Large Helical Device Plasmas Research Articles

Three-Dimensional Numerical Analysis of Pressure Driven Mode in RMP-Imposed LHD Plasma

Property of pressure driven modes in Large Helical Device (LHD) plasmas with a resonant magnetic perturbation (RMP) is numerically studied. Particularly, we analyze three-dimensional (3D) RMP effects on the linear magnetohydrodynamic (MHD) stability of the modes. For this purpose, 3D numerical codes are utilized for both calculations of an equilibrium including an RMP generating an m = 1/n = 1 magnetic island and the stability of the perturbations resonant at the ι = 1 surface. Here, m and n are the poloidal and the toroidal mode numbers, respectively, and ι denotes rotational transform. Owing to the RMP, the pressure driven mode is localized around the X-point of the island. The type of the mode structure changes from the interchange type to the ballooning type. This property is attributed to the fact that the equilibrium pressure gradient is larger at the X-point than at the O-point.

Control of 3D edge radiation structure with resonant magnetic perturbation fields applied to the stochastic layer and stabilization of radiative divertor plasma in LHD

It is found that resonant magnetic perturbation (RMP) fields have a stabilizing effect on the radiating edge plasma, realizing stable sustainment of radiative divertor (RD) operation in the Large Helical Device (LHD). Without RMP, thermal instability leads to radiative collapse. Divertor power load is reduced by a factor of 3–10 during the RMP-assisted RD phase, while maintaining relatively good core plasma confinement with confinement enhancement factor . It has also been demonstrated that the RMP field itself can initiate transition to RD operation by increasing perturbation strength, while keeping constant density and injection power. The results show a possibility of a new control knob for divertor power load in a 3D magnetic field configuration. It is also found that after the transition to RD, the energy confinement enhancement factor based on ISS04 scaling increases by a factor of 1.4 compared with the attached phase. The operation range of the RMP-assisted RD is identified in terms of RMP strength and radial location of the resonance layer of the RMP. A 3D edge radiation structure is analysed using the edge transport code EMC3-EIRENE and the results are compared with experiments. The comparison indicates that the application of RMP modulates the 3D edge radiation structure such that an intense radiation appears around the X-point of the m/n = 1/1 island in the case with RMP, while it is located at the inboard side without RMP.

Observation of visible forbidden lines from highly charged tungsten ions at the large helical device

Visible line emission from highly charged tungsten ions has been observed at the large helical device (LHD) using a tracer encapsulated solid pellet. One of the measured lines is assigned to a magnetic-dipole (M1) line of the ground-term fine-structure transition of W26+. The other line is unidentified but probably due to a highly charged tungsten ion. Photon emission was observed at 40 lines of sight divided along the vertical direction of a horizontally elongated poloidal cross section of the LHD plasma. The line-integrated intensity of the M1 line along each line of sight indicates a peaked profile at the plasma center where the electron temperatures are high enough so that tungsten ions are highly ionized.

Transport characteristics of tracer and intrinsic impurities depending on the density of LHD plasmas

By injecting a tracer-encapsulated solid pellet (TESPEL) with triple tracers, V (Z = 23), Mn (Z = 25) and Co (Z = 27), into a plasma in the Large Helical Device (LHD), it was found in the high-density case (typically ne = (5–7) × 1019 m−3) that the Kα emissions from the intrinsic impurities were strongly suppressed, while those from all the three tracers were retained for a long time. In the medium-density case (typically ne = (3–4) × 1019 m−3), Kα emissions from the intrinsic impurities were observed clearly, while those of the tracer impurities were observed to decay much faster than the high-density case. When the intrinsic impurities penetrate into the plasma core through the plasma periphery in the plasma build-up phase under relatively low-density conditions, then such impurities are found to be kept for a long time in the later phase under high-density conditions. By implementing supersonic Ar gas puffing in addition to the TESPEL injection, Ar Kα emission was clearly observed together with Kα emissions from the tracers in the medium-density case. In contrast to this, Ar Kα emission was completely suppressed in the high-density case. This result shows that the suppression of the intrinsic impurity coming from outside the plasma is indeed working in the high-density case, while the impurities deposited inside the plasma are kept for a long time.

