Search

In order to clarify the mass effect and the atomic number effect on the rotation of the interchange MHD mode, the relationship between the mode rotation and the plasma rotation in hydrogen-rich plasmas is compared with that in helium-rich plasmas and deuterium-rich plasmas. In all cases, the frequency of the m/n = 1/1 resistive interchange mode is almost the same as the sum of the E × B rotational frequency and the electron diamagnetic rotational frequency at the ι/2π = 1 resonant surface. This result indicates that the rotation of the resistive interchange mode does not depend on the ion species of which the plasma consists.

A new compact helical system CHS-qa has been designed based on the advanced stellarator concept of the quasi-axisymmetry in the magnetic field configuration. This new concept of quasi-axisymmetry makes it possible to realize the two-dimensional vacuum magnetic field structure (a tokamak-like configuration) without using plasma current. In this system, the neoclassical transport is greatly reduced compared with conventional helical systems because the helical ripples are very small. Good MHD stability for the interchange and ballooning modes are obtained. The possibility of finding the confinement improvement is also discussed from the viewpoint of plasma rotation and maximum-J criterion.

The transport/heating modeling and equilibrium/stability analysis have been carried out for LHD (Large Helical Device) plasmas. A new simulation code TOTAL (TOroidal Transport Analysis Linkage) is developed, which consists of the 3-dimensional equilibrium code VMEC including bootstrap current and 1-dimensional transport code HTRANS including helical-ripple transport determined as well as anomalous transport. This code clarified the favorable effect of bootstrap current on the neoclassical confinement in LHD. The 3-dimensional stability analysis using CAS3D code has been done and clarified the ballooning mode structure peculiar to the LHD high-beta plasmas. The 5-dimensional simulation code has been developed to analyze the NBI or ECH heating power depositions in LHD plasmas, and the particle orbit effects of high-energy particles are clarified. The plasma rotation analysis is also carried out related to the possibility of the electric-field transition and the plasma confinement improvement in LHD.

Magnetic islands excited by resonant magnetic perturbations (RMPs) in helical plasmas are investigated. A Rutherford-type equation is coupled with the time evolution equation of the radial electric field associated with the neoclassical particle diffusion due to helically rippled magnetic fields. Using the model, bifurcation between the excitation and the annihilation of non-rotating magnetic islands are newly observed, depending on the magnitude of RMPs and the anomalous plasma viscosity. It is found that the transition between these states is triggered by the change in the radial electric field profile in the vicinity of magnetic island.

Recent two major topics of Large Helical Device (LHD) towards fusion relevant conditions, high-density operation and high-ion-temperature operation, are reported. Super dense core plasma was obtained by the combination of repetitive hydrogen ice pellet injection and high power neutral beam injection (NBI) heating. A very peaked density profile with the highest central density of 1.1 × 1021 m-3 was produced showing that the particle transport was suppressed very well in the plasma core. The spatial density profile varies as the position of magnetic axis (Rax), and the steepest profile is obtained at Rax = 3.95 m. The highest central ion temperature of 5.6 keV was obtained in hydrogen plasma at electron density of 1.6 × 1019 m-3 by NBI, where a peaked ion-temperature profile with internal ion energy transport barrier was observed. The profile of electron temperature did not change much and was broad even when the ion temperature had a peaked profile. The central ion temperature is higher than the electron temperature, which is a new operation regime of LHD. High central ion temperature accompanied strong toroidal rotation and an extreme hollow profile of carbon ions (impurity hole). These steep temperature profiles were obtained so far at around Rax = 3.6 m. The compatibility between particle and energy confinement is a new issue of LHD to explore a new operation regime for attractive fusion reactor.

Intermittent turbulent potential fluctuations in ion-cyclotron wave heated plasmas and effects of their suppression due to strongly sheared plasma rotation with electron-cyclotron heating (ECH) for potential formation are observed in GAMMA 10. Such a shear effect on the central-cell potential data is demonstrated by the use of a heavy ion beam probe (HIBP) for the first time in GAMMA 10. The HIBP system with a gold neutral beam is employed for measuring profiles of radially formed potential as well as the associated radially sheared Er in the central cell. During the application of ECH, the produced stronger Er shear results in reduction of the turbulent fluctuations. The results are consistent with reported X-ray tomography data on the effect of Er shear on turbulence suppression.