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Enhancement of the output power per gyrotron has been planned in the Large Helical Device (LHD). Three 77-GHz gyrotrons with an output power of more than 1MW have been operated. In addition, a high power gyrotron with the frequency of 154 GHz (1 MW/5 s, 0.5MW/CW) was newly installed in 2012, and the total injection power of Electron cyclotron resonance heating (ECRH) reached 4.6MW. The operational regime of ECRH plasma on the LHD has been extended due to the upgraded ECRH system such as the central electron temperature of 13.5 keV with the line-averaged electron density ne_fir-=1×1019 m-3. The electron thermal confinement clearly improved inside the electron internal transport barrier, and the electron thermal diffusivity reached neoclassical level. The global energy confinement time increased with increase of ne_fir. The plasma stored energy of 530 kJ with ne_fir=3.2×1019 m-3, which is 1.7 times larger than the previous record in the ECRH plasma in the LHD, has been successfully achieved. [ABSTRACT FROM AUTHOR]

Measurement and control of divertor heat flux are a crucial issue for realization of fusion burning plasma. However, time-dependent heat flux is often difficult to be estimated experimentally, because thermal diffusion process in thermal probe tips takes time longer than the discharge duration of present experiments. A new reconstruction method for time dependent heat flux from the measurement of the thermal probe tip temperature is proposed in this paper. The heat flux of the Large Helical Device divertor leg was analyzed by the proposed method as the function of the time for the first time. This indicates that heat flux measurement with thermal probes would become a monitoring tools even for the future fusion devices. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Stable toroidicity-induced Alfvén eigenmodes (TAEs) with low toroidal mode number (n=1 and n=2) were excited by application of alternating magnetic field perturbations generated with a set of electrodes inserted into the edge region of neutral beam injection heated plasmas on the Compact Helical System [K. Nishimura, K. Matsuoka, M. Fujiwara et al., Fusion Technol. 17, 86 (1990)]. The gap locations of TAEs excited by the electrodes are in the plasma peripheral region of ρ>0.7 (ρ is the normalized minor radius) where energetic ion drive is negligibly small, while some AEs are excited by energetic ions in the plasma core region of ρ<0.4. The damping rate of these stable TAEs derived from plasma responses to applied perturbations is fairly large, that is, ∼9% to ∼12% of the angular eigenfrequency. This large damping rate is thought to be dominantly caused by continuum damping and radiative damping. [ABSTRACT FROM AUTHOR]

Extremely hollow profiles of impurities (denoted as “impurity hole”) are observed in the plasma with a steep gradient of the ion temperature after the formation of an internal transport barrier (ITB) in the ion temperature transport in the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. The radial profile of carbon becomes hollow during the ITB phase and the central carbon density keeps dropping and reaches 0.1%–0.3% of plasma density at the end of the ion ITB phase. The diffusion coefficient and the convective velocity of impurities are evaluated from the time evolution of carbon profiles assuming the diffusion and the convection velocity are constant in time after the formation of the ITB. The transport analysis gives a low diffusion of 0.1–0.2 m2/s and the outward convection velocity of ∼1 m/s at half of the minor radius, which is in contrast to the tendency in tokamak plasmas for the impurity density to increase due to an inward convection and low diffusion in the ITB region. The outward convection is considered to be driven by turbulence because the sign of the convection velocity contradicts the neoclassical theory where a negative electric field and an inward convection are predicted. [ABSTRACT FROM AUTHOR]

High-ion-temperature (exceeding 5 keV) hydrogen plasmas have been successfully produced in the Large Helical Device [Iiyoshi et al., Nucl. Fusion 39, 1245 (1999); Motojima et al., Nucl. Fusion 47, S668 (2007)] with the ion heat confinement improvement in the core region. The experimental ion heat diffusivity at the core region is found to be almost independent of the ion temperature, Ti (even decreasing as Ti increases). The neoclassical (NC) ripple transport is suppressed by the ambipolar radial electric field, Er (<0) predicted by NC transport fluxes. The temperature ratio, Ti/Te, is one of the key parameters to reduce the NC ambipolar particle and heat fluxes. Thus, it is suggested that the selective ion heating (making Ti/Te larger) is a plausible approach to further increase Ti. Spontaneous rotation is evaluated in these high-Ti plasmas, in which a co-directed component is recognized at the radial location with a large Ti gradient, in addition to the tokamak-like counter-directed component expected for Er<0. [ABSTRACT FROM AUTHOR]

The change of rotational transform due to noninductive plasma current driven by a negative neutral beam, which has a value typically less than 10% of total rotational transform determined by the external current in the helical coils, is measured in the Large Helical Device with motional Stark effect spectroscopy. Radial profiles of rotational transform are derived from the radial profiles of the polarization angle of the σ component in the Hα line emitted from the high energy hydrogen atom of the beam with four sets of linear polarizers, spectrometers, and CCD detectors. The radial profile of the change in the rotational transform due to the noninductive toroidal current driven by the neutral beam is measured. [ABSTRACT FROM AUTHOR]

