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Abstract The hyperspectral camera for auroral imaging (HySCAI), which can provide a two-dimensional (2D) aurora image with full spectrum, was developed to study auroral physics. HySCAI consists of an all-sky lens, monitor camera, galvanometer scanner, grating spectrograph, and electron multiplying charge coupled device (EM-CCD). The galvanometer scanner can scan a slit image of the spectrograph on the all-sky image plane in the direction perpendicular to the slit. HySCAI has two gratings; one is 500 grooves/mm for a wide spectral coverage of 400–800 nm with a spectral resolution (FWHM) of 2.1 nm, and the other is 1500 grooves/mm for a higher spectral resolution of 0.73 nm with a narrower spectral coverage of 123 nm. The absolute sensitivity is 2.1 count/s/R with 4 × 4 binning (256 × 340 image) at 557.7 nm. The exposure time depends on the brightness of the aurora emission and is typically 64 s for a 2D image (0.2 s per line scan). This system has been installed at the KEOPS (Kiruna Esrange Optical Platform Site) of the SSC (Swedish Space Corporation) in Kiruna, Sweden. All-sky images with a liquid crystal filter and a sky color camera have also been installed to compensate for the poor time resolution of HySCAI. 2D aurora monochromatic images for given wavelength are obtained by reconstructing the EM-CCD image over the scan period. HySCAI has the advantage of providing a 2D image of intensity for a weak emission line, which appears on top of a high background emission without the contamination from other emissions, which is usually difficult in a system with a bandpass filter. As the first light results, monochromatic images of $$\hbox {N}_2^+$$ N 2 + 1NG (0, 1) (427.8 nm), $$\hbox {N}_2^+$$ N 2 + 1NG (0, 2) (470.9 nm), $$\hbox {H}_{\beta }$$ H β (486.1 nm), N II (500.1 nm), N I ( $$^2$$ 2 D) (520.0 nm), O I ( $$^1$$ 1 S) (557.7 nm,), NaD (589.3 nm), O I ( $$^1$$ 1 D) (630.0 nm), and $$\hbox {N}_2^+$$ N 2 + 1NG (670.5 nm) emission intensity were measured. We estimated the precipitating electron energy from a ratio of I(630.0 nm)/I(427.8 nm) to be 1.6 keV. Graphical Abstract

The development of reduced models provide efficient methods that can be used to perform short term experimental data analysis or narrow down the parametric range of more sophisticated numerical approaches. Reduced models are derived by simplifying the physics description with the goal of retaining only the essential ingredients required to reproduce the phenomena under study. This is the role of the gyro-fluid code FAR3d, dedicated to analyze the linear and nonlinear stability of Alfvén Eigenmodes (AE), Energetic Particle Modes (EPM) and magnetic-hydrodynamic modes as pressure gradient driven mode (PGDM) and current driven modes (CDM) in nuclear fusion devices. Such analysis is valuable for improving the plasma heating efficiency and confinement; this can enhance the overall device performance. The present review is dedicated to a description of the most important contributions of the FAR3d code in the field of energetic particles (EP) and AE/EPM stability. FAR3d is used to model and characterize the AE/EPM activity measured in fusion devices as LHD, JET, DIII-D, EAST, TJ-II and Heliotron J. In addition, the computational efficiency of FAR3d facilitates performing massive parametric studies leading to the identification of optimization trends with respect to the AE/EPM stability. This can aid in identifying operational regimes where AE/EPM activity is avoided or minimized. This technique is applied to the analysis of optimized configurations with respect to the thermal plasma parameters, magnetic field configuration, external actuators and the effect of multiple EP populations. In addition, the AE/EPM saturation phase is analyzed, taking into account both steady-state phases and bursting activity observed in LHD and DIII-D devices. The nonlinear calculations provide: the induced EP transport, the generation of zonal structures as well as the energy transfer towards the thermal plasma and between different toroidal/helical families. Finally, FAR3d is used to forecast the AE/EPM stability in operational scenarios of future devices as ITER, CFETR, JT60SA and CFQS as well as possible approaches to optimization with respect to variations in the most important plasma parameters.

