H. Kawazome

Formation of electron internal transport barrier and achievement of high ion temperature in Large Helical Device

An internal transport barrier (ITB) was observed in the electron temperature profile in the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] with a centrally focused intense electron cyclotron resonance microwave heating. Inside the ITB the core electron transport was improved, and a high electron temperature, exceeding 10 keV in a low density, was achieved in a collisionless regime. The formation of the electron-ITB is correlated with the neoclassical electron root with a strong radial electric field determined by the neoclassical ambipolar flux. The direction of the tangentially injected beam-driven current has an influence on the electron-ITB formation. For the counter-injected target plasma, a steeper temperature gradient, than that for the co-injected one, was observed. As for the ion temperature, high-power NBI (neutral beam injection) heating of 9 MW has realized a central ion temperature of 5 keV with neon injection. By introducing neon gas, the NBI absorption power was incr...

Overview of confinement and MHD stability in the Large Helical Device

The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.

Plasma performance and impurity behaviour in long pulse discharges on LHD

The superconducting machine LHD has conducted long pulse experiments for four years to achieve long-duration plasmas with high performance. The operational regime was largely extended in discharge duration and plasma density. In this paper, the plasma characteristics, in particular, plasma performance and impurity behaviour in long pulse discharges are described. Confinement studies show that global energy confinement times are comparable to those in short pulse discharges. Long sustainment of high performance plasma, which is equivalent to the previous achievement in other devices, was demonstrated. Long pulse discharges enabled us to investigate impurity behaviour in a long timescale. Intrinsic metallic impurity accumulation was observed in a narrow density window (2–3×1019 m−3) only for hydrogen discharges. Impurity transport study by using active impurity pellet injection shows a long impurity confinement time and an inward convection in the impurity accumulation window, which is consistent with the intrinsic impurity behaviour. The pulsed neon gas injection experiment shows that the neon penetration into the plasma core is caused by the inward convection due to radial electric field. Finally, impurity accumulation control with an externally induced magnetic island at the plasma edge was demonstrated.

Radial electric field and transport near the rational surface and the magnetic island in LHD

The structure of the radial electric field and heat transport at the magnetic island in the large helical device (LHD) are investigated by measuring the radial profile of the poloidal flow with charge exchange spectroscopy and measuring the time evolution of the electron temperature with ECE. A vortex-like plasma flow along the magnetic flux surface inside the magnetic island is observed when the n/m = 1/1 external perturbation field becomes large enough to increase the magnetic island width above a critical range (15–20% of minor radius) in LHD. This convective poloidal flow results in a non-flat space potential inside the magnetic island. The sign of the curvature of the space potential (∂2Φ/∂r2, where Φ is the space potential) depends on the radial electric field at the boundary of the magnetic island. The heat transport inside the magnetic island is studied with a cold pulse propagation technique. The experimental results show the existence of radial electric field shear at the boundary of the magnetic island and a reduction in heat transport at the boundary and inside the magnetic island.

Characteristics of transport in electron internal transport barriers and in the vicinity of rational surfaces in the Large Helical Device

Characteristics of transport in electron internal transport barriers (ITB) and in the vicinity of a rational surface with a magnetic island are studied with transient transport analysis as well as with steady state transport analysis. Associated with the transition of the radial electric field from a small negative value (ion-root) to a large positive value (electron-root), an electron ITB appears in the Large Helical Device [M. Fujiwara et al., Nucl. Fusion 41, 1355 (2001)], when the heating power of the electron cyclotron heating exceeds a power threshold. Transport analysis shows that both the standard electron thermal diffusivity, χe, and the incremental electron thermal diffusivity, χeinc (the derivative of normalized heat flux to temperature gradient, equivalent to heat pulse χe), are reduced significantly (a factor 5–10) in the ITB. The χeinc is much lower than the χe by a factor of 3 just after the transition, while χeinc is comparable to or even higher than χe before the transition, which results...

Asymmetric divertor plasma distribution observed in Heliotron J ECH discharges

Abstract An asymmetric divertor plasma distribution observed in the standard configuration of Heliotron J is reported. The divertor plasma profiles were investigated with two Langmuir probe arrays, which were installed at the geometrically up–down symmetric positions, for three different heating schemes of ECH with 53.2 or 70 GHz microwaves. Although the position of the divertor plasma flux was almost consistent with the footprint position of the divertor field lines, the existence of two types of up–down asymmetry was revealed in the divertor plasma density and floating potential profiles. The first type of asymmetry was mainly observed near the boundary to the ‘private region’. This asymmetry seems to be independent of the heating schemes of the toroidal position of the heating source. As the direction of the confinement field was reversed, the feature of the plasma profile on the top array came to appear on the bottom array, and vice versa. This field-direction dependence indicates that the asymmetric B ×∇B drift motion of charged particles might cause this type of asymmetry. The second type of asymmetry was observed in the region away from the boundary and seemed to depend on the heating schemes.

