Search

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.

The Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) is the world’s largest superconducting helical fusion device, providing a scientific research center to elucidate important physics research such as plasma transport, turbulence dynamics, and other topics. Furthermore, many types of advanced diagnostic devices are used to measure the confinement plasma characteristics, and these valuable physical data are registered over the 131,000 discharges in the LHD database. However, it is difficult to investigate the experimental data even though much physical data has been registered. In order to improve the efficiency for investigating plasma physics in LHD, we have developed a new data visualization software, MyView2, which consists of Python-based modules that can be easily set up and updated. MyView2 provides immediate access to experimental results, cross-shot analysis, and a collaboration point for scientific research. In particular, the MyView2 software is a portable structure for making viewable LHD experimental data in on- and off-site web servers, which is a capability not previously available in any general use tool. We will also discuss the benefits of using the MyView2 software for in-depth analysis of LHD experimental data.

Most of the experiments of the LHD (Large Helical Device) Project are repetitive short pulse experiments. That is, a 10 seconds-long plasma discharge experiment is executed every three minutes. Therefore, it is important to grasp the results of the experiment as soon as possible to direct the ongoing experiment. The post analysis tools used for the LHD project provide important information to the scientists during the experiment. The first tool is the AutoAna. The AutoAna runs the programs automatically to maintain the dependency among the analyzed data. Another tool is MyView2. MyView2 is a python-based data visualization tool, and it has been developed mainly for visitors so that they may use MyView2 to retrieve and analyze the necessary data for their study soon after they arrive at NIFS. The graph layout and the data source can be flexibly customized using a GUI operation. In the central control room for the LHD project, approximately 10 computers are located at the center and display the latest results of the ongoing experiment using MyView2. The authors believe these tools are useful for other experiments other than the LHD project, and prove the usefulness by porting them to the KSTAR project.

Verification of carbon density profile measurements with charge exchange spectroscopy of CVI (Δn = 8 - 7, λ = 529.2 nm) using hydrogen and deuterium neutral beams is described. This is a key issue in the study of the comparison of the carbon density profile between hydrogen and deuterium plasma, because the hydrogen neutral beams are injected in the hydrogen plasma and the deuterium neutral beams are injected in the deuterium plasma. By applying the precise beam attenuation calculation using precise atomic data, identical carbon density profile is obtained for the plasma with hydrogen beam and deuterium beam. A Verification of the measurement is performed by comparison of carbon density profile measured with charge exchange spectroscopy and increment of electron density profile just after the carbon pellet injection.

An abrupt onset of the new tongue-shaped deformation of magnetic surface in magnetized plasmas, which was conjectured in since the 1960s but has not been observed, is experimentally identified just before an abrupt onset of a large-scale collapse event. Two novel properties of the event are identified. First, the transition of symmetry of perturbation (rather than a growth of linearly unstable MHD modes) was found to be a key for the onset of abrupt collapse, i.e., the transition of symmetry gives a new route to the collapse from stable state. Second, as a phase-space response of ions, the distortion from Maxwell-Boltzmann distribution of epithermal ions was observed for the first time.

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.

Spatial profile of deuterium/hydrogen (D/H) ratio is observed using the charge exchange emission of bulk deuterium/hydrogen ion and neutral beam perpendicularly injected into the torus plasma. Neutral beam injection (NBI) is turned on and off in order to subtract the deuterium/hydrogen emission from the plasma edge. The number of fitting parameters for the subtracted spectral signal is reduced from 12 to 3 by assuming that the bulk ion temperature and velocity are the same as those of the carbon ions (C6+). The initial result observed in Large Helical Device (LHD) plasma indicates that D/H ratio is constant in radial direction and is the same as that observed from passive spectroscopic measurement when the tangential NBI or the pellet injection are absent.