Isotope effects have been investigated in Neutral Beam Injection (NBI) heated plasmas on the Large Helical Device with similar operational parameters between Hydrogen (H) and Deuterium (D) plasmas. Experimental results show that the global energy confinement has no significant dependence on the isotope mass under similar discharge conditions with nearly the same heating power, line-averaged density ( ${\bar n_e}$ ) and magnetic field. For both electron and ion energy transport, the transport coefficients, which are obtained based on local power balance analysis, have analogous profiles between H and D dominant plasmas. For neoclassical ${\chi _e}$ and ${\chi _i}$ values, they are almost equal between H and D dominant plasmas in low ${\bar n_e}$ discharges, whereas in high ${\bar n_e}$ cases they are lower in H plasmas than those in D ones. At low ${\bar n_e}$ , the electron and ion thermal transport in both H and D plasmas are dominated by neoclassical transport at a certain zone ( $\rho \approx 0.6 - 0.85$ ), while the anomalous transport process has primary effects in the remaining area, and the density fluctuations exhibit ion temperature gradient mode nature. With increase of ${\bar n_e}$ , the anomalous transport becomes prevailing and the density fluctuations propagate along electron diamagnetic drift direction. Bispectral analysis reveals that the H plasma has stronger nonlinear coupling in edge density fluctuations in both low and high density discharges, which is probably due to that the D plasma has stronger damping rate for the nonlinear interaction of turbulence. For a comparative study, the present results have been compared with those observed in the ECRH discharges (Tanaka et al 2019 Nucl. Fusion 59 126040). The reasons for the similarities and dissimilarities between these two different heating manners are not clear yet. To unravel the underlying physics, essential inputs from theories and simulations are required.

We investigated the degradation of neutral beam (NB) fast-ion confinement depending on the NB power without magnetohydrodynamics instabilities in the Large Helical Device (LHD). In the LHD deuterium experiment, the neutron emission rate per NB power decreased by up to 20% with increasing injected NBs during a single discharge. Because there were no significant variations in the electron temperature and density, the NB shine-through rate, or the magnetic fluctuation due to the change in NB power, the reduction in the neutron emission rate indicates the degradation of the fast-ion confinement. In this paper, we formulated this degradation depending on the NB power and quantitatively estimated the degraded effective confinement time. In addition, we performed neutron emission rate simulations using the obtained effective confinement time. The simulation and experimental results were in good agreement, suggesting that the degraded effective confinement time is valid.

Recently, experiments on basic plasma physics issues for solving future problems in fusion energy have been performed on a Large Helical Device. There are several problems to be solved in future devices for fusion energy. Emerging issues in burning plasma are: alpha-channeling (ion heating by alpha particles), turbulence and transport in electron dominant heating helium ash exhaust, reduction of the divertor heat load. To solve these problems, understanding the basic plasma physics of (1) wave–particle interaction through (inverse) Landau damping, (2) characteristics of electron-scale (high- k ) turbulence, (3) ion mixing and the isotope effect, and (4) turbulence spreading and detachment, is necessary. This overview discusses the experimental studies on these issues and turbulent transport in multi-ion plasma and other issues in the appendix.

Sawtooth oscillations in tokamaks frequently lead to the redistribution of energetic ions, mainly on passing orbits, causing their expulsion from the core. This paper discusses the first measurements of the interaction of fast ions and sawteeth in the Large Helical Device. The crashes were caused by the plasma current induced by Electron Cyclotron Current Drive and Neutral Beam Current Drive. Despite these crashes, there was no detectable redistribution effect on fast ions in either the core or at the edge of the plasma.

Electron-scale turbulence, whose wavelength is the electron Larmor radius, is thought to have the potential to cause stiffness in an electron temperature gradient and degrade the confinement of future burning plasma in which the electron heating by alpha particles is dominant. The dependence of electron-scale turbulence and electron heat flux on the electron temperature inverse gradient length $R_\textrm{ax}/L_{T_\textrm{e}}$ , were investigated. The electron temperature gradient was successfully varied in the range of $-3 \lt R_\textrm{ax}/L_{T_\textrm{e}} \lt 12$ by controlling the injection power of on/off-axis electron cyclotron heating. The results show a significant increase in the electron-scale turbulence with increasing $R_\textrm{ax}/L_{T_\textrm{e}}$ , especially in conditions where Electron Temperature Gradient (ETG) instability is linearly unstable, suggesting the presence of ETG turbulence at high $R_\textrm{ax}/L_{T_\textrm{e}}$ . The electron heat flux also increases steeply with increasing $R_\textrm{ax}/L_{T_\textrm{e}}$ . In addition, the electron-scale turbulence is observed even at $R_\textrm{ax}/L_{T_\textrm{e}} \sim 0$ , which is stable in linear GKV calculations. Finding the cause of this phenomenon is an interesting task for the future.

