Satoru Sakakibara

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.

3D plasma response to the magnetic field structure in the Large Helical Device

The three-dimensional (3D) plasma response to the magnetic field structure is studied for high-β plasmas in the Large Helical Device (LHD). For the vacuum field in the LHD configuration, the boundary of the plasma is defined by the last closed flux surface. However, for high β, the magnetic field structure is changed by the 3D plasma response and the plasma boundary is shifted in the radial direction. The radial electric field, Er, to identify the plasma boundary, is measured in the peripheral region. The radial electric field shear from negative to positive appears in the region and this suggests the boundary between open and closed field lines. The position of the Er shear is always outside the vacuum boundary. A 3D magnetohydrodynamic modelling predicts the expansion of the stochastic region by the 3D plasma response. However, since the connection length of the stochastic field lines is larger than the electron mean free path, a confinement is expected in the region. In such a case, the edge of the stochastic region can be defined as the effective plasma boundary. The position of the appearance of the strong Er shear is almost comparable to the edge of the stochastic region of the modelling. That is, the 3D plasma response is identified in LHD experiments.

Initial results of local island divertor experiments in the large helical device

Abstract A local island divertor (LID) experiment has begun in the Large Helical Device (LHD) to demonstrate improved plasma confinement, and fundamental LID functions were demonstrated in the sixth experimental campaign in 2002–2003. It was clearly shown that when an m/n = 1/1 island is generated by adding a resonant perturbation field to the LHD magnetic configuration, the particle flow is guided along the island separatrix to the backside of the island, where carbon plates are located on a divertor head. The particles recycled there are pumped out efficiently so that the line-averaged core plasma density is reduced by a factor of ~2 at the same gas puff rate, compared with non-LID discharges. Obvious improvement of the global plasma confinement was, however, not observed yet, because the discharge could not be optimized, due to a large amount of outgas from the divertor head to the core plasma. The size of the divertor head was found to be larger than the optimum one; hence, the core plasma impacted slightly on the core plasma-facing portion of the divertor head with which the core plasma was not expected to collide.

Effect of Plasma Current on Magnetohydrodynamic Modes in Neutral Beam Heated Plasmas in Compact Helical System.

Effects of the net toroidal plasma currents on magnetohydrodynamic (MHD) modes have been investigated in neutral beam heated plasmas with the control of ohmic current. The characteristics of experimental observations are discussed in comparison with the calculated 3-D equilibria. The resonant modes are destabilized by the change of magnetic configuration due to the induced current in the co-direction. The modes with m/n=2/1 and 3/2 abruptly disappear at the specific current, which is consistent with the disappearance of the resonant surfaces. The behavior of these modes is phenomenologically consistent with that of the interchange mode.

Experimental Observation of Magnetic Fluctuations in NBI Heated Plasmas in CHS

The magnetic fluctuations with the frequency f ≤100 kHz in NBI heated plasmas have been investigated in the Compact Helical System (CHS). The observed coherent fluctuations are identified as global modes with m ≤4, n ≤3. The dependences of these modes on the magnetic configuration and the plasma beta are well interpreted phenomenologically by the theoretical models of the resistive and the ideal interchange modes. Both these modes and the incoherent fluctuations increase with the beta value , however, saturate when reaches about 1%. The present accessible limit of =2.0% is determined by the confinement of which a key mechnism is not the instabilities investigated here. The nonlinear saturation mechanism must be clarified, because the change of the fluctuation amplitude can not be explained from the model that the saturation level is proportional to the linear growth rate of an instability.

Progress of High-Beta Experiments in Stellarator/Heliotron

Abstract Recently, dramatic progress has been achieved in the study of helical systems with high-beta experiments. Discharges with more than 3% beta plasmas have been achieved in Large Helical Device (LHD) and Wendelstein 7-AS (W7-AS). Although magnetohydrodynamic (MHD) instabilities affect local pressure gradients, the global transport property does not seem to limit the achieved beta value in either device. We summarize the LHD high-beta properties in MHD stability, equilibrium, and transport, and we show the relationship between the experimentally achieved parameters and theoretical predictions. We contrast the LHD results with the W7-AS high-beta properties. In both devices, stationary discharges in the definitely MHD unstable region have not been observed. We mention the key issue for achievement of the beta values >5%.

Configuration Effect on Energy Confinement and Local Transport in LHD and Contribution to the International Stellarator Database

Abstract New stellarator experiments have been launched since the last compilation of the international stellarator database in 1995. Parameter regimes have been extended by Large Helical Device (LHD), and a variety of improved modes have been found since then. The revision of the international stellarator database has been initiated, driven by these emerging interests and by the requirements for a reactor assessment. Some provisional issues are discussed. An understanding of configurational effects is a prerequisite to the derivation of a unified scaling. Differences in magnetic geometry are influential in characterizing energy confinement. The results from the magnetic axis and elongation scans in LHD are highlighted. Comparison with tokamak confinement is also addressed. The revision of the database is in progress, and this paper is an interim report.

Comparative Study on Effect of Boronization and Titanium Gettering in Compact Helical System Heliotron/Torsatron Device

Boronization with decaborane which is solid at room temperature has been successfully applied to the Compact Helical System (CHS) experiment. The effect of boronization on neutral-beam-injection (NBI)-heated plasma has been compared with the optimized case of titanium gettering. The concentration of oxygen has been reduced by a factor of 2 with boronization. Metal impurities can also be completely suppressed. Plasma performance has been improved in the high-density region with \barn e> 5×1019 m-3. The effective lifetime covers at least 200 NBI discharges while the favorable condition with titanium gettering has an effective lifetime of only 20~30 discharges.

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.