Y. Narushima

Measurement of anisotropic pressure using magnetic measurements in LHD

In low density discharges of a Large Helical Device (LHD), anisotropic pressure is expected because the LHD has powerful tangential neutral beam injection systems. We study the behaviour of the ratio of the observed saddle loop flux to the diamagnetic flux, and the results are compared with the predicted beam pressure anisotropy by a Monte Carlo technique and the steady state Fokker–Planck solution. We show the possibility of the degree of pressure anisotropy being estimated by magnetic measurements in the LHD.

Overview of progress in LHD experiments

Abstract Remarkable progress to access the reactor-relevant regime has been made in a recent experiment in the Large Helical Device. Optimizing the rotational transform, the average beta value of 4.3%, which is the highest record among helical devices, was achieved. The high-performance plasma with a fusion triple product up to ~2.2 × 1019 m−3·keV·s was sustained for >7 s by repetitive hydrogen pellet injection. With regard to steady-state operation, which is one of the key issues to realize a fusion reactor, discharges for >30 min were successfully sustained by ion cyclotron range of frequency heating with the aid of the magnetic axis swing technique to reduce the heat load to the plasma-facing component. In the discharge, the total input energy to the plasma reached 1.3 GJ, which also established a new record.

Recent progress of MHD study in high-beta plasmas of LHD

Abstract This paper describes recent progress in magnetohydrodynamics (MHD) study of high-beta plasmas of the Large Helical Device. Control of the plasma aspect ratio (Ap) in the range of 6.3 to 8.3 was done in order to optimize the configuration for high-beta plasma production and to investigate the MHD characteristics. The experiments brought a maximum average beta of 4.3% at the Ap = 6.6 configuration. MHD activities in the periphery are dominantly observed in such a high-beta region, and their amplitudes increase with decreasing magnetic Reynolds number (S) and have clear dependence on the S parameter. When the plasma aspect ratio is increased, minor collapse due to the m/n = 1/1 mode without rotation occurs. It is enhanced further by the plasma current reducing magnetic shear and degrades the beta value by >50%. The results are expected to give important information on the operation regime and the future design of the helical fusion reactor and to contribute to experimental knowledge of ideal and resistive instability.

Characteristics of confinement and stability in large helical device edge plasmas

Recent progress in the heating capability in the large helical device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] has allowed the highest average β value (4.1%) obtained in the helical devices, and enables exploration of magnetohydrodynamics (MHD) stability in this β region. MHD activities in the periphery are found to become stable spontaneously from the inner region to the outer region when the averaged β value exceeds a threshold, and then a flattening of the electron temperature profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m∕n=1∕1 magnetic island, and the complete stabilization of the m∕n=1∕1 mode is demonstrated by the moderate island width. In addition, attempts to control peripheral plasmas are also performed by using a limiter and a local island divertor utilizing the m∕n=1∕1 island, to improve plasma confinement and, especially, to stabilize pressure-driven modes in the present study. The stabilization of peripheral MHD modes i...

Three-dimensional anisotropic pressure equilibria that model balanced tangential neutral beam injection effects

Fixed boundary three-dimensional anisotropic pressure equilibria are computed that are consistent with a family of distribution functions that load particles preferentially towards v parallel to similar or equal to v and model the effects of balanced tangential neutral beam injection. The total pressure surfaces remain coupled with the magnetic surfaces even when the parallel beam pressure greatly exceeds its perpendicular counterpart. The anisotropy factors and the Shafranov shift, which increases with the anisotropy, are calculated in a currentless two-field period quasiaxisymmetric stellarator device.

Nonlinear stability of magnetic islands in a rotating helical plasma

Coexistence of the forced magnetic reconnection by a resonant magnetic perturbation (RMP) and the curvature-driven tearing mode is investigated in a helical (stellarator) plasma rotated by helical trapped particle-induced neoclassical flows. A set of Rutherford-type equations of rotating magnetic islands and a poloidal flow evolution equation is revisited. Using the model, analytical expressions of criteria of spontaneous shrinkage (self-healing) of magnetic islands and sudden growth of locked magnetic islands (penetration of RMP) are obtained, where nonlinear saturation states of islands show bifurcation structures and hysteresis characteristics. Considering radial profile of poloidal flows across magnetic islands, it is found that the self-healing is driven by neoclassical viscosity even in the absence of micro-turbulence-induced anomalous viscosity. Effects of unfavorable curvature in stellarators are found to modify the critical values. The scalings of criteria are consistent with low-β experiments in...

Influence of resonant magnetic perturbation on a rotating helical plasma

Nonlinear stability of magnetic islands in a helical plasma with resonant magnetic perturbation (RMP) is investigated using reduced magnetohydrodynamic equations including neoclassical viscosity. Coexistence of RMP-driven islands and the resistive interchange mode is numerically simulated. The self-healing of locked magnetic islands by neoclassical flows is observed. It is found that the curvature effect modifies the threshold of the self-healing, where the unfavorable curvature drives not only the interchange mode but also the curvature-driven tearing mode. An analytical model based on a Rutherford equation with a momentum equation is also introduced to understand the simulation results. Criterion of the self-healing considering the curvature effect is newly obtained.

Experimental observation of response to resonant magnetic perturbation and its hysteresis in LHD

The magnetic island in the large helical device (LHD) shows the dynamic behaviour of the healing/growth transition with the hysteretic behaviour. The thresholds of plasma beta and poloidal flow for island healing are larger than that for growth. The threshold of resonant magnetic perturbation (RMP) for healing is smaller than that for growth. Furthermore, thresholds of the amplitude of RMP depend on the magnetic axis position Rax in the LHD. The RMP threshold increases as the magnetic axis position Rax increases. The poloidal viscosity may be considered as a candidate to explain the experimental observation from the viewpoint of the relationship between the electromagnetic torque and the viscous torque.

Characteristics of MHD instabilities limiting the beta value in LHD

Effects of low-n magnetohydrodynamic instabilities on plasma performance have been assessed in the regime where an achieved beta value is limited by instabilities. The unstable regime of an ideal interchange mode is characterized by enhanced magnetic hill and reduced magnetic shear. Experiments have clarified that (i) low-n modes are significantly destabilized in the ideal-unstable configurations and lead to degradation of central beta by at most 60%, and (ii) the degree of their damages strongly depends on the mode rotation velocity. The occurrence of the minor collapse is independent of an existence of an error field.

The effect of net toroidal current on the measurement of diamagnetic beta value in heliotron plasma

We study the effect of the net toroidal plasma current on the diamagnetic measurement for large toroidal current experiments in the Large Helical Device (LHD). In order to treat precisely the effect, we apply a new method that evaluates magnetic fluxes of magnetic measurements directly from plasma currents of a magnetohydrodynamic equilibrium state with net toroidal current. There is a large difference between the value calibrated by the new method and by a conventional method under the cylindrical and large aspect ratio plasma model. We calibrate the diamagnetic beta value using the net toroidal current distribution observed by motional Stark effect spectroscopy in a LHD discharge with a high net toroidal current, 40 kA T−1. We obtain the results that the temporal behaviour of the calibrated diamagnetic beta value is quite the same with the beta value based on the profile measurement by Thomson scattering and far-infrared interferometry.