Electron Bernstein wave heating by electron cyclotron wave injection from the high-field side in LHD

In the Large Helical Device (LHD), evident electron Bernstein wave (EBW) heating was successfully performed. The experiment was carried out using the electron cyclotron heating (ECH) system that was upgraded by installation of high-power, long-pulse 77 GHz gyrotrons. The EBW heating was achieved by a mode conversion from injected EC wave to EBW, by the so-called slow-XB technique where an X-mode wave is injected to the plasma from the high magnetic field side. The specific magnetic configuration of LHD provides a good opportunity to realize the slow-XB technique, which is generally difficult for tokamaks. With the slow-XB technique, increases in kinetically evaluated electron energy Wpe and electron temperature Te were observed in overdense plasmas. An electron heating in the so-called super dense core plasma in LHD, which is characterized with an internal diffusion barrier and a steep density gradient at the plasma core, was successfully demonstrated in the plasma core region where the central electron density ne0 of 17 × 1019 m−3 was about 1.2 times higher, at the beginning of the EC-wave injection, than the left-hand cut-off density of applied 77 GHz EC waves.

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

Characteristics of fast-ion losses induced by toroidal-Alfvén 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 certain fluctuation 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 simulation by 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 (ΔΓfast ion ∝ bθTAE) to a diffusive character as bθTAE increases.

Gyrokinetic turbulence simulations of high-beta tokamak and helical plasmas with full-kinetic and hybrid models

Turbulent transport in high-beta toroidal plasmas is investigated by means of an electromagnetic gyrokinetic model and a newly developed electromagnetic hybrid model consisting of the gyrokinetic equation for ions and drift-Landau-fluid equations for electrons. Full gyrokinetic simulation results for Cyclone base case tokamak and for Large Helical Device (LHD) plasmas are quickly and accurately reproduced by the hybrid simulation. In the kinetic ballooning mode (KBM)-driven turbulence the ion heat and particle fluxes are mainly caused by electrostatic perturbation, and the contribution of magnetic perturbation is small and negative. The electron heat flux is caused by both electrostatic and magnetic perturbations. The numerical solutions satisfy the entropy balance equation, and the entropy is transferred from ions to electrons through electrostatic and magnetic perturbations. An analysis based on the entropy balance equation shows that the zonal structure is produced by magnetic nonlinearity corresponding to the Maxwell stress in the fluid limit but is weakened by the electrostatic one related to the Reynolds stress. A linear analysis on the standard configuration of LHD plasmas shows the suppression of the ion temperature gradient mode by finite-beta effects and the destabilization of KBM at high beta.

Enhancement of hydrogen isotope retention in tungsten exposed to LHD plasmas

Abstract Tungsten (W) samples, undamaged and damaged by irradiation with 2.4 MeV Cu2+, were placed flush to the first wall of Large Helical Device (LHD) nearby the divertor target, and were exposed to 134 shots of hydrogen plasma discharges. Thereafter, to evaluate the enhancement of hydrogen isotope retention for W due to LHD hydrogen (H) plasma exposures, the W samples with and without LHD H plasma exposures were implanted with 1.0 keV D 2 + . For W samples without LHD H plasma exposures, the D retention in the damaged W was by a factor of ∼6 higher than that in the undamaged W. After LHD H plasma exposures, the carbon layers ∼4 nm in thickness were deposited on the W surfaces. Due to trapping of D atoms in the carbon layers, the D retention enhancement for undamaged and damaged W samples exposed to LHD H plasmas was significantly higher than that for the W samples without LHD H plasma exposures.

Development of Integrated Transport Analysis Suite for LHD Plasmas Towards Transport Model Validation and Increased Predictability

In this study, the integrated transport analysis suite, TASK3D-a, was developed to enhance the physics understanding and accurate discussion of the Large Helical Device (LHD) experiment toward facilitating transport model validation. Steady-state and dynamic (transient) transport analyses of NBI (neutral-beam-injection)-heated LHD plasmas have been greatly facilitated by this suite. This will increase the predictability of the transport properties of LHD plasmas toward reactor-relevant regimes and reactor-scale plasmas.