Abstract The preceding propagation of turbulence pulses has been observed for the first time in heat avalanche events during the collapse of the electron internal transport barrier (e-ITB) in the Large Helical Device. The turbulence and heat pulses are generated near the foot of the e-ITB and propagate to the peripheral region within a much shorter time than the diffusion timescale. The propagation speed of the turbulence pulse is approximately 10 km/s, which is faster than that of the heat pulse propagating at a speed of 1.5 km/s. The heat pulse propagates at approximately the same speed as that in the theoretical prediction, whereas the turbulence pulse propagates one order of magnitude faster than that in the prediction, thereby providing important insights into the physics of non-local transport.

Recently an improved confinement regime, characterized by reduced turbulent fluctuations has been observed in the Large Helical Device upon the injection of boron powder into the plasma (Nespoli et al 2022 Nat. Phys. 18 350–56). In this article, we report in more detail the experimental observations of increased plasma temperature and the decrease of turbulent fluctuations across the plasma cross section, on an extended database. In particular, we compare powders of different materials (B, C, BN), finding similar temperature improvement and turbulence response for the three cases. Modeling of the powder penetration into the plasma and of neoclassical electric field and fluxes support the interpretation of the experimental results. Additionally, we report evidence of the temperature improvement increasing with powder injection rates and decreasing for both increasing density and heating power. Though, plasma turbulence response varies depending on the initial conditions of the plasma, making it difficult to draw an inclusive description of the phenomenon.

Injection of impurities in the form of sub-millimeter powder grains is performed for the first time in the Large Helical Device (LHD) plasma, employing the Impurity Powder Dropper (IPD) (Nagy et al., 2018), developed and built by PPPL. Controlled amounts of boron (B) and boron nitride (BN) powder are injected into the helical plasma. Visible camera imaging, UV and charge exchange spectroscopy measurements show that the injected impurities effectively penetrate into the plasma in two different magnetic configurations. The prompt effects of the impurities on the plasma are characterized as the injection rate is scanned. The injected impurities provide a supplemental electron source, causing the plasma density to increase, together with the radiated power. Beneficial effects on the confined plasma temperature are observed at low plasma densities, due to an increased efficiency in NBI power absorption. For ne,av

Impurity behavior is studied in a variety of LHD (Large Helical Device) long pulse discharges, i.e. standard hydrogen plasmas, super dense core plasmas, helium plasmas with ICH (Ion Cyclotron Frequency Heating), multi-species plasmas mixed with H and He. Density scan experiments show a specific density range of impurity accumulation for only hydrogen discharges. Strong suppression of impurity accumulative behavior is observed in high temperature plasmas with high power heating. The main contributions to impurity transport are extracted by a comprehensive study on impurity behavior, i.e. investigating the critical conditions for impurity accumulation and the parameter dependences. It is found that the impurity behavior is determined by three dominant contributions, i.e. neoclassical transport mainly depending on radial electric field, turbulent transport increasing with heating power and impurity screening at high edge collisionality in the ergodic layer. The mapping of impurity behavior on n-T (electron density and temperature) space at the plasma edge shows a clear indication of the domain without impurity accumulation and provides operation scenarios to build up fusion-relevant plasmas. Keywords: Long pulse discharge, Impurity accumulation window, Radial electric filed, Impurity screening, Turbulent impurity transport

In this paper, we report new results of a beam switching experiment aiming at a reversed magnetic shear profile formation for the study of an MHD-mode-induced profile collapse event. A transient MHD mode, whose oscillation frequency chirps down, is observed. The electron temperature profile collapse is induced by the mode activity, leading to the flattening of the central electron temperature profile. The radial mode structure is the double-odd-parity at the beginning, but it transits to the even-parity in its final stage. The central electron temperature profile recovers after the radial mode structure changes to the even-parity, even though the mode itself does not disappear. [ABSTRACT FROM AUTHOR]

Positive isotope effects have been found in electron cyclotron resonant heating plasma of the Large Helical Device (LHD). The global energy confinement time (τE) in deuterium (D) plasma is 16% better than in hydrogen (H) plasma for the same line-averaged density and absorption power. The power balance analyses showed a clear reduction in ion energy transport, while electron energy transport does not change dramatically. The global particle confinement time (τp) is degraded in D plasma; τp in D plasma is 20% worse than in H plasma for the same line-averaged density and absorption power. The difference in the density profile was not due to the neutral or impurity sources, but rather was due to the difference in the transport. Ion scale turbulence levels show isotope effects. The core turbulence (ρ  =  0.5–0.8) level is higher in D plasma than in H plasma in the low collisionality regime and is lower in D plasma than in H plasma. The density gradient and collisionality play a role in the core turbulence level. [ABSTRACT FROM AUTHOR]