An impurity transport model based on diffusivity and the radial convective velocity is proposed as a first approach to explain the differences in the time evolution of Al XII (0.776 nm), Al XI (55 nm) and Al X (33.3 nm) lines following Al-injection by laser blow-off between normal confinement discharges and high density H-mode (HDH) discharges. Both discharge types are in the collisional regime for impurities (central electron temperature is 0.4 keV and central density exceeds 1020 m−3). In this model, the radial convective velocity is assumed to be determined by the radial electric field, as derived from the pressure gradient. The diffusivity coefficient is chosen to be constant in the plasma core but is significantly larger in the edge region, where it counteracts the high local values of the inward convective velocity. Under these conditions, the faster decay of aluminium in HDH discharges can be explained by the smaller negative electric field in the bulk plasma, and correspondingly smaller inward convective velocity, due to flattening of the density profiles.

A distorted Maxwell-Boltzmann distribution of epithermal ions is observed associated with the collapse of energetic ions triggered by the tongue shaped deformation. The tongue shaped deformation is characterized by the plasma displacement localized in the toroidal, poloidal, and radial directions at the non-rational magnetic flux surface in toroidal plasma. Moment analysis of the ion velocity distribution measured with charge exchange spectroscopy is studied in order to investigate the impact of tongue event on ion distribution. A clear non-zero skewness (3rd moment) and kurtosis (4th moment –3) of ion velocity distribution in the epithermal region (within three times of thermal velocity) is observed after the tongue event. This observation indicates the clear evidence of the distortion of ion velocity distribution from Maxwell-Boltzmann distribution. This distortion from Maxwell-Boltzmann distribution is observed in one-third of plasma minor radius region near the plasma edge and disappears in the ion-ion collision time scale.

Macroscopic behavior of a high-beta and supersonic plasma flow produced by a magneto-plasma-dynamic arcjet (MPDA) was investigated in the HITOP device. A macroscopic instability was observed as a bulk rotation of the plasma plume eccentrically around the center axis of the vacuum vessel. We investigated whether it was caused by current-driven or pressure-driven instability. A plasma current circulating in the plasma plume was controlled by changing discharge current, magnetic field configuration and mass flow rate and by inserting a copper-mesh grid. It was found that the plasma plume was macroscopically stable when the current extending into the plasma plume was low and the safety factor was more than unity. This indicates that the observed macroscopic instability was mainly caused by a current-driven one.

Low-frequency fluctuations related with a radial electric field are investigated in a magnetized plasma produced by an electron cyclotron resonance. It is observed that flute mode fluctuations are predominantly excited in a range of positive electric fields and drift mode fluctuations are stabilized with an increase of the radial electric-field strength.

Low-frequency fluctuations related with a radial electric field are investigated in a magnetized plasma column produced by the electron cyclotron resonance. Various radial profiles of space potential are formed by biasing a segmented endplate. Radial electric field and its shear are obtained by fitting the 6th order polynomials to the potential profiles measured. Two types of fluctuations are observed in this experiment. The flute mode fluctuations which are strongly excited in a range of large electric-field shear are considered to be a Kelvin-Helmholtz instability. The drift-wave mode is stabilized with an increase in the radial electric field regardless of its sign, which also tends to be stabilized by the ExB drift velocity shear.

A system of beam emission spectroscopy (BES) which has been proposed as a method for the measurement of long wavelength plasma density fluctuations has been applied to compact helical system (CHS). Spatial channels of BES consist of 16 optical fibers with object lenses focused on the plasma minor radius in CHS. Spatial resolution which corresponds to the width of a sightline and the spatial pitch between sightlines are 1.0 and 1.1 cm on the imaging surface, respectively. The hydrogen neutral beam for heating with the acceleration voltage of about 28 keV is used for the probe beam. The avalanche photodiode detectors combined with a 100 kHz low-pass filter are used as photon detectors. The temporal evolution of the inverse density scale length Ln−1 and the density fluctuation were measured simultaneously at the edge region where the beam attenuation was negligible. As an initial result, the rapid increase of the local plasma density gradient near the plasma edge accompanied by a coherent-like density fluctu...

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.

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.

Plasma flow velocity and its shear perpendicular to magnetic-field lines are generated by using the tungsten hot plate concentrically three segmented, which can form the radially-different plasma potential, or radial electric field even in a fully-ionized collisionless plasma. A drift-like instability which exists in the peripheral region, where the density gradient is relatively large, is observed to be suppressed by the perpendicular flow velocity shear in the central region.