Tritium Retention on Stainless Steel Surface Exposed to Plasmas in LHD (II)

Tritium retention of samples exposed to plasmas in the Large Helical Device (LHD) during each campaign in 12th, 13th and 14th cycles has been studied. Small sample plates made of stainless steel type 316L were fixed in advance at four different walls in LHD: location of a sample plate was 1.5U, 5.5U, 6.5L and 9.5L. After plasma exposure in each cycle, these samples were exposed to tritium gas at a temperature of 300 or 623 K. Retention behavior of tritium in surface layers of each sample was mainly examined using β-ray-induced X-ray spectrometry (BIXS) and X-ray photoelectron spectroscopy (XPS). The energy spectra observed by BIXS and XPS showed the depositions of boron, carbon, titanium, chromium, iron, nickel and molybdenum with oxygen. Tritium retention of the samples exposed to plasma increased than that of a bare SS316L sample, although it was largely different in the location of a sample. When the samples were exposed to tritium gas at 300 K, the order of magnitude of tritium retention was as follows: 9.5L� 5.5U>6.5L>1.5U for 12th cycle, 6.5L>9.5>1.5U>5.5U for 13th cycle, and 6.5L>1.5U∼5.5U>9.5L for 14th cycle. c

Linear properties of energetic particle driven geodesic acoustic mode

Linear properties of energetic particle driven geodesic acoustic mode (EGAM) in the large helical device plasmas are investigated using a hybrid simulation code for a magnetohydrodynamics fluid interacting with energetic particles. It is found that the EGAM is a global mode with the spatially uniform oscillation frequency despite the spatial variation of the local geodesic acoustic mode frequency. The poloidal mode numbers of poloidal velocity fluctuation, plasma density fluctuation, and magnetic fluctuation are m = 0, 1, and 2, respectively. Oscillation frequency, linear growth rate, and spatial width of EGAM are compared for different physics conditions. The EGAM frequency is proportional to the square root of the plasma temperature. The frequency is lower for higher energetic particle β value. The mode spatial width is larger for larger spatial width of the energetic particle distribution and for the reversed shear safety-factor profile than the normal shear profile. It is also found that the EGAM propagates radially outward in the linearly growing phase, and the propagation speed is slower for the spatially broadened modes.

Bifurcation-Like Behavior of Electrostatic Potential in LHD

The temporal evolution of the potential is measured with the heavy ion beam probe (HIBP) in the Large Helical Device (LHD). A rapid change in the potential is observed early in the tangential neutral beam injection (NBI) phase, just after the switch from tangential to perpendicular NBI, and in a low-power electron cyclotron heated (ECH) discharge in the LHD. The radial profile of the potential is measured before and after the potential change. This rapid potential change is considered to indicate the bifurcation of the radial electric field in the edge region of the LHD plasma.

Design for X-Ray Imaging Spectroscopy in Large Helical Device

An assembly of pin-hole camera has been designed for x-ray imaging spectroscopy in Large Helical Device (LHD). In the design the camera basically consists of a 2-D-pixel-array detector, a metallic filter, and a pin hole. The detector measures gray scale images through the filter, while the thickness of the filter changes shot by shot. The gray scale images are mathematically transformed to a color spectral image by means of Mellin transform. The available spectral range is depending on the material of the filter. The spectral energy resolution is also depending on the material of the filter and the intensity of incident x-ray. In the case of the filter made of aluminum, the spectral range is theoretically predicted to be from 3.0 keV to 30 keV. It is also theoretically expected in the case of typical LHD plasmas that 50 flames of color spectral 32 × 32-pixel-image is obtained per second with a spectral resolution of E/ΔE = 4.3 at a photon energy of E = 5.0 keV. c

Gyrokinetic simulations of entropy transfer in high ion temperature LHD plasmas

Gyrokinetic simulations of the ion temperature gradient (ITG) turbulence in non-axisymmetric configurations modeled on the Large Helical Device (LHD) are performed with the entropy transfer analysis (Nakata et al 2012 Phys. Plasmas 19 022303). It is clarified that a fluctuation spectrum elongated in the radial wavenumber direction is formed in a neoclassically optimized field configuration with the inward-shifted magnetic axis position, where the zonal flows are more effectively generated than in the standard case. The strong interaction between zonal flows and turbulence causes the successive entropy transfer from low to high radial wavenumber space and the spectral broadening of ITG turbulence.

Numerical magnetohydrodynamic analysis of Large Helical Device plasmas with magnetic axis swing

A partial collapse observed in magnetic axis swing experiments in the Large Helical Device (LHD) is analyzed with a nonlinear magnetohydrodynamic (MHD) simulation. Real time control of the background field in the operation is incorporated by means of a multi-scale numerical scheme in the simulation. The simulation result indicates that the changing background field accelerates the growth of an infernal-like mode and causes the partial collapse.

