Y. Mizuno

ECRH-Related Technologies for High-Power and Steady-State Operation in LHD

Abstract The electron cyclotron resonance heating (ECRH) system on the Large Helical Device (LHD) has been in stable operation for ~11 yr in numerous plasma experiments. During this time, many upgrades to the system have been made, such as reinforcement of the gyrotron tubes, modification of the power supply depending on gyrotron type, and increase in the number of transmission lines and antennas. These efforts allow the stable injection of millimeter-wave power in excess of 2 MW. In parallel, various transmission components were evaluated, and antenna performance was confirmed at a high power level. The coupling efficiency of the millimeter wave from the gyrotron to the transmission line and the transmission efficiency through the waveguide were further improved in recent years. The feedback control of the wave polarization has also been tried to maximize the efficiency of wave absorption. The gyrotron oscillation frequency was reconsidered in order to extend the flexibility of the magnetic configuration in plasma experiments. The development of 77-GHz gyrotrons with the output of 1 MW per few seconds in a single tube is currently taking place in collaboration with the University of Tsukuba. Two such gyrotron tubes already have been installed and were used for plasma experiments recently. An ECRH system with a capability of the steady operation is required, because the LHD can continuously generate confinement magnetic fields using superconducting magnets. Not only the gyrotron but also the transmission system and components must withstand continuous power operation. Further acceleration of both the power reinforcement and a steady-state capability will allow the sustainment of high-performance plasmas.

Electron cyclotron heating scenario and experimental results in LHD

A large helical device (LHD) experiment began at the end of March 1998. Fundamental and second harmonic electron cyclotron heating (ECH) are used as a plasma production and heating method with six gyrotrons whose frequencies are 82.6/84 and 168 GHz, respectively. Up to 0.9 MW power has been injected in LHD with long distance corrugated waveguide transmission systems. The maximum pulse width is achieved to 3.0 s/240 kW for the LHD experiments. Six antenna systems have been prepared at the horizontally and vertically elongated poloidal sections. The maximum stored energy using all six gyrotrons is 70 kJ at the averaged density of n e = 4 × 10 18 m -3 . The maximum central electron temperature T e0 = 3.5 keV is achieved at n e = 3 × 10 18 m -3 . The magnetic field structure in heliotron type devices like LHD, notably near the coil, is complicated. For this oblique injection, a wave is launched from the antenna, and then crosses the plasma in the complex field structure near the coil. The polarization ellipse of the wave is changed along the ray-path. The wave propagation in heliotron type devices has been analyzed in an ideal case that the magnetic field component along the propagation direction can be neglected. Even for perpendicular injection with our antenna systems, the field component along the propagation direction is not so small. Another treatment of the wave-propagation is introduced. Some calculations for the heating scenario with this treatment are shown.

The Development of a 77-GHz, 1-MW ECRH System for the Large Helical Device

Abstract A 77-GHz, 1-MW gyrotron is being newly installed in the Large Helical Device not only to enhance the total heating power but also to increase the possibility of controlling the local plasma parameters. Our progress in installing the new gyrotron and evaluating its properties is discussed. We have already finished the installation of the peripheral components, including the transmission line, and conducted a test at 1 MW for a short pulse. Our plan is to operate this gyrotron at a power of up to 1 MW for 5 s. The conditioning of the gyrotron has been smoothly conducted, and a gyrotron output power up to 810 kW for 3.6 s has been achieved so far. The total injected power of electron cyclotron resonance heating to the plasma reached a value of [approximately]2.5 MW.

Ion cyclotron range of frequency heating experiments on the large helical device and high energy ion behavior

Ion cyclotron range of frequency (ICRF) heating experiments on the Large Helical Device (LHD) [O. Motojima et al. Fus. Eng. Des. 20, 3 (1993)] achieved significant advances during the third experimental campaign carried out in 1999. They showed significant results in two heating modes; these are modes of the ICH-sustained plasma with large plasma stored energy and the neutral beam injection (NBI) plasma under additional heating. A long-pulse operation of more than 1 minute was achieved at a level of 1 MW. The characteristics of the ICRF heated plasma are the same as those of the NBI heated plasma. The energy confinement time is longer than that of International Stellarator Scaling 95. Three keys to successful ICRF heating are as follows: (1) an increase in the magnetic field strength, (2) the employment of an inward shift of the magnetic axis, (3) the installation of actively cooled graphite plates along the divertor legs. Highly energetic protons accelerated by the ICRF electric field were experimentally...

Real-time feedback control of millimeter-wave polarization for LHD

Electron cyclotron heating (ECH) is widely used in magnetic fusion devices, and the polarization of the injected millimeter-wave beams plays a crucial role in the propagation and absorption of the beam energy by the plasma. This polarization can be adjusted by grating mirror polarizers placed in the transmission lines which carry the microwaves from the power source to the plasma. In long-pulse devices such as the Large Helical Device (LHD) and ITER, it is desirable to track changes in the plasma and adjust the polarization of the ECH in real time such as to keep the absorption as high as possible and avoid shine-through which may lead to overheating of vessel components. For this purpose a real-time feedback control scheme is envisioned in which a measure of the absorption efficiency can be used to adjust the orientation of the polarizing mirrors toward an optimum. Such a setup has been tested in a low-power test stand as preparation for future implementation in the LHD ECH system. It is shown that a simple search algorithm is efficient and can in principle be used to control either the absorption efficiency or the linear polarization angle.

