Large Helical Device Plasmas Research Articles

Spectroscopic comparison between 1200grooves∕mm ruled and holographic gratings of a flat-field spectrometer and its absolute sensitivity calibration using bremsstrahlung continuum

A flat-field extreme ultraviolet (EUV) spectrometer with a varied line spacing groove grating (1200 grooves/mm at grating center) has been developed to study the emission spectra from highly ionized medium Z impurities in large helical device (LHD). It covers a wavelength range of 50-500 A using a mechanically ruled grating, which was later replaced by a newly developed laminar-type holographic grating for comparative studies. Differences in spectral resolution, intensities of higher order spectra, and sensitivities of the spectrometer were studied between the two gratings by observing the emission spectra of LHD plasmas. Although the achieved resolution was alike between them, i.e., deltalambda approximately 0.24 A at 200 A, the holographic grating was much superior in suppressing the higher order light than the ruled grating. The relative sensitivity between the two gratings was evaluated using continuum radiation from LHD plasmas. As a result, it was found that the holographic grating has a flat response in the full wavelength range, but the sensitivity of the ruled grating drops sharply below 200 A. A new technique for the absolute calibration of the EUV holographic grating spectrometer was tried by combining the continuum radiation with a branching ratio of C IV lines (3p-3s: 5800 A/3p-2s: 312 A), and an accurate absolute sensitivity has been successfully obtained.

Identification and intensity analysis on forbidden magnetic dipole emission lines of highly charged Al, Ar, Ti and Fe ions in LHD

Forbidden lines arising from magnetic dipole (M1) transitions in 2 s 2 2 p x ( x = 1 – 5 ) ground and 2s2p excited configurations of highly charged Al, Ar, Ti and Fe ions have been observed in the large helical device (LHD). Argon was puffed using a piezo-electronic valve and other metallic materials were injected using an impurity pellet injector into LHD plasma for the observation. The M1 transitions in vacuum ultraviolet (VUV) and visible regions are identified by analyzing their Doppler broadening, temporal intensity behavior and spatial intensity distribution. Wavelengths of these M1 transitions are determined with high accuracy and compared with previous experimental and calculated values. The line intensity ratios of the M1 transition to the allowed (E1) transition such as 2 s 2 2 p 5 P 3 / 2 2 – P 1 / 2 2 / 2 s 2 2 p 5 P 3 / 2 2 – 2 s 2 p 6 S 1 / 2 2 (ArX 5533 Å/165 Å, TiXIV 2118 Å/122 Å and FeXVIII 975 Å/94 Å) and 2s2p P 1 3 – P 2 3 / 2 s 2 S 0 1 – 2 s 2 p P 1 1 (ArXV 5944 Å/221 Å) are measured as a function of electron density. Results on the ArX and ArXV are compared with a simple level-population calculation. As a result, the density dependence of the ratio is well explained mainly by a competition between two decay processes of the collisional de-excitation and the radiative decay due to the forbidden line emission, although a clear discrepancy between the experimental and calculated absolute values appears for the ArX ratio. Contribution of fast proton impact excitation, which is brought by 180 keV neutral beam injection (NBI) for plasma heating, is also analyzed with the calculation for the F-like ions of ArX, TiXIV and FeXVIII.

New 20-Channel Diagnostic for Angle-Resolved Fast Particles Measurements in LHD

A new multi-channel diagnostic for fast particles has been developed and successfully tested on the Large Helical Device (LHD) during 2005-2006 experimental campaign. The number of simultaneously used channels was significantly improved from 2 to 20 channels and additional improvements for noise reduction have been made. Same time location of the diagnostic has been changed that allowed one to make measurements in a much wider range of pitch angles from perpendicular to tangential (90-160 degrees). All these improvements allow one to make time, energy, and angle-resolved measurements of charge exchange neutral particles in a single plasma discharge and to check the presence of fast particles loss-cones from LHD plasma in different heating regimes. This new diagnostic can be a very helpful and powerful tool in studying of fast particle distribution in such a complex helical plasma geometry like the one of LHD. Example data from plasma discharges are presented.

