Experimental Results for Electron Bernstein Wave Heating in the Large Helical Device

Electron cyclotron heating (ECH) using electron Bernstein waves (EBWs) was studied in the large helical device (LHD). Oblique launching of the slow extraordinary (SX-) mode from the high field side and oblique launching of the ordinary (O-) mode from the low field side were adopted to excite EBWs in the LHD by using electron cyclotron (EC) wave antennas installed apart from the plasma surface. Increases in the stored energy and electron temperature were observed for both cases of launching. These launching methods for ECH using EBWs (EBWH) is promising for high-density operation in future helical fusion devices instead of conventional ECH by normal electromagnetic modes

Characterization of Ion Cyclotron Wall Conditioning Using Material Probes in LHD

The ion cyclotron wall conditioning (ICWC) is one of the conditioning methods to reduce impurities and to remove tritium from the plasma facing components. Among the advantages of ICWC are the possible operation under strong magnetic field for fully torus area based on the charge exchange damage observed in thin SS samples arranged on a hexahxedron block holder with three different facings, the areas influenced by ICWC is estimated. On the plasma facing area of the material holder, high density of helium bubbles is observed by transmission electron microscope (TEM). But the other areas show no observable damage. The fact that the bubble were observed only in a sample facing the plasma implies that the effective particles, most probably charge exchange neutrals come to the wall straightly Thus, cleaning of the surfaces un-exposed to plasma directly and those in shadow area is difficult by ICWC

High Energy Particle Measurements during Long Discharge in LHD

The spatial resolved energy spectra can be observed during a long discharge of NBI plasma bycontinuously scanning the neutral particle analyzer. In these discharges, the plasmas are initiated by the ECH heating, after that NBI#2 (Co-injection) sustains the plasma during 40-60 seconds. The scanned pitch angle is from 44 degrees to 74 degrees. The injected neutral beam (hydrogen) energy of NBI#2 is only 130 keV because the original ion source polarity is negative. The shape of spectra is almost similar from 44 degrees to 53 degrees. However the spectra from 55 degrees are strongly varied. It reflects the injection pitch angle of the beam according to the simulation (53 degrees ot R* = 3.75 m in simulation). The beam keeps the pitch angle at incidence until the beam energy becomes to the energy, which the pitch angle scattering is occurred by the energy loss due to the electron collision. The low flux region can be observed around 10-15 keV, which is 15 times of the electron temperature. The energy region may be equal to the energy at which the pitch angle scattering is occurred. At the energy, the particle is scattered by the collision with the plasma ions and some of particles may run away from the plasma because they have a possibility to enter the loss cone. According to the simulation, the loss cone can be expected at the 10 keV with the small angular dependence. The depth of the loss cone is deep at the small pitch angle. The hollow in the spectrum may be concluded to be the loss cone as the tendency is almost agreed with the experimental result

Impurity emission characteristics of long pulse discharges in Large Helical Device

Line spectra from intrinsic impurity ions have been monitored during the three kinds of long-pulse discharges (ICH, ECH, NBI). Constant emission from the iron impurity shows no preferential accumulation of iron ion during the long-pulse operations. Stable Doppler ion temperature has been also measured from Fe XX, C V and C III spectra

Study of the Antenna Loading Resistance of the LHD ICRF Antenna

Ion cyclotron range of frequencies (ICRF) heating is used to heat plasma in magnetically confined fusion plasma experiments. ICRF heating has been used in the Large Helical Device (LHD) and contributes to high-power steady-state experiments. Antenna loading resistance is important in ICRF heating; a high loading resistance is required for high-power injection. Many elements influence the antenna loading resistance. Here, the dependence of the loading resistance on various parameters is investigated. The loading resistance is very low at lower wave frequencies. High-power injection using such frequencies was difficult in plasma heating experiments in the LHD. The loading resistance increases with the plasma density. The distance between the antenna and the plasma boundary is closely related to the plasma edge density. It is important to keep the antenna away from the plasma and also keep the loading resistance at a certain level in steady-state operation for the various types of plasma. The effect of additional heating and magnetic field strength are also investigated. These results will contribute to the design of new ICRF antennas, the ICRF heating experiment in the LHD, and ICRF heating in future plasma devices

An Analysis of an ICRF Antenna with Controllable Toroidal Wavenumber in LHD

In order to excite fast wave in the ion cyclotron range of frequencies (ICRF) using multiple antennas with the phase difference in the Large Helical Device (LHD), a controllable wavenumber antenna that consists of two single-strap antennas is designed, and the electrical characteristic features of the antenna are estimated by using a three-dimensional electromagnetic commercial code and the simplified antenna model. Controlling the radio-frequency (RF) phase difference between these two single-straps, reverse-phase excitation can be achieved in order to reduce the impurity production. According to this estimate, the RF current profile on the strap surface is strongly concentrated on the both horizontal strap edges, and the electrical strap length at frequency of 85 MHz is longer than the quarter wavelength of 85 MHz (λ85 MHz/4). Excitable wavenumber spectra are different in various RF phases and frequencies. At the low frequencies (< 60 MHz) with in-phase, effective wavenumbers between k = 0 m?1 and k = 10 m?1 can be excited. During the reverse-phase excitation, large (k = 6, 15 m?1) wavenumber spectra with low (k = 0 m?1) wavenumber kept small are obtained

A Method of Phase Control and Impedance Matching of Mutually Coupled ICRF Antennas in LHD

In the Large Helical Device (LHD), the installation of a pair of ion cyclotron range of frequencies (ICRF) antennas from upper and lower ports is planned. These antennas are geometrically symmetrical and located side by side. By changing the current phase on the straps, the wave number parallel to the magnetic field line can be controlled. However, antenna impedances will also be changed and reflected power will increase due to mutual coupling. For efficient power injection and the protection of tetrode tubes, the parameters of impedance matching devices must be controlled together with the current phase. A method was formulated and trials of phase control and impedance matching were successfully conducted with a simplified two-port dummy antenna