroidal positions as tokamaks. In the calculation, a flat electron density profile at the core region with ne0 = 1 × 1019 m−3 and a center-peaked electron temperature profile with Te0 = 3.5 keV are assumed. The EC wave beam direction is scanned mainly in the toroidal direction, aiming at the plasma axis. The vertical injection angle of the beam and magnetic field strength are varied and optimized to keep on-axis power deposition to maximize driven current at each toroidal direction of the EC wave beam. According to the calculation, the maximum driven current at optimum beam direction, with an expected maximum EC wave power of 400 kW, is approximately 80 kA. Meanwhile, approximately 26 kA of bootstrap current in CFQS with the volume-averaged β value of 1.2% is estimated using the BOOTSJ code. Hence, sufficient on-axis EC-driven current can be expected for compensation of the possible bootstrap current, although the current profiles are different. Moreover, a driven current of over 30 kA can be expected even in extreme cases where the magnetic field on-axis has ripples by modified modular coil currents by 20%. The possibility of compensation of bootstrap current in total amount and current profile is also discussed