ECH system and its application to long pulse discharge in large helical device

Abstract We have developed and constructed an ECH system for the large helical device (LHD). The ECH system consists of 0.5 MW, 84 GHz range and 168 GHz gyrotrons, high voltage power supplies, long distance transmission lines, and in-vessel quasi-optical antennas. It has been improved step by step. At the third campaign of LHD experiments, three 84 GHz range (two 82.6 GHz and one 84 GHz) and three 168 GHz gyrotrons are operated and ECH power can be injected from four antennas vertically and two horizontally. This complicated system is remotely controlled and monitored by fully GUI (Graphical User Interface) control panels realized on PC via TCP (transmission control protocol) communication. Over 10 000 shots of gyrotron power have been injected steadily into the LHD during the experimental campaigns on this system. One line of the system (84 GHz line) is specially prepared for the experiments of steady state plasma production. Using this line, plasma sustainment for 2 min was successfully achieved by only ECH power. Injected ECH power was 50 kW with 95% duty factor. The electron density and temperature of the sustained plasma are measured to be 0.3–0.5×1018 m−3 and ∼650 eV. Ion temperature measured by Doppler broadening of the impurity radiation line was kept constant at ∼300 eV during RF injection.

Real time polarization monitor developed for high power electron cyclotron resonance heating and current drive experiments in large helical device

The polarization state of a wave is an important factor in electron cyclotron resonance heating (ECRH) and current drive (ECCD), for it strongly affects the propagation and absorption of the wave in the plasma. A real-time monitor of the polarization of the EC beam has been developed for use in ECRH/ECCD experiments in large helical device (LHD). Two orthogonal components of the wave field are measured in one of the miter-bends by use of a specially designed coupler and a waveguide circuit with a 0°–90° phase switch to deduce the polarization parameters: the polarization angle α and the ellipticity β. Since fast-response pin diodes are used for the switches, the polarization is determined every 3 ms, facilitating real time acquisition of the polarization. This article reports on the design and the principle of this monitor as well as on the algorithm used to calculate α and β. This article also reports on the method of calibration, for the accuracy of this measurement depends on it. Finally, a comparison ...

Design of polarizers for a mega-watt long-pulse millimeter-wave transmission line on the large helical device.

The polarizer is one of the critical components in a high-power millimeter-wave transmission line. It requires full and highly efficient coverage of any polarization states, high-power tolerance, and low-loss feature. Polarizers with rounded shape at the edge of the periodic groove surface are designed and fabricated by the machining process for a mega-watt long-pulse millimeter-wave transmission line of the electron cyclotron resonance heating system in the large helical device. The groove shape of λ/8- and λ/4-type polarizers for an 82.7 GHz transmission line is optimally designed in an integral method developed in the vector theories of diffraction gratings so that the efficiency to realize any polarization state can be maximized. The dependence of the polarization states on the combination of the two polarizer rotation angles (Φλ/8, Φλ/4) is examined experimentally in a low-power test with the newly developed polarization monitor. The results show that the measured polarization characteristics are in good agreement with the calculated ones.

Handling TecHnology of Mega-WaTT MilliMeTer-Waves for opTiMized HeaTing of fusion plasMas

Millimeter-wave components were re-examined for high power (Mega-Watt) and steady-state (greater than one hour) operation. Some millimeter-wave components, including waveguide joints, vacuum pumping sections, power monitors, sliding waveguides, and injection windows, have been improved for high power CW (Continuous Waves) transmission. To improve transmission efficiency, information about the wave phase and mode content of high power millimeter-waves propagating in corrugated waveguides, which are difficult to measure directly, were obtained by a newly developed method based on retrieved phase information. To optimize the plasma heating efficiency, a proof-of-principle study of the injection polarization feedback control was performed in the low power test stand.

Progress on Electron Cyclotron Heating and Electron Cyclotron Current Drive Experiments in LHD

Abstract The electron cyclotron resonance heating (ECH) system in the Large Helical Device consists of nine gyrotrons: two that are 82.7 GHz, 0.45 MW, and 2 s; two that are 84 GHz, 0.8 MW, and 3 s; one that is 84 GHz, 0.2 MW, and 1000 s; and four that are 168 GHz, 0.5 MW, and 1 s. ECH and electron cyclotron current drive (ECCD) experiments using this system have been conducted not only for plasma heating and current drive experiments but also for transport and power deposition studies with power modulation. The configuration of the recent ECH system including gyrotrons, high-voltage power supplies, and the transmission system is overviewed. The outstanding progress on the ECH/ECCD experimental results is described in detail, which includes an electron transport study in the plasma with an electron internal transport barrier, electron Bernstein wave heating through the mode conversion process, preliminary current drive experiments, and a steady-state plasma sustainment >1 h by only ECH.