A clear transition of edge particle transport was observed for the neutral beam heated plasmas in CHS. The heating power threshold for the transition is about 1 MW for a plasma with 2 × 10 19 m ‐3 average density. The Hα emission drops within 1 msec and the increase of the local edge density at the transition was confirmed by means of YAG Thomson scattering and beam emission spectroscopy. When the heating power is well above the threshold, the transport barrier is maintained for the full duration of NBI heating (100 msec). A clear back transition appears when the heating power is decreased during the discharge.

The bifurcation of magnetic topology is identified by a significant change in the heat pulse propagation properties in magnetized plasmas. Clear evidence of stochastization of the magnetic surfaces near a rational surface is observed in the core plasma with weak magnetic shear in the Large Helical Device by slowly decreasing the magnetic shear and measured by applying heat pulses driven by modulated electron cyclotron heating (MECH). Three topologies of the magnetic flux surfaces (a nested magnetic island and partial and full stochastization) are identified by the patterns of heat pulse propagation observed in the flat temperature region in the plasma. Slow heat pulse propagation exhibiting a non-monotonically increasing delay time is evidence of a magnetic island, while the fast heat pulse propagation observed in the plasma with medium magnetic shear is evidence of the stochastization of the magnetic surfaces. The region with the fast heat pulse propagation varies with a slight change of magnetic shear. There are two types of stochastization of the magnetic surfaces. In one, the stochastization region is localized near the rational surface (partial stochastization) and in the other, the region is extended to the magnetic axis (full stochastization). The appearance of a stochastic magnetic field is not caused by MHD instability. The significant increase of the ratio of electron thermal diffusivity to ion thermal diffusivity is consistent with that expected by stochastization of the magnetic field.

In this study, the effects of resonant magnetic perturbation (RMP) on particle transport are investigated in Large Helical device (LHD). The magnetic configuration is selected to be the outwardly shifted configuration, for which the magnetic axis position (Rax) is 3.9 m. At Rax = 3.9 m, the main plasma is surrounded by a thick ergodic layer, with width of about 30% of the plasma minor radius. The perturbation mode m/n = 1/1, where m and n are poloidal and toroidal mode numbers, is applied. The resonant layer is around the last closed flux surface. With RMP, a region in which both the connection and Kolmogorov lengths are finite and the magnetic field is ergodic forms; this region extends inside the main plasma. In the low-collisionality regime, where νh* < 1 (νh* = 1 is the boundary between the 1/ν and plateau regimes in stellarator/helical neoclassical transport), there is no apparent difference in particle transport with and without RMP. However, in the high-collisionality regime (νh* > 1), a clear difference in particle transport is found. A clear difference in turbulence is also observed, suggesting that turbulence plays a significant role in particle transport in the high-collisionality regime both with and without RMP.

A new fast-ion diagnostic based on the spectroscopy of the Doppler shifted Balmer-alpha line of Deuterium is considered for JT-60SA. Two sight lines, one tangential and the other perpendicular, are examined as candidates. It was found that the tangential one has better sensitivity than the perpendicular one. A trial of the FIDA measurement was performed on JT-60U. Signals due to the tangential-NB injection were obtained. It was found that about the one third of the signal would be due to the active FIDA signal, which was produced by fast-ions and a probe beam, while the remainder would be due to passive signal. The results indicate the FIDA measurement on JT-60SA would be possible with proper background measurements. c

Spontaneous toroidal flows driven by radial electric fields and ion temperature gradients are studied in the Large Helical Device (LHD). The positive radial electric field drives spontaneous flow in the counter-direction at the plasma edge and in the co-direction near the magnetic axis. The difference in the direction of the spontaneous flow between the core and the edge is considered to be due to the difference in the ratio of the helical ripple to the toroidal ripple. It is clearly observed that the ion temperature gradient causes a spontaneous toroidal flow and is expected to be one of the dominant components of the toroidal flows in the high ion temperature discharges in the LHD.

An extremely hollow profile of carbon impurity (denoted as an ‘impurity hole’) is observed associated with an increase in the ion temperature gradient after an impurity injection in the Large Helical Device. The central carbon density drops to 0.3% of the plasma density due to a strong outward convection driven by the ion temperature gradient, while an inward convection is predicted by neoclassical theory. Transport analysis gives a low diffusion coefficient of 0.4 m2 s−1 and an outward convection velocity of 3 m s−1 at half of the minor radius.

