LHDヘリカル閉ダイバータのための新コンセプト真空容器内クライオソープションポンプの開発

The in-vessel cryo-sorption pump for the Closed Helical Divertor (CHD) in the Large Helical Device (LHD) has been developed at the National Institute for Fusion Science (NIFS). An organic adhesive-free bonding technique for attaching activated carbon pellets to a copper cold panel was invented, which employs the indium solder with intermediate materials. The prototype of the CHD with the newly developed cryo-sorption pump was installed in the LHD. Performance of the cryo-sorption pump was estimated in the LHD vacuum vessel. A satisfactory result of the maximum pumping speed up to 9 m3/s was obtained with one divertor module in one toroidal section (10% of the torus), which is equivalent to the required pumping speed of the CHD

Effect of divertor legs on neutral particle and impurity retention for a closed helical divertor configuration in the Large Helical Device

A closed helical divertor (CHD) has been designed for efficient particle control in the plasma periphery and for retaining neutral particles and impurity ions in the divertor region. The effect of impurity retention by divertor legs for the CHD configuration is investigated from the viewpoints of neutral impurity transport and force balance of impurity ions along magnetic field lines. A fully three-dimensional neutral particle transport simulation proves that the plasma on the divertor legs is effective for retaining neutral particles/impurities in the CHD region. A one-dimensional impurity ion transport analysis predicts that friction force by plasma flow from the main plasma sweep impurity ions toward the divertor plates even in high neutral density case in which a steep temperature gradient is formed. It shows that the CHD configuration is promising for enhancing LHD plasma performance by effective control of the neutral particles and the impurity ions in the plasma periphery

First EMC3-EIRENE Simulations with Divertor Legs of LHD in Realistic Device Geometry

An extended mesh system for EMC3-EIRENE has been developed to simulate peripheral plasma including the ergodic and the divertor leg regions of LHD. Both the open and the closed divertor configurations are available. A series of simulations for 8MW input power, five different electron densities at the LCFS (last closed flux surface) and the open/closed configurations were carried out. Approximately 10 times larger neutral pressure was observed under the dome structure compared with the open configuration, which is in good agreement with experimental measurements. In the case of the closed configuration, the leg regions have a large contribution of ionization to hydrogen recycling. In the case of high density discharges, however, electron temperature in the legs becomes low and the major contribution of ionization moves to the ergodic region. Significant influence of configurations is observed in the inboard side of LHD, where closed divertor components are installed but little influence is seen near the LCFS. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Effect of a baffle divertor structure on neutral hydrogen and helium transport in the Large Helical Device

Control of the peripheral plasma density is one of urgent tasks for improving the plasma performance in the Large Helical Device. Test modules with a baffle divertor structure have been installed in the inboard side of the torus for controlling the neutral density in the plasma periphery. Neutral hydrogen and helium transport in an original open divertor and the baffle divertor configurations have been monitored with filtered CCD cameras to observe the intensity profile of visible emission by neutral hydrogen (Hα) and helium (HeI). A detailed analysis using a three-dimensional neutral particle transport simulation code reasonably explains the dependence of the observed intensity profiles on the divertor and the magnetic configurations. It also predicts that the baffle divertor significantly reduces the emission of neutral hydrogen in the plasma periphery in the inboard side, and the compression of neutral helium density by the baffle divertor is comparable to that of hydrogen

Analysis of the impurity flow velocity in a wide plasma parameter range for deuterium and hydrogen plasmas in the divertor legs of the stochastic layer in LHD

Impurity flow velocity measurements have been conducted for different magnetic field configurations in a wide plasma parameter range in the divertor leg region of LHD for understanding of the edge impurity transport. In all cases (densities, magnetic configurations, hydrogen (H) & deuterium (D) discharges), flows of several tens of km/s are observed. It is found that the flow in thick stochastic layer is faster than in thin stochastic layer configuration by a factor of 3. Different velocities of different charge states of carbon impurity are observed. The simulation with EMC3-EIRENE code shows similar trend as the experiments, but only qualitatively: faster flow in H compared to D discharges due to the mass effect, faster flow in the case of thick stochastic layer. However, synthetic spectra show discrepancy with experiments in the absolute Doppler shift, where the impurity velocity in the experiments is one order faster compared to the simulations