The tongue-shaped deformation (TSD), a possible trigger for the evolution of quasi-stable magneto hydrodynamic (MHD) modes into MHD bursts, was first experimentally identified in the large helical device (LHD) (Ida et al 2016 Sci. Rep. 6 36217). Further analysis has been based on the experimental data in low density (<) and low () hydrogen plasmas Ida et al (2017 Phys. Plasmas24 122502); Ida et al 2018 (Sci. Rep. 8 2804). However, since the mechanism causing the tongue-shaped deformation has not been clarified, it is important to study the characteristics of this nonlinear instability in a more broad spectrum of experiments to stimulate the study in simulation and theory for this event. Identification of tongue-shaped deformations in both hydrogen and deuterium plasmas is automatised and characterisation of the TSDs is done by quantifying their size, timing and location using ECE, RF and magnetic probe diagnostics. The dependence of these characteristics on plasma parameters such as density, toroidal rotation and NBI power is given. [ABSTRACT FROM AUTHOR]

An asymmetric parallel return flow, which modifies the parallel component of the flow, is expected to meet the zero divergence of the flow on a flux surface based on the common neoclassical theory for torus plasma. The full flow structure is measured by charge exchange spectroscopy on the Large Helical Device. Inboard/outboard asymmetry of the parallel flow is observed according to the full flow profile measurement. Flow asymmetry is considered to be induced by the Pfirsch–Schlüter flow closely associated with the radial electric field. A linear relationship between the integrated flow asymmetry and the electric potential difference is obtained in different magnetic fields and configurations. A model based upon the incompressibility of the flow is applied to acquire a geometric factor hB, which only connects to the magnetic configuration from the experiment. The asymmetric component of the parallel flow measured is compared with the asymmetric component of parallel flow calculated in the incompressibility conditions of flow on the magnetic flux surface. The measured asymmetric flow is consistent with the calculation in plasma with a small toroidal torque input in the inward shifted configuration. However, the measured asymmetric flow is significantly smaller than that calculated for plasma with a large toroidal torque or in the outward shifted configuration. One possible explanation for this variation could be radial transport due to anomalous perpendicular viscosity as well as strongly poloidally asymmetric radial flow. [ABSTRACT FROM AUTHOR]

A remarkable extension of the high-ion-temperature (high-Ti) regime was obtained in deuterium plasma experiments in the Large Helical Device. In order to clarify transport characteristics in the ion internal transport barrier (ITB) formation with an isotope effect, a dataset of pure deuterium (nD/ne  >  0.8) and pure hydrogen (nH/ne  >  0.8) plasmas in the high-Ti regime were analyzed, and two mechanisms of transport improvement were characterized. A significant reduction of ion heat transport in the core of both deuterium and hydrogen plasmas was observed, indicating ion ITB formation. The dependence of the ion heat diffusivity on temperature ratio (Te/Ti) and normalized Ti-gradient (R/LTi  =  −(R/Ti)(dTi/dr)) was investigated in the core region, in which gyrokinetic simulations with the GKV code predict the destabilization of ion temperature gradient (ITG) modes. The Te/Ti dependence shows ITG-like property, while a significant deviation from the ITG-like property is found in the R/ dependence, indicating suppression of the ITG mode in the large R/ regime and resultant ion ITB formation. In the comparison between deuterium plasma and hydrogen plasma, the lower transport in the deuterium plasma is observed in both ion and electron heat diffusivities, indicating a significant isotope effect. It was found with the nonlinear turbulent transport simulation with GKV that the zonal flow enhancement contributes to the ITG suppression in the deuterium plasma. [ABSTRACT FROM AUTHOR]

We investigate the penetration threshold of resonant magnetic perturbation (RMP) by the external coils in the Large Helical Device (LHD) for various plasma aspect ratio configurations. The qualitative dependence on the collisionality is opposite to that in a high plasma aspect configuration; this is a quite unique property first found in the LHD. We also investigate the threshold dependence on the ion species, and find that the threshold in deuterium discharges is much smaller than that in hydrogen discharges. In all cases the thresholds are higher as the poloidal rotation becomes faster, which shows that poloidal rotation is the dominant driver to the RMP shielding. This is qualitatively consistent with the torque balance model between the electromagnetic and poloidal viscous torques. In a configuration of the LHD, the dependence of the threshold on the density is qualitatively similar to that in Ohmic tokamak plasmas, but the beta dependence is opposite to that of tokamaks. The difference arises from the cause of the viscous torque. [ABSTRACT FROM AUTHOR]

