Takashi Mutoh

Recent Heliotron E physics study activities and engineering developments

Recent studies of transport, magnetohydrodynamic stability, and divertor action on Heliotron E are summarized. A pellet injector and a new diagnostic system are developed. Moreover, the Heliotron groups is conducting research and development on heating and other new systems for the Large Helical Device.

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.

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.

Steady-State Amplifier at Megawatt Level for LHD ICRF Heating

Abstract A high-power, wide-band, steady-state amplifier was developed as a part of research and development for ion cyclotron range of frequency (ICRF) heating in the Large Helical Device at the National Institute for Fusion Science. A double coaxial cavity was adopted to cover the wide frequency range of 25 to 100 MHz. An analysis of this cavity is compared with results of static tests, and good agreement is shown. In a high-power test, long-pulse operation of 5000 s at an output power of 1.6 MW, which is a world record for steady-state operation of an ICRF amplifier, has been achieved as a low-impedance-mode operation is adopted. Cooling of various elements of the amplifier is important in the steady-state operation. This paper reports how the steady-state operation is obtained through cooling. An analysis of heat removal in response to the temperature rise of a coaxial cable is also reported.

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.

Development of 28GHz and 77GHz 1MW Gyrotron for ECRH of Magnetically Confined Plasma

Abstract We are developing a new 28GHz 1MW and a 77GHz 1MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. The detail design study of 28GHz 1MW gyrotron such as cavity, magnetron injection gun (MIG) has been done. We obtained the oscillation power of 1.37MW and the oscillation efficiency of 42.7% with the pitch factor of 1.2. Two 77GHz 1MW gyrotrons have been fabricated and tested. The maximum output power of 1.1MW was obtained. The pulse width with 0.46MW extended to 5s with the short aging time of only 65 hours. A plasma injection for LHD with MOU output of 0.81MW 3.6s was performed.

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.

Feedback control of ECRH polarization on LHD

The polarization of electron cyclotron resonance heating (ECRH) waves, set by the orientation of a pair of corrugated mirror polarizers in the transmission line, determines the degree of coupling to O- and X-modes in the plasma and has an important effect on the first-pass absorption. Existing methods for determining the required polarization have been found adequate in most experiments. However, as the pulse length is increased it becomes increasingly important to maximize the first-pass absorption while the plasma or injection conditions change or when there can be significant O- to X-mode power coupling during propagation, particularly in the edge plasma region of a stellarator. This has motivated the development of a dedicated feedback control system which is able to adjust the polarizers angles settings during the discharge in order to maintain the highest possible absorption. An extremum seeking controller is shown to successfully recover the optimum polarization setting during long-pulse ECRH experiments on the Large Helical Device (LHD).Corrections were made to this article on 02 September 2010. The was removed before LHD in several places.

Steady-State Tests of High-Voltage Ceramic Feedthroughs and Coaxial Transmission Line for ICRF Heating System of the Large Helical Device

Steady-state ion cyclotron range offrequency (ICRF) heating technologies have been developed to heat plasma for >30 min in the Large Helical Device (LHD Steady-state-operation tests of high voltages up to 40 kV 0p for >30 min were carried out on radio-frequency (rf) vacuum feedthroughs and a coaxial transmission line in a test set. Four types of ceramic feedthroughs, each having a 240-mm diameter, were tested. Cone-type alumina ceramic and cylinder-type silicon nitride composite-ceramic feedthroughs produced good performances of 40 kV/30 min and 50 kV/10 s. The others had vacuum leaks when subjected to long-pulse duration. The temperature of the cone-type alumina ceramic feedthrough was measured during the ICRF operations. By using gas-cooling techniques, the temperature increase of the ceramic was substantially reduced and saturated within 20 min. Without any gas-cooling techniques, the temperature increased linearly and did not saturate. Therefore, this approach could not be used for steady-state operation. The rf dissipation on the ceramic was calculated using the ANSYS finite element computer code. It was found that damaged feedthroughs had local high heat spots, which could result in vacuum leaks. A 240-mm-diam water-cooled coaxial transmission line was designed and tested for steady-state operation. Specially designed connector components and Teflon insulator disks were tested. During the test operation, the insulation gases of nitrogen, sulfur hexafluoride, and carbon dioxide were used to compare their insulation capabilities for steady state. For the duration of a 10-s pulse, these gases performed well up to 60 kV 0p . However, for steady-state operation, carbon dioxide gas could not withstand voltages >40 kV 0p . The connector components of the transmission line performed without problems below 50 kV 0p and I kA 0p for 30-min steady-state operation. The performance of the feedthroughs and transmission line exceeded the specifications for steady-state heating in the LHD.

Two Pulse and Multiple Position Thomson Scattering System

A Thomson scattering system for simultaneous measurements of the electron temperature and density at ten different positions at two times during a single plasma shot has been developed for Heliotron E. The apparatus consists of two ruby lasers, each delivering two pulses whose temporal separation is typically 100 ms; light collection optics including automatic positioning mechanisms; 80 detectors; and a data processing system. We have obtained spatial profiles of Te (0.05–1.3 keV) and ne (0.2-9.0×1019 m-3) in Heliotron E for each plasma shot.