A mechanism of ion temperature peaking by impurity pellet injection in a heliotron plasma

Experiments on the Large Helical Device with the injection of carbon pellets into discharges of low density have demonstrated a significant reduction of the ion heat conduction in the plasma core and an increase in the central ion temperature by a factor of up to 2. These results are interpreted in the framework of a transport model elaborated on the basis of those applied previously to explain the improvement in confinement by impurity seeding into the tokamak devices TEXTOR and JET. The calculations performed reproduce well the strong peaking of the ion temperature profile with increasing carbon density nZ and the consequent drop in the confinement as nZ exceeds a certain critical level. The importance of different elements in the model, such as braking of the main ion rotation by friction with impurity ones and the shape of the density profiles, are investigated. A qualitative assessment of the applicability under fusion reactor conditions, e.g. of much higher plasma density and heating power, is performed

Investigation of the Helical Divertor Function and the Future Plan of a Closed Divertor for Efficient Particle Control in the LHD Plasma Periphery

The function of the divertor plasmas on the particle control in the plasma periphery is investigated from viewpoints of magnetic field line structures and neutral particle transport in the Large Helical Device (LHD). It shows that the particle and heat deposition on the divertor plate arrays are qualitatively explained by the distribution of strike points calculated by magnetic field line tracing including a particle diffusion effect. Control of neutral particle fueling from the divertor plates is a critical issue for sustaining long-pulse discharges and achieving superdense core plasmas. The behavior of neutral particles in the plasma periphery has been investigated by Hα emission measurements and a neutral particle transport simulation. It reveals that gas fueling from the toroidally distributed divertor plates heated by protons accelerated by ion cyclotron resonance frequency wave is necessary for explaining measurements in a long-pulse discharge, and the spatial profile of the neutral particle density in the plasma periphery in various magnetic configurations is explained by the strike point distribution. Based on these analyses, a closed helical divertor configuration optimized for the intrinsic magnetic field line structure in the plasma periphery is proposed for efficient particle control and heat load reduction on the divertor plates

高ベータプラズマ運転領域の低衝突性領域への拡大, 高ベータプラズマ運転領域の低衝突性領域への拡大

In the large helical device, plasma with more than 4% average beta was successfully produced by multi-pellet injections in a regime with a collisionality one order of magnitude lower than that in previous high-beta operations. An improvement in particle confinement was observed during a high-beta discharge produced by a gas puff, and particle flux to the divertor was reduced by more than 40%. High instabilities at the plasma edge occurred and suppressed the increment of the average beta to 3.4%. A spontaneous change in the magnetic topology contributes to an increase in the average beta value while triggering the excitation of edge MHD instabilities

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 Reynold\u27s 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, which we believe is the MHD instability. It is possible that type-I oscillations are caused by the asymmetric concentration of the impurities. The type-I oscillations are similar to the impurity snake phenomena observed in tokamaks though type-I oscillations survive only several tens of milliseconds in LHD

Particle control in long-pulse discharge using divertor pumping in LHD

Density control is crucial for maintaining stable confined plasma. Divertor pumping, where neutral particles are compressed and exhausted in the divertor region, was developed for this task for the Large Helical Device. In this study, neutral particle pressure, which is related to recycling, was systematically scanned in the magnetic configuration by changing the magnetic axis position. High neutral particle pressure and compression were obtained in the divertor for a high plasma electron density and the inner magnetic axis configuration. Density control using divertor pumping with gas puffing was applied to electron cyclotron heated plasma in the inner magnetic axis configuration, which provides high neutral particle compression and exhaust in the divertor. Stable plasma density and electron temperature were maintained with divertor pumping. A heat analysis shows that divertor pumping did not affect edge electron heat conductivity, but it led to low electron heat conductivity in the core caused by electron-internal-transport-barrier-like formation