In this paper, a new criterion for the internal transport barrier (ITB) formation is proposed by defining a unique scalar parameter, the profile gain factor. The profile gain factor shows degree of the confinement improvement with respect to an arbitrary reference temperature profile in the L-mode. As the reference L-mode profile for the large helical device (LHD), the edge ion temperature profile data is extrapolated to the core by the L-mode profile function, which is characterized by the thermal diffusion coefficient being proportional to the local ion temperature. The profile gain factor is defined as the ratio of the ion stored energy experimentally measured to that evaluated from the reference L-mode profile. The proposed method is applied to the LHD experimental data, and its capability for quantification of the ITB strength is demonstrated. [ABSTRACT FROM AUTHOR]

The mode slowing down mechanism of the locked-mode-like instability without a large magnetic island is investigated, based on the LHD experimental analysis. The mode frequency coincides with E  ×  B rotation frequency at the resonant surface and the slowing down is caused by two processes. One is the resonant surface moving to the small E  ×  B rotation frequency region and the other is the slowing down of the E  ×  B rotation frequency near the resonant surface. Both processes are almost the same as those of the locked-mode-like instability with a large magnetic island. The results suggest that the slowing down occurs even though the precursor does not have a large magnetic island. However, when the external RMP is imposed, the mode frequency in the slowing down phase sometimes does not coincide with the E  ×  B rotation frequency. Moreover, the mode amplitude during the slowing down phase increases with the decrease of the mode frequency both with and without the imposed external RMP, which suggests that the instability growth in the slowing down phase is more strongly related to the mode frequency than the E  ×  B rotation frequency because the mode sometimes does not rotate with the E  ×  B rotation. [ABSTRACT FROM AUTHOR]

The isotope effect on impurities and bulk ion particle transport is investigated by using the deuterium, hydrogen, and isotope mixture plasma in the Large Helical Device (LHD). A clear isotope effect is observed in the impurity transport but not the bulk ion transport. The isotope effects on impurity transport and ion heat transport are observed as a primary and a secondary effect, respectively, in the plasma with an internal transport barrier (ITB). In the LHD, an ion ITB is always transient because the impurity hole triggered by the increase of ion temperature gradient causes the enhancement of ion heat transport and gradually terminates the ion ITB. The formation of an impurity hole becomes slower in the deuterium (D) plasma than the hydrogen (H) plasma. This primary isotope effect on impurity transport contributes the longer sustainment of the ion ITB state because the low ion thermal diffusivity can be sustained as long as the normalized carbon impurity gradient R/Ln,c, where , is above the critical value (∼−5). Therefore, the longer sustainment of the ITB state in the deuterium plasma is considered to be a secondary isotope effect due to the mitigation of the impurity hole. The radial profile of H and D ion density is measured using bulk charge exchange spectroscopy inside the isotope mixture plasma. The decay time of H ion density after the H-pellet injection and the decay time of D ion density after D-pellet injection are almost identical, which demonstrates that there is no significant isotope effect on ion particle transport. [ABSTRACT FROM AUTHOR]

Exhaust of turbulence cloud at the tongue-shaped deformation which triggers MHD bursts is observed in the Large Helical Device in the low density plasma with significant contribution of trapped particles injected by perpendicular neutral beam injection. The exhaust of turbulence cloud is characterized by the abrupt large increase of turbulence amplitude in the frequency range of 150–500 kHz measured with Doppler reflectometer at the edge region of the plasma (). The increase of turbulence amplitude is significantly large and is by one order of magnitude. This abrupt increase of turbulence level is transient and disappears within one milli-second (typically  ∼600 μs). In contrast, the turbulence level slightly inside the plasma edge () decreases by a factor of 2 after the MHD bursts. [ABSTRACT FROM AUTHOR]

The deuterium (D) operation was initiated in the LHD in 2017. In the first campaign of the D experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature (Ti) of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The thermal confinement characteristics of ITB plasmas were compared between hydrogen and D discharges. The effective ion thermal diffusivity of the ion-ITB plasmas was found to be smaller in the D discharges than that in the H discharges. The profiles of the Ti, the electron density, and the impurity of the high Ti plasmas strongly depended on the magnetic configuration and these profiles tended to peaked in the inward-shifted configuration. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case. The GKV simulation showed the linear growth rate of TEM/ITG reduced in the plasmas with D both for the ion ITB and the electron ITB plasmas and qualitatively agreed with the tendency of the change in the thermal diffusivity obtained from the power balance analysis. [ABSTRACT FROM AUTHOR]