Line analysis of EUV Spectra from Molybdenum and Tungsten Injected with Impurity Pellets in LHD

Spectroscopic data on high-Z materials for impurity diagnostics are important due to its possible use as a plasma facing component in the next generation fusion device. For this purpose molybdenum and tungsten are injected by an impurity pellet injector into the large helical device (LHD) plasmas. Emissions from such highly ionized elements mostly fall in extreme ultraviolet (EUV) and soft X-ray ranges. The EUV spectra in a range of 20-500 A are recorded using a flat-field EUV spectrometer. The observed emissions are identified with the help of its temporal evolution and detailed analysis is done with electron temperature profiles. At high central electron temperature (∼2.2 keV) molybdenum appears as an Al-, Mg- and Na-like ionization stages. Typical examples of identified transitions are Mo XXXI 190.46 A (3 s2 1S-3s3p 3P) and Mo XXXII 176.63 A (3s 2S-3p 2P). For tungsten, on the other hand, three well-separated bands appear in wavelength range of 24-80 A. The transitions around 33 A have been tentatively identified with the help of calculated values. Most of the isolated lines on the top of pseudo-continuum bands around 50 and 60 A are identified, and the wavelengths are compared with previous experimental studies and also with calculated values.

Effect of Neoclassical Transport Optimization on Electron Heat Transport in Low-Collisionality LHD Plasmas

Electron heat transport in the low-collisonality electron cyclotron heating plasma is investigated to clarify the effect of neoclassical transport optimization on the thermal plasma transport in the Large Helical Device (LHD). Five configurations are realized by shifting the magnetic axis position in major radius: 3.45, 3.53, 3.6, 3.75, and 3.9 m. A clear effective helical ripple (which is a quantitative measure of the neoclassical transport optimization) dependency on the enhancement factor of the global energy confinement relative to ISS95 is observed. Local heat transport analyses show a higher electron temperature and a lower heat transport in the neoclassical transport optimized configuration at half the minor radius. The comparisons of the experimental total heat fluxes with that of the neoclassical transport by DCOM/NNW suggest that the neoclassical transport plays a significant role in the heat transport and that the neoclassical transport optimization is effective in improving the plasma confinement in the low-collisionality LHD plasma.

Multi-scale approach to the solution of nonlinear MHD evolution of heliotron plasma

A multi-scale nonlinear magnetohydrodynamic (MHD) evolution scheme is developed for the numerical analysis of a heliotron plasma as beta increases. In this scheme, the fast time scale dynamics is given by the nonlinear MHD code based on the reduced MHD equations and the slow dynamics is carried out with three-dimensional static equilibrium code. The time evolution is calculated iteratively with a linear interpolation technique for the equilibrium quantities. This scheme is applied to the analysis of the Large Helical Device plasma. Self-organization of the pressure profile induced by the interchange mode is obtained.

Remote experiment of ultrashort-pulse reflectometry for large helical device plasmas

An ultrashort-pulse reflectometer (USPR) has been applied to the large helical device plasmas in National Institute for Fusion Science for edge density profile measurement. Remote control system using super science information network has been introduced to the USPR system. The remote console at Kyushu University having graphical user interface is prepared to control the instruments of the USPR via the general-purpose interface bus. The operations such as the adjustment of supply voltage fed to amplifiers and the frequency doubler, timing control of the impulse, data acquisition, and monitoring can be performed from the remote site. The position of transmitter and receiver antennas can also be controlled remotely in order to observe the cutoff layer depending on various plasma conditions. The directly recorded signal by a sampling scope is analyzed and reconstructed by means of the signal record analysis method.