Magnetic configuration effects on TAE-induced losses and a comparison with the orbit-following model in the Large Helical Device

Fast-ion losses from Large Helical Device (LHD) plasmas due to toroidal Alfvén eigenmodes (TAEs) were measured by a scintillator-based lost fast-ion probe (SLIP) to understand the loss processes. TAE-induced losses measured by the SLIP appeared in energy E ranges of around 50–180 keV with pitch angles χ between 35°–45°, and increased with the increase in TAE amplitudes. Position shifts of the magnetic axis due to a finite plasma pressure led not only to an increase in TAE-induced losses but also to a stronger scaling of fast-ion losses on TAE amplitudes. Characteristics of the observed fast-ion losses were compared with a numerical simulation based on orbit-following models in which the TAE fluctuations are taken into account. The calculation indicated that the number of lost fast ions reaching the SLIP increased with the increase in the TAE amplitude at the TAE gap. Moreover, the calculated dependence of fast-ion loss fluxes on the fluctuation amplitude became stronger in the case of large magnetic axis shifts, compared with the case of smaller shifts, as was observed in the experiments. The simulation results agreed qualitatively with the experimental observations in the LHD.

On influences of long-range fluctuations on transport in Large Helical Device plasmas

The competing effects of the long-range fluctuation, which has been found on the Large Helical Device (Inagaki et al 2011 Phys. Rev. Lett. 107 115001), on the net turbulent transport are discussed. The transport, which is driven directly by this mode, is evaluated. Then, the associated reduction of fluctuations in the range of drift wave turbulence is analyzed, by employing the predator–prey model. The transport driven by microscopic turbulence is reduced by the appearance of long-range fluctuations. Comparing these two competing processes, the condition is derived for the net transport to be reduced by the appearance of this long-range fluctuation. In addition, the experimental observations of microscopic fluctuations are discussed. The reduction of high-frequency fluctuations occurs in conjunction with the long-range fluctuations.

Current status of the LHD Thomson scattering system

The large helical device (LHD) Thomson scattering system measures electron temperature (Te) and density (ne) profiles of LHD plasmas, along the LHD major radius (R). The total length of plasma measured is 3 m (R = 2.325–5.386 m), the number of observation points is 144, and the spatial resolution is 12–25 mm. The sampling frequency is 10–100 msec (10–100 Hz). The measurable temperature and density ranges have been estimated to be 5 eV–20 keV and 1018–1022 m−3, respectively. The LHD Thomson scattering system consists of several subsystems, yttrium-aluminum-garnet (YAG) lasers, light collection optics, polychromators, and data acquisition system. In usual plasma experiments, we use three types of YAG lasers: 2 J/10 Hz, 1.6 J/30 Hz, and newly developed 1.2/50 Hz YAG lasers. Thomson scattering signals are analyzed with the FASTBUS-based data acquisition system. Recently, a hardware technique and three data analysis methods have been tested to improve data quality. By using these methods, the data quality has been increased by more than an order of magnitude in high-Te, low-ne plasma experiments. In the paper, we describe the current status of the LHD Thomson scattering system.

Studies of reflectivity degradation of retroreflectors in LHD and mitigation of impurity deposition using shaped diagnostic ducts and protective windows

Maintaining the reflectivity of first mirrors is indispensable in future fusion devices. While a retroreflector (corner cube mirror) is useful for laser diagnostics, impurities tend to accumulate and form a thick deposition layer in the central region, which causes degradation of reflectivity, due to the hollow shape of the retroreflector. Two mirror structures are tested to retain the reflectivity in the Large Helical Device (LHD). One is a bending mirror structure with a protective cylinder with fins and it could maintain the reflectivity over a three-month experimental campaign. The other is a cover window just in front of the reflector. Candidates of the window materials were exposed to the LHD plasmas and the degradation of the transmissivity of ZnSe and silicon, which are used for infrared and far infrared laser light, respectively, were small.

Two-dimensional measurement of edge impurity emissions using space-resolved extreme ultraviolet spectrometer in Large Helical Device

A space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 50-500 Å has been developed to measure two-dimensional distribution of impurity spectral lines emitted from edge plasma of Large Helical Device (LHD), in which the magnetic field is formed by stochastic magnetic field with three-dimensional structure called ergodic layer. The two-dimensional measurement of edge impurity line emissions is carried out by scanning horizontally the observation chord of the space-resolved EUV spectrometer during single LHD discharge. Images of CIV (312.4 Å) and HeII (303.78 Å) are presented as the first result. The results are compared with ones calculated from the edge chord length in the ergodic layer of LHD plasma.