燃料粒子制御に向けた磁場閉じ込め核融合プラズマにおける 粒子排気のこれまでとこれから

本解説では，粒子制御に向けたこれまでの取り組みとして，主として粒子排気という視点に立ち，プラズマ対向壁の燃料粒子吸蔵，放出現象のほか，ダイバータによる効率的な燃料排出について筆者が行ってきた研究を中心に紹介する．また，粒子制御を行うための重要な計測手法として，中性粒子圧力計測の一つである高速イオンゲージ開発の進展についても触れる．そのほか，燃焼プラズマで想定される水素同位体，ヘリウム混合プラズマ中の同位体分離や，効率的な粒子排出を可能にする粒子制御手法についても紹介し，これからの粒子制御に向けた展望を述べる

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

Twenty barrel in situ pipe gun type solid hydrogen pellet injector for the Large Helical Device

A 20 barrel solid hydrogen pellet injector, which is able to inject 20 cylindrical pellets with a diameter and length of between 3.0 and 3.8 mm at the velocity of 1200 m/s, has been developed for the purpose of direct core fueling in LHD (Large Helical Device). The in situ pipe gun concept with the use of compact cryo-coolers enables stable operation as a fundamental facility in plasma experiments. The combination of the two types of pellet injection timing control modes, i.e., pre-programing mode and real-time control mode, allows the build-up and sustainment of high density plasma around the density limit. The pellet injector has demonstrated stable operation characteristics during the past three years of LHD experiments

Transition between Isotope-Mixing and Nonmixing States in Hydrogen-Deuterium Mixture Plasmas

The transition between isotope-mixing and nonmixing states in hydrogen-deuterium mixture plasmas is observed in the isotope (hydrogen and deuterium) mixture plasma in the Large Helical Device. In the nonmixing state, the isotope density ratio profile is nonuniform when the beam fueling isotope species differs from the recycling isotope species and the profile varies significantly depending on the ratio of the recycling isotope species, although the electron density profile shape is unchanged. The fast transition from nonmixing state to isotope-mixing state (nearly uniform profile of isotope ion density ratio) is observed associated with the change of electron density profile from peaked to hollow profile by the pellet injection near the plasma periphery. The transition from nonmixing to isotope-mixing state strongly correlates with the increase of turbulence measurements and the transition of turbulence state from TEM to ion temperature gradient is predicted by gyrokinetic simulation

Experimental study of non-inductive current in Heliotron J

It is important to control non-inductive current for generation and steady-state operation of highperformance plasmas in toroidal fusion devices. Helical devices allow dynamic control of non-inductivecurrent through a wide variety of magnetic configurations. The reversal of non-inductive current consisting of bootstrap current and electron cyclotron driven current in electron cyclotron heating plasmas has been observed in a specific configuration at low density in Heliotron J device. By analyzing thenon-inductive current for normal and reversed magnetic fields, we present experimental evidence for the reversal of bootstrap current. Our experiments and calculations suggest that the reversal is caused bya positive radial electric field of about 10 kV/m. Moreover, we show that the typical electron cyclotron current drive efficiency in Heliotron J plasma is about 1.0 × 1017 AW?1m?2, which is comparable to other helical devices. We have found that the value is about 10 times lower than that of tokamak devices. This might be due to an enhanced Ohkawa effect by trapped particles

Characterized divertor footprint profile modification with the edge pressure gradient in the Large Helical Device

The first attempt to characterize the divertor footprint profile in the heliotron device LHD was done, by using a number of Langmuir probes and the multivariable analysis technique. In order to clarify the generation mechanism of the private-side peak on the footprint profile, which has not been reproduced in the modeling study, over 6000 time points were extracted by excluding time points with profile modifications due to already-known reasons. A characterization index ${r}_{2/1}$ was newly defined from the multivariable analysis result, and its dependences on upstream parameters were investigated. As a result, it was found that the footprint profile correlates with the pressure gradient at the edge inside the core region with a fixed beta, suggesting that change of the plasma pressure profile could modify the edge magnetic field structure even if the volume integral of the plasma pressure was constant

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

Improved performance of electron cyclotron resonance heating by perpendicular injection in the Large Helical Device

A real-time interlock system for power injection in electron cyclotron resonance heating(ECRH) was developed to be applied to Large Helical Device (LHD) plasma. This systemenabled perpendicular injection, thus improving the performance of ECRH more than has everbeen achieved before in LHD. Perpendicular propagation of the electron cyclotron wave at77 GHz became more insensitive to the effect of refraction in comparison to the conventionaloblique propagation. The achieved central electron temperature in the case of perpendicularinjection was approximately 2 keV higher than that in the case of standard oblique injectionfor a central electron density of 1 × 1019 m−3 by 1 MW injection.With such improvedperformance of ECRH, high-density ECRH plasma of 8 × 1019 m−3 was successfullysustained after the injection of multiple hydrogen ice pellets for the first time in LHD

Imaging of radiation during impurity gas puffing in LHD

In LHD, several methods of detachment have been attempted, including impurity gaspuffing [1], and the application of an m/n=1/1 magnetic perturbation [2]. LHD is equipped with an imaging bolometer (IRVB) [3] that views the plasma from an upper port. Two scenarios are shown and compared, Ne puffing and N2 puffing. In the case of Ne puffing, radiation becomes more intense near the helical divertor X-point as the radiation increases. In the case of N2 puffing, a double stripe pattern evolves around the upper helical divertor X-point, which appears to be localized near the gas puff inlet. In addition, probe data also indicates that the drop in divertor flux with N2 is localized, while uniform with Ne