Iron Kα measurement of LHD plasmas using a wide band and compact x-ray crystal spectrometer

X-ray spectra of FeKα transitions have been obtained on the Large Helical Device (LHD) using a wide band and compact x-ray spectrometer. The spectrometer consists of a Johann-type LiF(220) crystal with a curvature of 430mm and a back-illuminated charge-coupled device (CCD) detector with a size of 26.6×6.7mm2. A wide energy range of 6.4–7.0keV can be observed with high brightness and high temporal resolution, which enables us the measurement of Kα transitions from all charge states of Fe ions. An energy resolution of the spectrometer was 10eV at full width at half maximum. Time-developed Kα spectra after injection of Fe-coated impurity pellet were also measured with a time interval of 10ms in the full binning mode of CCD in order to analyze the impurity transport at the central column of LHD plasmas. The system can be modified to have better time response up to 1ms to analyze the ionization and recombination processes after the pellet injection.

Microwave imaging reflectometry in LHD

A multichannel reflectometry with an imaging optical system is under development for the measurement of the electron density fluctuations in the Large Helical Device (LHD). The right-hand cutoff layer is utilized as a reflection surface. The angle of an ellipsoidal mirror installed inside the vacuum chamber is remotely adjustable with the ultrasonic motor in order to optimize the illumination angle for the wider range of the plasma parameters. An oscillation due to density fluctuation was observed using the microwave imaging reflectometry for the first time in LHD plasma experiment.

Development of Integrated Simulation System for Helical Plasmas

Development of an integrated simulation system for helical plasma is described that draws new experimental plans, including those in new devices, and does experimental data analysis from the viewpoint of integrated physics. The integrated simulation system to be developed has a modular structure that consists of modules for calculating magnetohydrodynamic (MHD) equilibrium/stability, transport, and heating. Each module can be selected in accordance with a user’s request and can be combined with other modules. When we want to perform the integrated simulation during the entire plasma duration, a transport module is to be a core module. An integrated tokamak transport code will be extended for the helical configuration and used as a transport module. As the first step of the extension, time evolution of the plasma net current, which is consistent with the three-dimensional MHD equilibrium, is planned to be solved for Large Helical Device plasmas by taking into account the bootstrap current and the beam-driven current.

Radial profile measurement of impurity line emissions using space-resolved 3m vacuum ultraviolet spectrometer in LHD

A space-resolved 3m vacuum ultraviolet (vuv) spectrometer has been developed to measure a radial (vertical) profile of impurity line emissions in the Large Helical Device (LHD). The system consists of a 3m normal incidence spectrometer equipped with a space-resolved slit and a back-illuminated charge-coupled device (CCD) detector, view-angle adjustable mirrors, and a toroidal slit. A full vertical profile of the vuv line radiated from the LHD plasma is observed with a convex cylindrical mirror by extending the view angle. The vertical profiles of intrinsic impurities of carbon (C III–C V) and externally injected impurities of neon (Ne VIII), aluminum (Al XI), and argon (Ar VIII) are successfully obtained with a time interval of 125ms in the VUV range of 500–3100Å. The ion temperature profiles are also measured from the C IV line emitted in the ergodic layer of LHD.

A dependence of carbon impurity transport coefficients on fuel ions in hydrogen and helium plasmas of Large Helical Device

Impurity transport of carbon has been studied using a new method combined carbon pellet injection with high-spatial resolution bremsstrahlung measurement on the Large Helical Device [A. Iiyoshi et al., Fusion Technol. 17, 169 (1990)]. The carbon pellets are injected into a steady phase in neutral beam heated discharges with a standard configuration of Rax=3.6m. The particle transport coefficients (diffusion coefficient D and convective velocity V) are inferred using a diffusive/convective model. The results are compared between hydrogen and helium plasmas. As a result, it is found that the analyzed D has a constant radial profile with almost closed values of 0.2m2∕s in both plasmas. On the other hand, the inward V is required only at the plasma outer region (ρ>0.6) where the electron density gradient exits, and the inward V in helium plasmas (−0.4m∕s at ρ=0.8 and ne∼4.0×1019m−3) is nearly half as much as that in hydrogen plasmas (−0.7m∕s). This difference of the inward V between hydrogen and helium plasmas suggests a dependence on the charge state of fuel ions predicted from neoclassical theory.

