Modeling of the resonant magnetic perturbation effect on detachment in the Large Helical Device

An approach to describe the plasma parameter variation with the effective minor radius across the edge region in the heliotron Large Helical Device (LHD) in configurations without and with a resonant magnetic perturbation (RMP) is elaborated, by averaging fluid equations for transport of particles, momentum and heat over the flux surfaces. Numerical solutions of one-dimensional, time-dependent equations derived and analytical estimates performed allow us to interpret the LHD experiments on the density limit. Calculations reproduce qualitatively the principal difference between situations without and in the presence of RMP: in the former case a thermal collapse of the discharge occurs immediately after the plasma detachment from the divertor target plates at a radiation level of 0.4–0.5; in the latter the radiating layer is localized at the plasma edge even if the power radiated exceeds 90% of the input power. A prominent role for such a behavior of plasma particle flows along magnetic field lines perturbed by RMP, leading to a positive radial gradient of the plasma density inside the magnetic island, is demonstrated

Self-Organized Rotating Filament Structure in Plasma in the Large Helical Device After Tracer Encapsulated Solid Pellet Injection

A small tungsten grain encapsulated in a polystyrene pellet was injected into plasmas in the large helical device. The ionized tungsten was transported and accumulated in the plasma center to drastically drop the central electron temperature. A tangentially viewing fast framing camera observed a bubble-like structure expanding from the plasma center after the pellet injection. After that, a self-organized filament appeared on the surface of the bubble, and the filament began to rotate around the plasma center, which was stably sustained for ∼0.22 s

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

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

A study on the TAE-induced fast-ion loss process in LHD

Characteristics of fast-ion losses induced by toroidal-Alfv en eigenmodes (TAEs) are investigated over wide parameter ranges of Large Helical Device (LHD) plasmas to reveal the fast-ion loss process. To study fast-ion losses, a scintillator-based lost-fast ion probe is used, and an increment of fast-ion loss flux due to TAEs from the neoclassical orbit loss level (ΔΓfast ion) is measured. The dependence of ΔΓfast ion on the TAE magnetic fluctuation amplitude(bθTAE) changes from a linear to a quadratic and finally a third power with an increase in the magnetic axis shift. It is found that the dependence of fast-ion loss flux on TAE magnetic fluctuation amplitudes changes at a certainfluctuation level in a fixed configuration. Experimental results show that in the small bθTAE regime, ΔΓfast ion is proportional to bθTAE, whereas ΔΓfast ion increases with the square of bθTAE in the larger bθTAE regime. A simulationby orbit-following codes that incorporate magnetic fluctuations with frequency chirping-down due to TAEs suggests the change in the fast-ion loss process from a convective ( ΔΓ ion ∝ bθTAE) to a diffusive (ΔΓfast ion ∝ b2 θTAE) character as bθTAE increases

Investigation of remaining tritium in the LHD vacuum vessel after the first deuterium experimental campaign

Remaining tritium in the vacuum vessel after the first deuterium plasma experimental campaign conducted over four months was investigated in the large helical device (LHD) for the first time in stellarator/heliotron devices by using the tritium imaging plate technique. In-vessel components such as divertor tiles and first wall panels, and long-term material probes retrieved from the vacuum vessel were analyzed. The in-vessel component in which tritium remained most densely is the baffle part of divertor tiles made of graphite retrieved from the inboard-side divertor. Asymmetric tritium retention is observed on divertor tiles located at magnetically symmetric positions, and can be attributed to the toroidal field direction dependence of the asymmetric loss of energetic tritons generated by deuterium–deuterium nuclear fusion reactions. On the first wall, tritium remained in a deposited layer, which mainly consists of carbon

亜臨界高速イオン励起モードの非線形励起, 亜臨界高速イオン励起モードの非線形励起

In collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase-space. The growth of such structures is a nonlinear, kinetic mechanism, which provides a channel for free-energy extraction, different from conventional inverse Landau damping. However, such nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave–wave coupling can provide a seed, which can lead to subcritical instability by either one of two mechanisms. Both mechanisms hinge on a collaboration between fluid nonlinearity and kinetic nonlinearity. If collisional velocity diffusion is low enough, the seed provided by the supercritical mode overcomes the threshold for nonlinear growth of phase-space structure. Then, the supercritical mode triggers the conventional subcritical instability. If collisional velocity diffusion is too large, the seed is significantly below the threshold, but can still grow by a sustained collaboration between fluid and kinetic nonlinearities. Both of these subcritical instabilities can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. These results were obtained by modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave–wave coupling equations. This model is applied to bursty onset of geodesic acoustic modes in an LHD experiment. The model recovers several key features such as relative amplitude, timescales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, with sustained collaboration between fluid and kinetic nonlinearities

The title of this Accepted manuscrpt was "Observation of strong destabilization of stable modes with a half frequency associated with chirping EGAMs in the LHD".

Abrupt and strong excitation of a mode has been observed when the frequency of a chirping energetic-particle driven geodesic acoustic mode (EGAM) reaches twice the geodesic acoustic mode (GAM) frequency. The frequency of the secondary mode is the GAM frequency, which is a half-frequency of the primary EGAM. Based on the analysis of spatial structures, the secondary mode is identified as a GAM. The phase relation between the secondary mode and the primary EGAM is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. The results suggest that the primary mode (EGAM) contributes to nonlinear destabilization of a subcritical mode

Observation of subcritical geodesic acoustic mode excitation in the large helical device

The abrupt and strong excitation of the geodesic acoustic mode (GAM) has been found in the large helical device (LHD), when the frequency of a chirping energetic particle-driven GAM (EGAM) approaches twice that of the GAM frequency. The temporal evolution of the phase relation between the abrupt GAM and the chirping EGAM is common in all events. The result indicates a coupling between the GAM and the EGAM. In addition, the nonlinear evolution of the growth rate of the GAM is observed, and there is a threshold in the amplitude of the GAM for the appearance of nonlinear behavior. A threshold in the amplitude of the EGAM for the abrupt excitation of the GAM is also observed. According to one theory (Lesur et al 2016 Phys. Rev. Lett. 116 015003, Itoh et al 2016 Plasma Phys. Rep. 42 418) the observed abrupt phenomenon can be interpreted as the excitation of the subcritical instability of the GAM. The excitation of a subcritical instability requires a trigger and a seed with sufficient amplitude. The observed threshold in the amplitude of the GAM seems to correspond with the threshold in the seed, and the threshold in the amplitude of the EGAM seems to correspond with the threshold in the magnitude of the trigger. Thus, the observed threshold supports the interpretation that the abrupt phenomenon is the excitation of a subcritical instability of the GAM

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