[en] Two experiments are described: 1) the first measurements of enhanced RF electric field at the RF plug section and resulting formation of the RF plug potential, 2) the mechanism of the plasma potential formation in the central section. (author)

[en] RFC-XX-M is an MHD stable and axisymmetric mirror-cusp device with radio frequency (rf) plugging. The plasma is produced by ion cyclotron resonant heating (ICRH) with gas puffing at the central mirror. The total ion energy confinement time including axial loss, electron drag and charge exchange is improved by a factor of three by the rf plugging and reaches 1 ms. The energy confinement time due to the axial particle loss is estimated to be 7.5 ms from a detailed analysis of ion energy loss channels. An rf plugging potential (ψ) is measured from the energy analysis of the escaping ions. It is determined that ψ is proportional to the square of an applied rf voltage and the maximum value reaches 250 V. This value agrees with a plugging potential deduced from the Pastukhov formula within a factor of 2. (author)

[en] Ion Bernstein Wave Heating experiment was conducted in JIPP T-IIU tokamak. A relatively high frequency, 130 MHz, was used to reduce impurity influx and IBW power up to 400kW was injected without plasma disruption. It was found that the radial profiles of electron density, electron temperature, and ion temperature are all peaked during the IBWH. It was also found that ion distribution function does not have high energy tail above certain critical energy. These are favorable and useful features in optimizing fusion reactivity in reactor applications. (author)

[en] Wave-heating at the fundamental ion-cyclotron frequency was applied to a hydrogen plasma in the Large Helical Device (LHD) over a range of plasma densities from 0.2-8x10¹⁹ m^-3. Substantial heating was observed for all densities. In the low-density plasma (less than 0.4x10¹⁹ m^-3) ion-cyclotron-wave (shear Alfven wave) heating was effective. For high-density plasmas, a fast-wave should be excited, and in this case also, effective heating was observed with the presence of the NBI beam component. The wave damping mechanism may be attributed to the finite gyro-radius effect on beam ions by the right-handed polarized wave. The experimental results were compared with an analysis using the full-wave code. The heating performance was a little worse than that of the usual two-ion hybrid-heating mode. (author)

[en] This paper describes recent experimental results of ICRH plasma production, RF plugging and ambipolar potential plugging in RFC-XX-M (IPP, Nagoya). Results of ICRF heating and RF stabilization in HIEI (Kyoto) are also described. (author)

[en] An experiment in a new regime of ion Bernstein wave (IBW) heating has been carried out using 130 MHz high power transmitters in the JIPP T-IIU tokamak. The heating regime utilized the IBW branch between the 3rd and 4th harmonics of the hydrogen ion cyclotron frequencies. This harmonic number is the highest among those used in the IBW experiments ever conducted. The net radio-frequency (RF) power injected into the plasma is around 400 kW, limited by the transmitter output power. Core heating of ions and electrons was confirmed in the experiment and density profile peaking was found to feature the IBW heating (IBWH). The peaking of the density profile was also found when IBW was applied to the neutral beam injection heated discharges. An analysis by use of a transport code with these experimental data indicates that the particle confinement should be improved in the plasma core region on the application of IBWH. It is also found that the ion energy distribution function observed during IBWH has less high energy tail than those in conventional ion cyclotron range of frequency heating regimes. The observed IBWH-produced ion energy distribution function is in a reasonable agreement with the calculation based on the quasi-linear RF diffusion / Fokker-Planck model. (author)

[en] A series of experiments using a newly constructed 130 MHz RF system has been conducted on the JIPP T-IIU tokamak since 1989. It is predicted theoretically that the fast wave in this range of frequency weakly interacts with particles. Though weak, two mechanisms of wave absorption were identified in the experiment: electron Landau/transit-time damping and 3rd ion cyclotron harmonic heating. The former has an intimate connection with fast wave current drive and the latter can provide a new regime of plasma heating or a possible method of controling transport of α-particles. It was found that the efficiency of the 3rd cyclotron heating is improved by using it in the combination with NBI and ICRF heating. The heating efficiency obtained is as high as that of conventional heating. The experimental results are also analyzed on the basis of a theory of the global wave which takes into account wave particle interactions. These mechanism of wave particle interactions are competing with each other as they will be under more realistic reactor conditions. (author)

[en] The Lame Helical Device, LHD, is in its final construction phase. It is a 1=2, m=10 Heliotron/Torsatron type helical system with a major radius of 4 m. The compact helical system, CHS, is a 1=2, m=8 helical system of the same type with a major radius of 1m. CHS has been used for supporting experiments to clarify physics issues of helical systems and to examine the key ideas which will be applied to LHD. This paper summarizes the experimental results of those supporting experiments in CHS and how this knowledge is incorporated in the design of RF heating in LHD. ICRF Heating results in CHS by use of loop antennas are described in section II. The results of using a Nagoya type-III coil is described in Section III. Two types of antennas used in the initial phase of the LHD ICRF Heating: a loop antenna designed for steady state heating and a folded wave guide antenna designed for EBW, are described in section IV

[en] Significant progress has been made with Ion-Cyclotron Range-of-Frequencies (ICRF) heating in the Large Helical Device (LHD). This is mainly due to better confinement of the helically trapped particles, and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 MW to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating, and high-density fast-wave heating at the fundament cyclotron frequency. This fundamental heating mode extended the plasma-density range of effective ICRF heating to a value of 1x10²⁰ m^-3. This was the first successful result of this heating mode in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis shift was common to both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations using the power-modulation technique. It confirmed that the confinement of high-energy particles with the inward-shifted configuration was better than that with the normal configuration. The impurity problem was not serious when the plasma boundary was sufficiently far from the chamber wall. By reducing the impurity problem, it was possible to sustain the plasma for more than two minutes using ICRF alone. (author)

[en] ICRF heating experiments with five poloidal half-turn antennas have been carried out in Compact Helical System (CHS). These antennas designed for the inward shifted magnetic configuration (R_ax=92.1cm) were installed in the high field side of helical field. A high power RF pulse is applied to a deuterium plasma with hydrogen minority initiated by ECH or NBI. The plasma performance was mainly affected by the oxygen radiation which was reduced by more than 2 times by boronization. The plasma stored energy reached 2.2 kJ with 590 kW of RF power from 5 antennas and was sustained to the end of the RF pulse(60msec). Two-component ion energy spectrum was observed by NPA. The combined heating with NBI was also successful and achieved the increase in the stored energy of 0.8 kJ with 450 kW of RF power from 5 antennas. (author)