High-power and long-pulse injection with negative-ion-based neutral beam injectors in the Large Helical Device

A negative-ion-based neutral beam injection (NBI) system, in which large caesium-seeded negative-ion sources are utilized, has operated reliably in the Large Helical Device (LHD) since it was operational in 1998. The injection power of the hydrogen beam has been increased up to 13.1 MW with three injectors. In one injector with modified ion sources utilizing a multi-slotted grounded grid, the injection power reached 5.7 MW with an energy of 184 keV, which exceeds the designed value of 180 keV and 5 MW. Individual control of the local arc-discharge with independently divided arc and filament power supplies is effective in improving beam uniformity in a large negative ion source. The injection duration has been extended to 120 s, during which the LHD plasma is sustained by the NBI alone with a reduced power of 0.2–0.3 MW using one ion source with a cooled plasma grid. These results demonstrate that the injection performance of the negative-ion-based NBI is comparable to that of the conventional positive-ion-based NBI. The progress of the negative-NBI in the LHD is reviewed in detail from the point of view of the improvement of the negative ion sources.

Abrupt reduction of core electron heat transport in response to edge cooling on the Large Helical Device

Cold pulse inversion in a helical plasma is observed in the Large Helical Device (LHD) and thus the strong non-local effects are evident in the helical device as well as in tokamaks. A hydrogen pellet or tracer encapsulated solid pellet is injected into the edge of the LHD plasmas. A significant rise of the electron temperature is observed in the central region in response to the edge cooling. Transient analysis indicates a heat flux jump despite the absence of a change in the local temperature gradient. The non-local temperature rise takes place in the low density and high temperature regime just as predicted by the TFTR scaling.

Radial structure of edge MHD modes in LHD plasmas with L–H transition

In the Large Helical Device (LHD), edge coherent MHD modes such as m/n = 2/3 or 1/2 (m, n: poloidal and toroidal mode numbers), of which the rational surface is located in the plasma edge region of the magnetic hill, are strongly enhanced after an L–H transition. In this case, an edge MHD mode localized in a region further in than the location of the enhanced MHD modes is often stabilized because of the decrease in the pressure gradient there. Enhancement of the edge MHD mode amplitude stops a substantial rise in the edge pressure gradient after the transition. In an L–H transition plasma, soft x-ray (SX) fluctuations, δIsx, related to edge MHD modes are clearly detected. The radial profiles of the amplitude and the phase differences among the SX-detector channels are sensitively dependent on the m-number. The relative amplitudes of the SX fluctuations, δIsx/Isx, which may reflect the eigenfunction of the MHD mode, rapidly increase towards the plasma edge in which the relevant rational surface exists.

Self-sustained detachment in the Large Helical Device

Self-sustained detachment has been obtained in the Large Helical Device (LHD). Strong hydrogen gas puffing of ∼200 Pa m3 s−1 after a density feedback phase detaches the plasma from the divertor plates with high reproducibility. High electron density of over 1 × 1020 m−3 is sustained without gas puffing until the heating beam stops and a high-density flat top for 2 s has been demonstrated. Throughout the self-sustained detachment phase, the minor radius of the hot plasma column shrinks to ∼90% of the last-closed-flux-surface, which corresponds to the rational surface. This new state has been named the ‘Serpens mode’, for self-regulated plasma edge 'neath the last-closed-flux-surface. Global energy confinement of the Serpens mode is compared with the international stellarator scaling 1995 (ISS95) and the recently established scaling for high-density LHD plasmas (HD scaling), where shrinking confinement volume and shallow penetration of the heating beams are taken into account. Although the energy confinement of the Serpens mode seems deteriorated compared with ISS95, as in the case of high-density attached plasmas, it is consistent with the HD scaling. This suggests that the energy confinement properties of detached plasmas in LHD are similar to those in high-density attached plasmas.