Experimental study on ion temperature behaviours in ECH, ICRF and NBI H2, He and Ne discharges of the Large Helical Device

Ion heating experiments have been carried out in the large helical device using ECH (82.5, 84.0, 168 GHz, ≤1 MW), ICRF (38.5 MHz, ≤2.7 MW) and NBI (H° beam: 160 keV, ≤9 MW). The central ion temperature has been observed from the Doppler broadening of Ti XXI (2.61 A) and Ar XVII (3.95 A) x-ray lines, which are measured using a newly installed crystal spectrometer with a charge-coupled device. Recently, in ECH discharges, on-axis heating became possible. As a result, a high Te(0) of 6–10 keV and a high ion temperature of 2.2 keV were obtained at ne = 0.6×1013 cm−3. A clear increment of Ti was also observed with the enhancement of the electron–ion energy flow when the ECH pulse was added to the NBI discharge. These results demonstrate the feasibility towards ECH ignition. A clear Ti increment was observed also in ICRF discharges at low density ranges of (0.4–0.6)×1013 cm−3 with appearance of a new operational range of Ti(0) = 2.8 keV > Te(0) = 1.9 keV. In low power ICRF heating (1 MW), the fraction of bulk ion heating is estimated to be 60% of the total ICRF input power, which means Pi>Pe. Higher Ti(0), up to 3.5 keV, was obtained for a combined heating of NBI ( Te(0), whereas the Ti(0) remained at relatively low values of 2 keV in H2 and He NBI discharges due to less Pi. The main reasons for the high Ti achievement in the Ne discharges are: (1) 30% increment of deposition power, (2) increase in Pi/ni (five times, Pi/niPe/ne, Pi<Pe) and (3) increase in τei (three times). The obtained Ti(0) data can be plotted by a smooth function of Pi/ni. This result strongly suggests that the ion temperature increases even in the H2 discharge if the Pi can be raised up.

Experimental study of plasma breakdown by second harmonic electron cyclotron waves in Heliotron J

Second harmonic plasma breakdown using electron cyclotron (EC) waves has been studied experimentally in the helical-heliotron device Heliotron J. The magnetic field and the injected EC beam parameters such as power, polarization and injection angle are scanned. The experimental results show that the plasma starts up around the crossing point between the injected EC beam and the resonance layer. The initial plasma moves as the crossing point is shifted. The breakdown is earliest when the beam crosses the resonant layer around the magnetic axis with the X-mode polarization. These results indicate that the single pass absorption has the dominant role on the breakdown rather than the multi-reflection absorption, although the linear absorption is quite low. The bumpiness component in the magnetic field spectrum is also scanned from negative to positive values with the resonance located on-axis. The breakdown is most delayed when the bumpiness component is zero, suggesting that the confinement of accelerated electrons is important.

Observation of H-mode operation windows for ECH plasmas in heliotron J

Abstract The H-mode transition properties of 70-GHz, 0.4-MW electron cyclotron heating (ECH) plasmas in Heliotron J have been studied with special reference to their magnetic configuration dependences, such as the edge iota dependences. Two edge iota windows for the H-mode transition were observed to be (a) 0.54 < ɩ(a)/2π < 0.56 in separatrix discharge plasmas and (b) 0.62 < ɩ(a)/2π < 0.63 in partial wall-limiter discharge plasmas if a certain threshold line-averaged electron density ([overbar]ne = 1.2-1.6 × 1019 m-3) is achieved, where ɩ(a) is the vacuum edge iota value and a is the plasma minor radius, respectively. A strong dependence of the quality of the H-mode on the edge topology conditions was revealed. The energy confinement time for the separatrix discharge plasmas was found to be enhanced beyond the normal ISS95 scaling in the transient H-mode phase, being 50% longer than that in the “before transition” phase. The window characteristics are discussed on the basis of the calculated geometrical poloidal viscous damping rate coefficient in a collisional plasma, indicating that the behavior of the viscous damping rate coefficient alone could not explain the observed characteristics. The bootstrap current properties of ECH plasmas and the relevant electron cyclotron current drive experimental results are also discussed.

High resolution measurements of the Hα line shape in LHD plasmas

Abstract The shift and fine line shape of H α emissions in large helical device (LHD) plasmas have been analyzed by a high resolution spectroscopic system. The minimum detectable velocity for hydrogen atoms is 10 3 m/s. This is confirmed by a newly developed light source with a magnetic field of 1.13 T. In a long-pulse NBI plasma of LHD, hydrogen atoms move toward the plasma from the divertor with velocity of 10 4 m/s. Fine spectral profile of H α was measured with a linear polarizer and it shows an asymmetric structure. The spectrum consists of cold component shifted to the blue side and tail extended to 656.0 nm. The energy corresponding to the tail is from 8 to 100 eV.