Y. Takemura

Response of MHD stability to resonant magnetic perturbation in the Large Helical Device

Penetration of m/n = 1/1 resonant magnetic perturbation (RMP) in different magnetic configurations was investigated in the Large Helical Device (LHD). In the experiments with constant plasma parameters and heating condition, it was found that the mode penetration threshold increased linearly with increase in the magnetic shear. Also the threshold of penetration was increased by mitigating the magnetic hill. The amplitude of the perturbation field after the penetration was larger than that given by the RMP field. When the magnetic shear was further reduced by the plasma current and the plasma entered the ideal unstable regime, m/n = 1/1 minor collapse occurred after the mode rotation was decelerated and stopped. The occurrence of the collapse was independent of the existence of the error field.

Modification of the magnetic field structure of high-beta plasmas with a perturbation field in the Large Helical Device

The effect of resonant magnetic perturbation (RMP) on MHD characteristics is investigated in high-beta plasmas of the Large Helical Device. The ramp-up and static m/n = 1/1 RMP field are applied in medium- (~2%) and high- (~4%) beta plasmas in order to find beta dependences of mode penetration, MHD activities and confinement. The results show that the threshold of mode penetration linearly increases with the beta value and/or plasma collisionality. The threshold of mode penetration in the RMP ramp-up experiments is roughly consistent with the static RMP case. The beta value gradually decreases with the RMP field strength before mode penetration, which is caused by a reduction in the pressure inside the ι/2π = 1 resonance. The width of the magnetic island after the penetration becomes larger than the given RMP field, and it is further enhanced by the increment of the beta value.

Effect of pressure-driven MHD instabilities on confinement in reactor-relevant high-beta helical plasmas a)

Through the experiment data analysis in the large helical device (LHD), the influence of the global MHD instability and the relatively short wave length MHD instabilities driven turbulence on the confinement performance in reactor-relevant high-beta helical plasmas is studied. The comparison of the energy confinement time between just before global MHD instability disappears and after that, and the estimation of the saturated mode structure by the multi-channel soft x-ray measurement enable us to quantitatively estimate the influence of the global interchange type MHD instability with different saturated mode structures on the confinement performance. According to the comparison between thermal conductivities in experiments and those predicted by theoretical transport models, the transport properties in the peripheral region of high beta LHD plasmas are quite similar with anomalous transport model based on an interchange type MHD instability driven turbulence, and that result is supported by the dependenc...

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.

Behavior of MHD instabilities of the Large Helical Device near the effective plasma boundary in the magnetic stochastic region

In the high beta experiments of the Large Helical Device (LHD), the plasma tends to expand from the last closed flux surface ( LCFS ) determined by the vacuum magnetic field. The pressure / temperature gradient in the external region is finite. The scale length of the pressure profile does not change so much even when the mean free path of electrons exceeds the connection length of the magnetic field line to the wall. There appear MHD instabilities with amplitude of 10 -4 of the toroidal magnetic field. From the mode number of the activities (m/n = 2/3, 1/2, 2/4), the location of the corresponding rational surface is outside the vacuum LCFS. The location of the mode is consistent with the fluctuation measurement, e.g., soft X-ray detector arrays. The MHD mode localized in the magnetic stochastic region is affected by the magnetic field structure estimated by the connection length to the wall using 3D equilibrium calculation.

Simultaneous excitation of the snake-like oscillations and the m/n = 1/1 resistive interchange modes around the iota = 1 rational surface just after hydrogen pellet injections in LHD plasmas

Two types of oscillation phenomena are found just after hydrogen ice pellet injections in the Large Helical Device (LHD). Oscillation phenomena appear when the deposition profile of a hydrogen ice pellet is localized around the rotational transform ι = 1 rational surface. At first, damping oscillations (type-I) appear only in the soft X-ray (SX) emission. They are followed by the second type of oscillations (type-II) where the magnetic fluctuations and density fluctuations synchronized to the SX fluctuations are observed. Both oscillations have poloidal/toroidal mode number, m/n = 1/1. Since the type-II oscillations appear when the local pressure is large and/or the local magnetic Reynolds number is small, it is reasonable that type-II oscillations are caused by the resistive interchange modes. Because both types of oscillations appear simultaneously at slightly different locations and with slightly different frequencies, it is certain that type-I oscillations are different from type-II oscillations, whi...

Magnetohydrodynamic stability at the edge region in H-mode plasmas with long edge-localized-mode-free phases in the large helical device

Clear suppression of magnetic fluctuations associated with resistive interchange modes (RICs) is observed during long edge-localized-mode (ELM)-free phases of the H-mode plasma in an outward-shifted configuration of the Large Helical Decice, in which a steep pressure gradient is generated at the plasma edge in the magnetic hill. The ELM-free H-phase is interrupted by large amplitude ELMs which are thought to be induced through nonlinear evolution of the RICs having m = 1/n = 1 dominant component (m: poloidal mode number, n: toroidal one). The m = 1/n = 1 RIC amplitude is enhanced about 10 times compared with the H-phase level during each ELM. In most of the H-mode shots, the final ELM-free phase returns to L-phase by a large amplitude ELM. In the L-phase, the RIC amplitude is enhanced by a factor of ~3 compared with that in the H-phase, although the edge pressure gradient is reduced considerably. Linear resistive magnetohydrodynamic stability analysis is attempted using experimentally obtained equilibrium profiles. From the numerical analysis, the distance between the location of the steepest pressure gradient and the main mode resonance surface, i.e. the rotational transform ι = 1, is found to be important for a large growth of the m = 1/n = 1 RIC in the H-phase.

Characteristics of Low-n Instability Observed in Ideal Unstable Regime of Helical Plasmas

Impact of “ideal” interchange instability to plasma confinement has been investigated in various magnetic configurations of Large Helical Device (LHD) for a design of helical fusion reactor. MHD theory suggests that growth rate of resistive interchange mode is proportional to S -1/3 , which is qualitatively consistent with experimental observation [1], where S is magnetic Reynolds number. It means that the suppression of low-n mode can be expected in the reactor with high S if the ideal mode is stable. Here we report characteristics of instabilities observed in ideal unstable regime where the growth rate is almost independent of S. In the experiments, we brought the plasma to ideal unstable regime by reduction of magnetic shear due to beam-driven toroidal current and/or control of magnetic configurations. In the beginning of a discharge, the m/n = 1/1 mode appeared with a finite rotation. When the plasma current exceeded a threshold, the amplitude of the mode started to increase and the mode rotation started to slow down. After the stop of the rotation, the mode abruptly grew and led to minor collapse [2] and the averaged beta decreased by about 50 % then. Experiments also show that the minor collapse occurred even if an error field is reduced, which suggests that the deceleration of the mode rotation be due to reduction of equilibrium flows by formation of the magnetic island [3]. The regime where the minor collapse occurred has been already identified through the various operations, and the collapse is concentrated in the ideal unstable regime with the weak magnetic shear [4]. The mode has a strongest impact to plasma confinement in all operations of LHD.