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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. [ABSTRACT FROM AUTHOR]

• We developed an optimized power combiner for high-power and long-pulse ICRF heating in the LHD. • We also developed a control system for the power combination in order to minimize the power loss. • Power combination of over 2 MW for 6 s and 1 MW for 10 min were achieved. In the Large Helical Device (LHD), the development of high-power and long-pulse Ion Cyclotron Range of Frequencies (ICRF) heating system is ongoing. The developed Field-Aligned-Impedance-Transforming (FAIT) antenna has the potential for high-power injection of more than 1.8 MW. Here, to achieve this injection power, a power combination system was developed. An optimized power combiner was designed by repeated simulations, and then was fabricated and installed in the ICRF transmission system. Control of the power and the phase of incident waves into the input ports of the power combiner is important for the power combination. Therefore, a real-time control system was developed, and prompt reduction of power loss was demonstrated. As a result, combined powers of more than 2 MW for 6 s and 1 MW for 10 min were successfully achieved. [ABSTRACT FROM AUTHOR]

The transmission line is one of the most important components of ion cyclotron range of frequencies (ICRF) heating devices. In the case of unexpected trouble on the line, such as a breakdown, immediate power-off is necessary in order to avoid severe damage on the line. Breakdowns are difficult to detect with a reflection monitor, since the reflection may originate from a change in the antenna-plasma coupling. In the Large Helical Device (LHD), a Fault Detection System (FDS) for the transmission line was developed, which detects the breakdown utilizing the unbalance of three signals from the both ends of the line. For the precise balancing in the normal condition, the calibration is iteratively conducted. FDS is insensitive to the change of the antenna impedance, therefore, FDS can detect breakdown clearly. Frequency shift is also detectable with the FDS applied to a long transmission line. Therefore, the self-oscillation accompanying frequency shift could be detected in addition to breakdown. [ABSTRACT FROM AUTHOR]

Newly developed ion cyclotron range of frequencies (ICRF) antennas were fabricated and installed in the Large Helical Device (LHD). The antennas, called field-aligned-impedance-transforming (FAIT) antennas, were designed based on former handshake form (HAS) antennas for high power and steady-state operation. The FAIT antennas demonstrated higher loading resistance and lower temperature on ceramic feed-through than HAS antennas. Antenna impedance and injected power were measured precisely using a directional coupler installed between the impedance matching device and antenna. It was shown that an injection power of 1.8 MW or a power density of 15 MW/m 2 would be possible for short-pulse discharges. The mutual coupling between two FAIT antennas is small enough, since any current phase is possible without adjusting the impedance matching device. Although the FAIT antennas are aligned in a poloidal direction, a high heating efficiency of 85% was achieved with the proper minority concentration ratio. [ABSTRACT FROM AUTHOR]

Abstract: A pair of ion cyclotron range of frequencies (ICRF) antennas in the large helical device (LHD), HAS antennas showed high efficiency in minority ion heating. However the low loading resistance must be increased to prevent breakdown in transmission line. Moreover, the voltage and the current around the feed-through must be reduced to protect it. For these purpose, we developed a design procedure of the impedance transformer for HAS antennas. To optimize the transformer, the inner conductors were divided into several segments and the radii of them were given discretely and independently. The maximum effective loading resistance was obtained by using all combinations of radii within the limitations of the voltage and current at the feed-through and the electric field on the transformer. To get a precise solution, this procedure was repeated several times by narrowing the range of radii of inner conductors. Then the optimized impedance transformer was designed by smoothing the radii of inner conductors. For the typical discharge, the voltage and current at the feed-through were reduced to the half of the original values with the same power. The effective loading resistance was increased to more than four times. High performance is expected in wide impedance region. [Copyright &y& Elsevier]

Abstract: New ion cyclotron range of frequencies (ICRF) antennas have just been installed in the large helical device (LHD). These side-by-side ICRF antennas are symmetrical and designed to launch fast waves with various wave numbers parallel to the magnetic field line. The wave number can be controlled by changing the current phase on the straps; however, the mutual coupling between antennas changes antenna impedances, even if the plasma parameters are constant, leading to an increase in the reflected power. In addition to the current phase control, impedance matching devices must be tuned for the protection of tetrode tubes and efficient power injection. For this purpose, the impedance matrix of ICRF antennas must be determined, and it can be deduced from the forward and reflected waves at the outlet of the power amplifier by assuming geometric symmetry and reciprocity of the antennas. Using half-scale antennas, we successfully demonstrated simultaneous impedance matching and current phase control. [Copyright &y& Elsevier]

Abstract: Ion cyclotron emissions (ICEs) were clearly detected in the Large Helical Device (LHD) during perpendicular neutral beam (NB) injection. Antennas for the ion cyclotron range of frequencies (ICRF) heating were used as probes. The frequencies of ICEs were proportional to magnetic field strength. The location of ICE excitation was a peripheral region of plasma and the source particles were lost-ions injected by the perpendicular NB. [Copyright &y& Elsevier]

