Y. Takita

Compact Helical System physics and engineering design

This paper reports on the Compact Helical System designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable effort was devoted to reducing error fields from current feeds and crossovers. The final machine parameters are as follows: major radius of 1 m; minor radius of the helical field coil of 0.313 m; plasma aspect ratio A{sub p} = 5; pole number and toroidal period number of the helical field coil of l = 2 and m = 8, respectively; and helical pitch modulation of {alpha}{sup *} = 0.3.

Coupling of tilting Gaussian beam with hybrid mode in the corrugated waveguide

The mode-conversion loss in the matching between the gaussian beam emanated from the gyrotron and the hybrid mode in the circular corrugated waveguide with the diameter of 2a is discussed. By numerical calculation, it is found that the loss considerably increases and optimum waist sizew0 changes when TEM00 mode with the wavelength λ is injected with offset or tilt. By fitting numerical data to the polynomial function, it becomes evident that the scaling formulas of the losses for the off-axis shiftrd and for the tilt angle θ are derived to be 2.3(rd/a)2 – 2.2(rd/a)4 and 3.9(aα/λ)2 – 5.6(aθ/λ)4 for fixedw0/a=0.643, respectively. To keep the mode-conversion loss ≤1% for the frequency of 168 GHz and 2a=88.9 mm, tilting angle and offset should be less than 0.1 degrees and 2.9 mm, respectively.

Shafranov shift in the low aspect ratio heliotron/torsatron Compact Helical System

The MHD equilibrium properties of neutral beam heated plasmas have been experimentally investigated in the Compact Helical System (CHS)-a low aspect ratio (Ap ~ 5) heliotron/torsatron. This configuration is characterized by a strong breaking of helical symmetry. The radial profiles measured by various diagnostics have shown a significant Shafranov shift due to the plasma pressure. The deviation of the magnetic axis from is vacuum position has become as large as 50% of the minor radius. When the three-dimensional equilibrium code VMEC is used to reconstruct the equilibrium from the experimental data, the result is in good agreement with the experimentally observed Shafranov shift as well as with the diamagnetic pressure in plasmas with β ≤ 1.2% and β0 ≤ 3.3%. This beta values corresponds to half of the conventional equilibrium β limit defined by the Shafranov shift reaching a value of half of the minor radius. Although tangential neutral beam injection causes pressure anisotropies, p||/p⊥ ≤ 3, the description of the equilibrium assuming isotropic pressure is consistent with the experiment

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.

Plasma confinement studies in LHD

The initial experiments on the Large Helical Device (LHD) have extended confinement studies on currentless plasmas to a large scale (R = 3.9 m, a = 0.6 m). Heating by NBI of 3 MW produced plasmas with a fusion triple product of 8 × 1018m-3keVs at a magnetic field strength of 1.5 T. An electron temperature of 1.5 keV and an ion temperature of 1.1 keV were achieved simultaneously at a line averaged electron density of 1.5 × 1019 m-3. The maximum stored energy reached 0.22 MJ with neither unexpected confinement deterioration nor visible MHD instabilities, which corresponds to β = 0.7%. Energy confinement times reached a maximum of 0.17 s. A favourable dependence of energy confinement time on density remains in the present power density (~40 kW/m3) and electron density (3 × 1019 m-3) regimes, unlike the L mode in tokamaks. Although power degradation and significant density dependence are similar to the conditions on existing medium sized helical devices, the absolute value is enhanced by up to about 50% from the International Stellarator Scaling 95. Temperatures of both electrons and ions as high as 200 eV were observed at the outermost flux surface, which indicates a qualitative jump in performance compared with that of helical devices to date. Spontaneously generated toroidal currents indicate agreement with the physical picture of neoclassical bootstrap currents. Change of magnetic configuration due to the finite β effect was well described by 3-D MHD equilibrium analysis. A density pump-out phenomenon was observed in hydrogen discharges, which was mitigated in helium discharges with high recycling.

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...

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 ...

H-mode transition in the CHS heliotron/torsatron

Characteristics of the H-mode transition observed in CHS are described. The transition is achieved with modification of the rotational transform profile by inducing a small ohmic heating current. The transition is often initiated by a sawtooth crash caused by a coherent mode with the poloidal mode number m=2 and toroidal one n=1.