The behavior of carbon impurities in deuterium plasmas and its impact on thermal confinement were investigated in comparison with hydrogen plasmas in the Large Helical Device (LHD). Deuterium plasma experiments have been started in the LHD and high-ion-temperature plasmas with central ion temperature (Ti) of 10 keV were successfully obtained. The thermal confinement improvement could be sustained for a longer time compared with hydrogen plasmas. An isotope effect was observed in the time evolution of the carbon density profiles. A transiently peaked profile was observed in the deuterium plasmas due to the smaller carbon convection velocity and diffusivity in the deuterium plasmas compared with the hydrogen plasmas. The peaked carbon density profile was strongly correlated to the ion thermal confinement improvement. The peaking of the carbon density profile will be one of the clues to clarify the unexplained mechanisms for the formations of ion internal transport barrier and impurity hole on LHD. These results could also lead to a better understanding of the isotope effect in the thermal confinement in torus plasma. [ABSTRACT FROM AUTHOR]

Surveys of the ion and electron heat transports of neutral beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma at LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality. In low collisionality, with low density and high heating power, an ion internal transport barrier (ITB) was formed. The ion heat conductivity (χi) is lower than electron heat conductivity (χe) in the core region at ρ  <  0.7. On the other hand, in high collisionality, with high density and low heating power, χi is higher than χe across the entire range of plasma. These different confinement regimes are associated with different fluctuation characteristics. In ion ITB, fluctuation has a peak at ρ  =  0.7, and in normal confinement, fluctuation has a peak at ρ  =  1.0. The two confinement modes change gradually depending on the collisionality. Scans of concentration ratio between He and H were also performed. The ion confinement improvements were investigated using gyro-Bohm normalization, taking account of the effective mass and charge. The concentration ratio affected the normalized χi only in the edge region (ρ ~ 1.0). This indicates ion species effects vary depending on collisionality. Turbulence was modulated by the fast ion loss instability. The modulation of turbulence is higher in H rich than in He rich plasma. [ABSTRACT FROM AUTHOR]

A simultaneous high ion temperature (Ti) and high electron temperature (Te) regime was successfully extended due to an optimized heating scenario in the LHD. Such high-temperature plasmas were realized by the simultaneous formation of an electron internal transport barrier (ITB) and an ion ITB by the combination of high power NBI and ECRH. Although the ion thermal confinement was degraded in the plasma core with an increase of Te/Ti by the on-axis ECRH, it was found that the ion thermal confinement was improved at the plasma edge. The normalized ion thermal diffusivity at the plasma edge was reduced by 70%. The improvement of the ion thermal confinement at the edge led to an increase in Ti in the entire plasma region, even though the core transport was degraded. [ABSTRACT FROM AUTHOR]

Response of the plasma toroidal flow to the forward and backward transition between the nested and the stochastic magnetic field is studied using the charge exchange spectroscopy in the large helical device (LHD). Abrupt damping of toroidal flow associated with a transition from nested magnetic flux surface to a stochastic magnetic field is observed when the magnetic shear at the rational surface decreases to 0.5 after the exchange of the neutral beam injection (NBI) direction from co- to counter-direction in LHD. The stochastization of magnetic field occurs only in a narrow range of magnetic shear near 0.5 and spontaneous back-transition from stochastic to nested magnetic field (healing) is observed in the steady-state phase of magnetic shear. When the NBI direction is changed from counter- to co-direction, the healing of magnetic field occurs associated with the increase of magnetic shear. [ABSTRACT FROM AUTHOR]

Radial profiles of bulk ion and impurity ions density are simultaneously measured and quantified using charge exchange spectroscopy and peaking parameter, . The peaking parameter is positive for inward and negative for outward convection in a plasma with an impurity hole in the Large Helical Device. Following the formation of an impurity hole associated with the transition from L-mode to ion ITB plasma, the bulk ion becomes peaked by the inward convection , while impurities (helium and carbon) become hollow due to the outward convection . In contrast to the ion ITB plasma, in the L-mode plasma the impurity profiles are peaked , where the bulk ion density profile remains flat . Understanding of the impurity behavior in the ion ITB plasma could lead to a self cleaning plasma reactor concept, with an efficient impurity exhaust. [ABSTRACT FROM AUTHOR]

We report the first observation of the formation of a magnetic island before the occurrence of mode locking in helical plasma. New analysis and observation techniques applied to the ECE signal and poloidal flow in LHD experiments yield the following results. (i) A magnetic island structure is present, rotating at the end of the rotating phase. (ii) The rotation speed of the island is not uniform in space and time. The rotation of the island changes significantly at the end of the rotating phase, and the deformation increases until the mode is locked. [ABSTRACT FROM AUTHOR]