Global confinement scaling for high-density plasmas in the Large Helical Device

Density and power scan experiments have been carried out at various magnetic field strengths on the Large Helical Device (LHD), to investigate the temperature and magnetic field dependences of the thermal diffusivity. In the moderate density regime, the thermal diffusivity shows gyro-Bohm-like parameter dependences. In the high-density and low-temperature regime, on the other hand, the thermal diffusivity increases with the square root of the local electron temperature and decreases with a 1.2 power of the magnetic field strength. The turning point temperature where the weak temperature dependence changes to the gyro-Bohm type is also found to increase with a 0.8 power of the magnetic field strength. Based on the experimental results, dimensionally correct scalings for the energy confinement time, τE, and the plasma stored energy for the high-density LHD plasmas have been derived. Compared with the gyro-Bohm model (τE ∝ (n/P)3/5, where n and P denote the density and the heating power, respectively) and the international stellarator scaling 1995 (τE ∝ n0.51P−0.59), our scaling has a weaker positive dependence on the density together with the mitigated power degradation as τE ∝ (n/P)1/3, owing to the weak temperature dependence of the thermal diffusivity.

Observation of Minor Collapse of Current-Carrying Plasma in LHD

A minor collapse observed in current-carrying plasma has been investigated in Large Helical Device (LHD). The magnetic configuration with high central rotational transform has ι/2π = 1 surface at the core region and is relatively unstable for the m/n = 1/1 mode (here, m and n are the poloidal and toroidal mode number, respectively). When the beam-driven current exceeds a certain value, the m/n = 1/1 mode grows with a growth time of ˜30 ms and causes a sudden drop of the plasma stored energy and the electron temperature, and it also limits the plasma current itself. A local flattening in an electron temperature profile appears just after the minor collapse. The mode does not rotate and stays at the same spatial location. The possibility of pressure- and current-driven magneto-hydro dynamics (MHD) instabilities is discussed.

Observation of the Effect of Energetic Ions on Pellet Ablation in the Large Helical Device

Ablation of a solid hydrogen pellet in hot plasmas was investigated in the Large Helical Device (LHD). The penetration depth of the injected pellets in the experiment was compared to a theoretical model employing ablation due to the heat flux of fast ions as well as thermal electrons. Shallower penetration than that predicted based on the model considering only electrons was observed in LHD plasmas in the presence of highly energetic (up to 180 keV) fast ions due to neutral beam injection (NBI). This discrepancy can be quantified by the contribution of fast ions in terms of the stored energy of fast ions. The ablation model calculation taking into consideration the effect of thermal electrons and fast ions agrees with the experimental results obtained by LHD. Scaling which includes the fast-ion effect on penetration depth in LHD was compared with the wider multi-experiment database (IPADBASE) [L.R. Baylor et al., Nucl. Fusion 37, 445 (1997)].

Two-Dimensional Measurement of Electron Density Profile in the Edge Region of the Large Helical Device Plasma by a Sheet-Shaped Thermal Lithium Beam

In the Large Helical Device (LHD), of which the magnetic surfaces are surrounded by an ergodic magnetic field layer, the two-dimensional profile of electron density was for the first time measured by a newly developed sheet-shaped thermal lithium beam probe. The sheet beam was formed by guiding lithium vapor through three orifices placed in front of an aperture of an oven and injecting the vapor nearly vertically into the LHD plasma. The two-dimensional (2D) image of the LiI line was measured by using a CCD camera with an interference filter for LiI (670.8nm). From the 2D image of LiI, we successfully obtained a two-dimensional profile of electron density in the plasma edge region. c