A tripolar vortex, three aligned vortices with alternate signs of polarity of rotation, has been observed in a plasma for the first time. The tripolar vortex always appears with a deep density depression in the neutral particles, and the rotation direction of each vortex is opposite to that of the E×B rotation due to the ambipolar electric field. It is shown that a net momentum transfer during the charge-exchange interaction produces an effective force acting on the ions. The present experiment shows that this effective force may dominate the ambipolar-electric field and drive the anti-E × B vortical motion of ions. [ABSTRACT FROM AUTHOR]

A new multiantenna system for large area radio-frequency H- ion source using a filamentless ion source was tested to extract ions from dense plasmas with high rf input power. A 1/6 scale large helical device ion source was equipped inside a plasma chamber with four linear antenna rods. Positive-ion extraction with a single hole extractor was studied in detail, while a negative one only showed preliminary results. The positive-ion current density of 34 mA/cm2 to the collector plate was achieved with a new multiantenna after optimization of the antenna number and source gas pressure, where the number of the antennae was 2 with ∼36 kW of rf input power and with a gas pressure of ∼0.1 Pa with H2. If the external magnetic filter optimized for negative-ion production in the multifilament discharge source is taken off, the current density is estimated to reach about 120 mA/cm2. This corresponded closely to the value measured with a Langmuir probe near the center. Two antennae appear to be required to generate uniform plasmas as well as dense plasmas in the present size of the 35×35 cm2 source. A negative-ion current density of 0.3 mA/cm2 to the collector plate was detected with hydrogen without optimization. This multiantenna rf ion source is expected to be adaptable advantageously to large area positive- and negative-ion sources, owing to not only a filamentless source but also potentially due to uniform plasmas as well as probable low-electron-temperature plasmas, compared to that with a one-turn coil system. [ABSTRACT FROM AUTHOR]

A multiantenna rf ion source is studied. The new antenna system is developed to reduce the rf voltage on the antenna compared to that of the conventional loop antenna, which can reduce high voltage rf breakdown on antennas. The antenna was installed in a 1/6 scale ion source for the neutral beam injector (NBI) (H-) used in large helical devices. The characteristics of the ion density current of the plasma produced by using the segmented antennas were studied. A dense plasma with ion saturation current density of 92 mA/cm2 was generated at 45 kW of rf power at 9 MHz for the two-antenna system. At around 10 kW of rf power the plasma density jumped and became a high density mode. The threshold power for the density jump seems to be shifted by the antenna segments and the rf frequency. The current density of the positive ion beam with a small accelerator voltage is 26 mA/cm2 for 10.3 keV of extraction energy, 45 kW of rf power, and 0.16 Pa of plasma source pressure of hydrogen. This multiantenna rf source could potentially be scaled up to the large NBI source. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Effects of carbon impurities on the thermal confinement properties were discussed for carbon-pellet-injected high-Ti discharges in the Large Helical Device (LHD). To clarify their role, the amounts of carbon impurities introduced into plasmas were scanned by varying the number of injections in a discharge or the size of the pellet, and the changes of thermal confinement properties with these variations were examined. In all cases, strong correlations between the densities of carbon impurities and the thermal diffusivities of plasmas were found. Thermal diffusivities were small when the carbon densities were greater than a certain value, e.g. ∼1.5 × 1017 m3 at r/a ≈ 0.72, and the thermal diffusivity degraded drastically below this density. This indicates the existence of a threshold density of carbon impurity for an improved confinement in plasmas. Combined with the formation of the impurity hole, the variation of confinement property with carbon density can explain the degradation of ion temperature in high-Ti discharges with carbon pellet injection in the LHD. [ABSTRACT FROM AUTHOR]

The wall conditioning using radio frequency (RF) plasma was carried out using Ion Cyclotron Range of Frequency (ICRF) heating and/or Electron Cyclotron Resonance Heating (ECRH) with the working gas of helium under the established confinement magnetic field. After sufficient numbers of repetitive wall discharge conditioning (DC, ICDC for ICRF and ECDC for ECRH), the formation of the parabolic electron density profile and the increase of the central ion temperature T i were observed. There was no difference in the attained central T i of the NBI discharge just after the conditioning using a similar value of input energy regardless of the applied RF source. Thus, it is concluded that both ICDC and the ECDC are effective for the higher T i plasma production under the established magnetic field in the Large Helical Device. The effective RF wall conditioning scenarios are also investigated for high T i plasma production. [ABSTRACT FROM AUTHOR]