New observations in the large helical device (LHD) show that the magnetic islands externally imposed by m/n  =  1/1 resonant magnetic perturbation (RMP) can be maintained in an intermediate state with a finite phase shift away from the value present in vacuum. Given the previous experimental observation that the saturated magnetic islands show either growth or healing, the intermediate states are realized in the ‘healing region’ in the beta and collisionality space, which implies that a parameter other than beta and collisionality should exist in order to determine the island state. Theories based on the competition between electromagnetic torques and poloidal flow-induced viscous torques provide a prediction for the intermediate state. These two types of torques might be balanced to realize the steadily maintained intermediate state whereas the islands are placed in the growth state or healing state in the case in which the balance is broken. The experimental observation shows that there is a possibility for the magnetic island phase to deviate from its designed position. If the parameters are controlled properly, it is possible to control the phase of the magnetic island, which may permit continued utilization of the island divertor concept. [ABSTRACT FROM AUTHOR]

Strong suppression of impurity accumulation is observed in long pulse hydrogen discharges with high power NBI (neutral beam injection) heating (Pnbi  >  10 MW) on the large helical device (LHD), even in the impurity accumulation window where the intrinsic impurities such as Fe and C are always accumulated into the plasma core. Density scan experiments in these discharges demonstrate to vanish the window and a new operational regime without impurity accumulation is found in steady state hydrogen discharges. Impurity pinch decreases with increasing ion temperature gradient and carbon Mach number. The peaking of the measured carbon profiles shows strong anti-correlations with the Mach number and its radial gradient. An external torque has a big impact on impurity transport and strong co-current rotation leads to an extremely hollow carbon profile, so-called ‘impurity hole’ observed in high ion temperature modes. Impurity pinch in the plasmas with net zero torque input (balanced NBI injection) is also strongly reduced by increasing ion temperature gradient, which can drive turbulent modes. The combination effect of turbulence and toroidal rotation plays an important role in the impurity transport. [ABSTRACT FROM AUTHOR]

The mitigation of a tracer impurity accumulation in the core region of high-temperature helical plasma was clearly observed by applying electron cyclotron heating (ECH) in the large helical device (LHD). In the LHD, the accumulation of impurities toward the centre of the plasma has been observed in a high-density regime. In this study, for observing clearly the behaviour of impurity ions in the plasma core, the extrinsic ‘tracer’ impurity was injected into that region by means of a tracer-encapsulated solid pellet (TESPEL). The high-density LHD plasma without ECH definitely shows the strong impurity accumulation, and then it causes the reduction in electron and ion temperatures in the core region. When ECH was applied just after the TESPEL injection, the accumulation of the tracer impurity ions was mitigated. Even after ECH was switched-off, the intensities of the line emissions from the highly-ionized tracer impurity were increased very slightly. The micro-turbulence measurement with a 2-dimensional phase contrast imaging diagnostic during ECH does not support the view that the change in the micro-turbulence would enhance the outward flow (an increase in a diffusive flux, a decrease in an inward convective flux and/or a change the direction of the convective flux from inward to outward) of the impurity ions. Moreover, at this moment, there is no conclusive data regarding a radial electric field measured with a charge exchange spectroscopy diagnostic to support the view that the change in the radial electric field would be attributed to the increment in the outward flow of the impurity ions from the core region of the LHD plasma. [ABSTRACT FROM AUTHOR]

The ability to exhaust helium as the fusion born plasma impurity is a critical requirement for burning plasmas. We demonstrate in this paper that resonant magnetic perturbation (RMP) fields can be used to actively manipulate helium exhaust characteristics. We present results from puff/pump studies at TEXTOR as example for a tokamak with a pumped limiter and from the Large Helical Device (LHD) with the closed helical divertor as example for a heliotron/stellarator device. For LHD, the effective helium confinement time is a factor of 7–8 higher in the low and high density regimes explored when compared to TEXTOR discharges. This is attributed to ion root impurity transport which is one particular impurity transport regime assessed experimentally at LHD and which facilitates helium penetration to the plasma core. However, when an edge magnetic island is induced by externally applied RMP fields, is decreased by up to and hence values closer to those of TEXTOR can be established. The combination of TEXTOR and LHD results suggest that a magnetic island induced by the RMP field in the plasma source region is an important ingredient for improving helium exhaust. The reduction in seen is caused by a combination of improved helium exhaust due to an enhanced coupling to the pumping systems, increased outward transport and a reduced fueling efficiency for the helium injected and recycling from the wall elements. [ABSTRACT FROM AUTHOR]