Abstract: In the Large Helical Device (LHD), long-pulse plasma experiments have been conducted using an ion cyclotron range of frequencies (ICRFs) heating system. Real-time impedance matching is necessary to keep injecting ICRF heating power into the plasma against the variation of the antenna impedance that changes gradually during a long-pulse plasma discharge. A feedback control system for the real-time impedance matching was constructed and utilized for long-pulse plasma discharges. As a component of the impedance matching device, liquid stub tuners were used. The reflected power ratio was reduced and kept sufficiently low by controlling the liquid heights during long-pulse plasma discharges, and the problem of the gradual increase in the reflected power was solved. [Copyright &y& Elsevier]

Abstract: During long-pulse discharges by minority ion cyclotron range of frequencies (ICRF) heating (majority: He++ and minority: H+) in the large helical device (LHD), phenomena of hot spots on antenna side protectors, localized divertor plate temperature increase and sparks from inner side of torus were observed. Plasma duration time was limited by the gradual or sudden density increase. The gradual density increase was due to outgassing from the graphite divertor plates or antenna side protectors with hot spots. The spark resulted in sudden density increase. It is thought that the localized divertor plate temperature increase and sparks are attributed to the high-energy ions accelerated in front of ICRF antennas. By suppressing these phenomena the world largest input energy was achieved. [Copyright &y& Elsevier]

Abstract: For steady-state operation with ion cyclotron range of frequencies (ICRF) heating, real-time impedance matching is necessary in order to inject ICRF power into the plasma against variations in the antenna impedance. Two methods of the real-time impedance matching were employed during a long pulse discharge in the large helical device (LHD). One was the manual-frequency control method and the other was the automatic control of liquid heights in liquid stub tuners using trial-and-error method. Both of these methods entailed disadvantages; a limitation of bandwidth in impedance matching by a frequency control, and in trial-and-error method an abrupt increase of the reflected power fraction and a slow reduction rate of the reflected power fraction. A new real-time impedance matching method solving these problems was developed utilizing liquid stub tuners. The impedance of the resistor attached to the outlet of the stub tuners was measured using a directional coupler attached to the coaxial line near the outlet of the final power amplifier. Optimum liquid heights were predicted and liquid heights were adjusted in real-time using pulse motors. The reduction of the reflected power fraction was demonstrated and was kept low against variations in the impedance of the resistor. [Copyright &y& Elsevier]

Abstract: In the large helical device (LHD), the plasma duration time was extended up to 150s by ion cyclotron range of frequencies (ICRF) heating. Time-integrated total input power reached 71MJ. However, this discharge terminated due to radiation collapse accompanied by an increase of electron density. The temperature of the divertor plates and the intensity of Hα were locally increased in the same toroidal section, near the ICRF antenna. One of the possible causes of the increase of radiation power is an outgassing from the divertor plates that were heated by particles accelerated in the cyclotron resonance layer near the antenna. Another possible cause is the outgassing from the ICRF antenna itself due to a temperature increase of the ICRF antenna owing to high-energy particles, or the formation of an RF (radio frequency) sheath. [Copyright &y& Elsevier]

Abstract: A three-dimensional neutral particle transport simulation code is applied to identify a source of the hydrogen outgas in a LHD long pulse discharge by ICRF heating. Radiation collapse induced by uncontrollable plasma density rise terminated the long pulse discharge. Toroidally non-uniform increases in the Hα emission and divertor temperature were observed. A CCD camera detected local heating of a vertically installed divertor plate, which is consistent with the strike points of the trajectories of protons accelerated by ICRF waves. The neutral particle transport simulation with the proton trajectory analyses shows that outgas from the divertor plates heated by the protons is necessary for explaining the observed toroidal non-uniform distribution of the Hα emission in the long pulse discharge. [Copyright &y& Elsevier]

Abstract: In the large helical device (LHD), a steady-state operation (SSO) is carried out using ion cyclotron range of frequencies (ICRF) heating. Feedback control systems aiming for stable RF power injection, constant plasma density and real-time antenna impedance matching (RTIM) are required for long-duration discharges. When feedback control systems are employed, injected RF power and plasma density are successfully kept constant. Furthermore, magnetic axis sweeping is important for reducing the temperature of hot spots on ICRF antenna protectors and graphite divertor plates; the produced heat flux around the ICRF antennas is locally concentrated on these. By moving the magnetic axis outward, the hot spot temperatures on the divertor plates are decreased, and the plasma duration is extended to more than 300s. [Copyright &y& Elsevier]

Abstract: In order to choose the best conditioning methods to remove hydrogen or tritium from the wall, the ion cyclotron conditioning (ICC) has been tested in fusion devices. The ICC is operated under strong magnetic fields of a few tesla and it is one of the important potential methods for future devices with superconducting coils, such as ITER. The successful initiations of ICC in TEXTOR, Tore Supra, HT-7, AUG, W7-AS and JET using standard ICRF antennas demonstrate the feasibility of this alternative technique. In the large helical device (LHD), which is a current-less steady-state helical device with all superconducting coils, a new conditioning method, the ICC has been started in 2005. An initial experiment of long-term ICC during a strong magnetic field was successfully carried out in LHD. The removal rate depends on the wide energy distributions of high-energy particles accelerated by ICRF power, and controlling this distribution is also important. [Copyright &y& Elsevier]