ヨウリョクタイガタ フェレドキシン ノ コウゾウ カイセキ 2.8Å ブンカイノウ

Remarkable progress in the physical parameters of net-current free plasmas has been made in the Large Helical Device (LHD) since the last Fusion Energy Conference in Chengdu, 2006 (O.Motojima et al., Nucl. Fusion 47 (2007) S668). The beta value reached 5 % and a high beta state beyond 4.5% from the diamagnetic measurement has been maintained for longer than 100 times the energy confinement time. The density and temperature regimes also have been extended. The central density has exceeded 1.0 x 10^21 m^-3 due to the formation of an Internal Diffusion Barrier (IDB). The ion temperature has reached 6.8 keV at the density of 2 x 10^19m^-3, which is associated with the suppression of ion heat conduction loss. Although these parameters have been obtained in separated discharges, each fusion-reactor relevant parameter has elucidated the potential of net-current free heliotron plasmas. Diversified studies in recent LHD experiments are reviewed in this paper

Recent Advance in LHD Experiment

In the first four years of LHD experiment, several encouraging results have emerged, the, most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that plasma with an average beta of 3 % is stable in this configuration even though the theoretical stability conditions of Mercier modes and pressure driven low n modes are violated. In the low density discharges heated by NBI and ECR heatings, ITB(internal transport barrier) and an associated high central temperature (> 10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory The width of the externally imposed island (n/m=1/1) is found to decrease when the plasma is collisionless with finite beta and it increases when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail ( up to 500keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2minute) discharge, with an ICRF power of 0.4 MW has been demonstrated and energy confinement characteristics are almost the same as those in short pulse discharges

ICRF Heating and High Energy Particle Production in the Large Helical Device

"Significant progress has been made with lon-Cyclotron Range-of-Frequencies (lCRF) 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 fundamental cyclotron frequency. This fundamental heating mode extended the plasma-density range of effective ICRF heating to a value of 1 x 10^20 m^-3. This was the first successful result of this geating 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 confimement 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 wal1. By reducing the impurity problem, it was possible to sustain the plasma for more than two minutes using ICRF alone.

Impact of heat deposition profile on global confinement of NBI heated plasmas in the LHD

Energy confinement and heat transport of net-current-free NBI heated plasmas in the large helical device (LHD) are discussed with emphasis on density and power deposition profile dependences. Although the apparent density dependence of the energy confinement time has been demonstrated in a wide parameter range in LHD, the loss of this dependence has been observed in the high density regime under specific conditions. Broad heat deposition due to off-axis alignment and shallow penetration of neutral beams degrades the global energy confinement while the local heat transport maintains a clear temperature dependence, lying between Bohm and gyro-Bohm characteristics. The central heat deposition tends towards an intrinsic density dependence like tau(E) alpha ((n) over bar (e)/p)(0.6) from the state where density dependence is lost. The broadening of the temperature profile due to the broad heat deposition profile contrasts with the invariant property that has been observed widely as profile resilience or stiffness in tokamak experiments. The confinement improvement as a result of the inward shift of the magnetic axis is obvious in the core region, which emphasizes the improvement of transport because of the geometry being unfavourable for the central heating of NBI in this configuration. The edge pressure, clearly, does not depend on the magnetic axis position. Unlike a tokamak H-mode, the edge pressure is determined by transport and can be increased by increasing the heating power

MHD instabilities and their effects on plasma confinement in the large helical device plasmas

MHD stability of NBI heated plasmas and impacts of MHD modes on plasma confinement are intensively studied in the Large Helical Device (LHD). Three characteristic MHD instabilities were observed, that is, (1) pressure driven modes excited in the plasma edge, (2) pressure driven mode in the plasma core, and (3) Alfvén eigenmodes (AEs) driven by energetic ions. MHD mode excited in the edge region accompanies multiple satellites, and is called Edge Harmonic Modes (EHMs). EHM sometimes has a bursting character. The bursting EHM transiently decreases the stored energy by about 15 percents. In the plasma core region, m=2/n=1 pressure driven mode is typically destabilized. The mode often induces internal collapse in the higher beta regime more than 1 percent. The internal collapse appreciably affects the global confinement. Energetic ion driven AEs are often detected in NBI-heated LHD plasmas. Particular AE with the frequency 8-10 times larger than TAE-frequency was detected in high beta plasmas more than 2 percent. The AE may be related to helicity-induced AE. Excitation of these three types of MHD instabilities and their impacts on plasma confinement are discussed