The radial electric field in the plasma edge is studied in the Large Helical Device (LHD) experiments. When magnetic field lines become stochastic or open at the plasma edge and connected to the vessel, electrons are lost faster than ions along these field lines. Then, a positive electric field appears in the plasma edge. The radial electric field profile can be used to detect the effective plasma boundary. Magnetic topology is an important issue in stellarator and tokamak research because the 3D boundary has the important role of controlling MHD edge stability with respect to ELMs, and plasma detachment. Since the stochastic magnetic field layer can be controlled in the LHD by changing the preset vacuum magnetic axis, this device is a good platform to study the properties of the radial electric field that appear with the different stochastic layer width. Two magnetic configurations with different widths of the stochastic layer as simulated in vacuum are studied for low-β discharges. It has been found that a positive electric field appeared outside of the last closed flux surface. In fact the positions of the positive electric field are found in the boundary between of the stochastic layer and the scrape-off layer. To understand where is the boundary of the stochastic layer and the scrape-off layer, the magnetic field lines are analyzed statistically. The variance of the magnetic field lines in the stochastic layer is increased outwards for both configurations. However, the skewness, which means the asymmetry of the distribution of the magnetic field line, increases for only one configuration. If the skewness is large, the connection length becomes effectively short. Since that is consistent with the experimental observation, the radial electric field can be considered as an index of the magnetic topology. [ABSTRACT FROM AUTHOR]

Bifurcation physics of a magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in a large helical device (LHD) and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between the magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport. [ABSTRACT FROM AUTHOR]

Radial profiles of the density ratio of helium to hydrogen ions are measured using charge exchange spectroscopy with a two-wavelength spectrometer in the large helical device. Helium transport at the last closed flux surface (LCFS) and stochastic magnetic field layer outside the LCFS as well as in the core plasma is studied for a wide range of helium fractions, i.e. from hydrogen-dominated plasmas up to helium-dominated plasmas. The helium density profile becomes more peaked and inward convection velocity increases in the hydrogen-dominant plasma, while it becomes flat or hollow and the convection velocity is in the outward direction in the helium-dominant plasmas. The density gradient of helium at the LCFS is twice that of hydrogen and becomes steeper as the hydrogen becomes more dominant. [ABSTRACT FROM AUTHOR]

A resistive interchange mode of the structure (, : poloidal and toroidal mode numbers, respectively) with a bursting character and rapid frequency chirping in the range less than 10 kHz is observed for the first time in the edge region of the net current-free, low beta LHD (Large Helical Device) plasmas during high power injection of perpendicular neutral beams. The mode resonates with the precession motion of helically trapped energetic ions (EPs), following the resonant condition. The radial mode structure is recognized to be similar to that of the pressure-driven resistive interchange mode, of which radial displacement eigenfunction quite localizes around the mode rational surface, and evolves into an odd-type (or island-type) during the late of frequency chirping phase. This beam driven mode is excited when the beta value of helically trapped EPs exceeds a certain threshold. This instability is thought to be a new branch of resistive interchange mode destabilized by the trapped energetic ions. The radial transport, i.e. redistribution and losses, of helically trapped energetic ions induced by the mode transiently generates significant radial electric field near the plasma peripheral region. The large shear of thus generated radial electric field is thought to contribute to the observed suppression of micro-turbulence and transient increases of the temperature of fully ionized carbon impurity ions and electron density, suggesting improvement of bulk plasma confinement. [ABSTRACT FROM AUTHOR]

The discharge scenario of high temperature plasma with a helical configuration has significantly progressed. The increase of central ion temperature due to the reduction of wall recycling was clearly observed. The peaking of the ion heating profile and the reduction of charge exchange loss of energetic ions play an important role for further improvement of ion heat transport in the ion internal transport barrier (ITB) core. The ion ITB and electron ITB have been successfully integrated due to the superposition of centrally focused electron cyclotron heating to the ion ITB plasma, and the high temperature regime of the ion temperature comparable to the electron temperature (Ti ~ Te) has been significantly extended. The width of the ion ITB formed with electron ITB is wider than the width of electron ITB. The positive radial electric field was observed in the integrated ITB plasma by a heavy ion beam probe, while the negative radial electric field was observed in ion ITB plasmas. The ion temperature gradient decreases with the increase of the temperature ratio (Te /Ti). [ABSTRACT FROM AUTHOR]

A correction to the article "Integrated Transport Simulations of High Ion Temperature Plasmas of LHD" by S. Murakami, H. Yamaguchi and A. Sakai that was published in the August 26, 2015 issue is presented.

The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion53 095024), is reviewed. Plasma with high ion and electron temperatures (Ti(0) ∼ Te(0) ∼ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 1019 m−3 and with high electron and ion temperatures (Ti(0) ∼ Te(0) ∼ 2 keV), resulting in 3.36 GJ of input energy, is achieved. [ABSTRACT FROM AUTHOR]