An experimental technique to investigate fast ion confinement based on charge exchange spectroscopy of H(alpha)-light was applied to evaluate the confinement property of perpendicular fast ions in large helical device (LHD). Sensitivities of the H(alpha) spectra to the pitch angles of injected neutral beams (NBs) and these to the angle between the sight line of the measurement and NB injection path are examined. The energy dependence of the charge exchange cross section significantly affects the observed spectra since the driving NB is injected perpendicular to the magnetic field lines in the geometry of LHD. The measured spectra are compared to the spectra of GNET simulation results and the simulated spectra agreed well with the experimental measurement when we take into account the contribution of halo neutrals. Although it is difficult to obtain the fast ion distribution functions directly, this technique provides useful experimental data in benchmarking simulation codes.

In order to investigate the formation of toroidal flow in helical systems, both NBI driven flow and spontaneous toroidal flow were observed in Large Helical Device (LHD). The toroidal flow driven by NBI is dominant in plasma core while its contribution is small near plasma edge. The spontaneous toroidal flow changes its direction from co to counter when the radial electric field is changed from negative to positive at plasma edge. The direction of the spontaneous toroidal flow due to the radial electric field near plasma edge is observed to be opposite to that in plasma core where the helical ripple is small.

An active current drive is a promising technique for improving plasma performances by controlling rotational transform and/or magnetic shear profiles in helical devices. A current drive based on electron cyclotron resonance heating is the most appropriate scheme for this purpose in terms of locality of driven current. Optimum conditions for an efficient electron cyclotron current drive (ECCD) in Large Helical Device (LHD) are being investigated using three-dimensional ray-tracing code, which can simulate propagation and power dissipation of electron cyclotron waves with large parallel refractive index. In the present experiment, inversion of directions of driven plasma current corresponding to injected ECCD modes was demonstrated successfully, and the results could be elucidated by the Fisch-Boozer theory. In addition, clear shifts of rotational transform were observed by motional Stark effect polarimetry. Our findings verified that the ECCD can be used as an effective actuator for controlling the rotational transform and magnetic shear profile in LHD.

Plasmas with an electron internal transport barrier (ITB), which is characterized by peaked electron temperature profiles, are obtained in the JT-60U tokamak and in the large helical device (LHD) when the electron cyclotron heating (ECH) is focused on the magnetic axis. The maximum values of R/L Te , where R is the major radius and L Te is the scale length of the electron temperature gradient, are similar for the LHD and JT-60U ITB plasmas. However, there is a clear jump of R/L Te observed in LHD but not in JT-60U in the ECH power scan. This result is consistent with the fact that the trigger mechanism of the electron ITB is the fast transition of the radial electric field from a small negative E r to a large positive E r in LHD and a change of the magnetic shear from positive to negative is required for the formation of the electron ITB in JT-60U. There are also differences in the electron temperature profiles inside the ITB. The flattening of the electron temperature profile inside the strong ITB could be explained by the sharp increase of q values observed in JT-60U, while no flattening of the electron temperature profile is observed in LHD, where the central q values stay low.

The structure of the radial electric field and heat transport at the magnetic island in the large helical device (LHD) are investigated by measuring the radial profile of the poloidal flow with charge exchange spectroscopy and measuring the time evolution of the electron temperature with ECE. A vortex-like plasma flow along the magnetic flux surface inside the magnetic island is observed when the n/m = 1/1 external perturbation field becomes large enough to increase the magnetic island width above a critical range (15–20% of minor radius) in LHD. This convective poloidal flow results in a non-flat space potential inside the magnetic island. The sign of the curvature of the space potential (∂2Φ/∂r2, where Φ is the space potential) depends on the radial electric field at the boundary of the magnetic island. The heat transport inside the magnetic island is studied with a cold pulse propagation technique. The experimental results show the existence of radial electric field shear at the boundary of the magnetic island and a reduction in heat transport at the boundary and inside the magnetic island.