Search

The shape of a plasma meniscus is a key factor to determine the beam focusing. The physics model of the meniscus formation for hydrogen negative ion sources has not been established yet. A backward trajectory calculation based on experimental observation is performed in order to derive the particle information at the meniscus. It is observed that the negative ion density is spatially nonuniform in the direction parallel to the magnets for suppression of co-extracted electrons. A nonuniformity of the negative ion density in the vicinity of the meniscus is taken into account in the forward trajectory calculation. It reveals that the nonuniform negative ion distribution leads to degradation of the beam focusing and the beam splitting in phase space. The importance of the spatial distribution of negative ions on meniscus modelling is discussed with a comparison to uniform extraction model.

The extraction mechanism of negative ion beams is affected by the behavior of positive and negative ions near the extraction apertures. Flow of the charged particles in a hydrogen discharge were measured with a newly developed directional photodetachment probe which has a single probe tip. Compared with our previous multiple-tip directional probe, the single-tip directional probe was able to measure fine flow structure of the positive and the negative ions without spatial ambiguity associated with electron deflection magnetic field and density gradient. The single-tip probe observed the turning of the flow of the negative hydrogen ion by increasing the bias voltage in the extraction region of the negative ion source.

An isotope effect of negative ion motion in plasma near a plasma grid (PG), which is a plasma-beam boundary grid, has been investigated in a negative hydrogen-ion source with the surface production process of the negative ion on the PG surface. Negative deuterium-ion (D-) density was higher than negative hydrogen-ion (H-) density in the condition without extraction-grid bias (V egb) and approached the H- density in the condition with V egb. Thus, the D- density responded stronger than the H- density to the V egb. The density of the negative hydrogen-ion isotopes and the density response to the V egb have been organized into respective identical trends by momenta of isotopes emitting from the PG. The Larmor motion can be a dominant mechanism of the negative ion transport from the PG to the plasma.

It is reported from the measurement of a negative ion beamlet emittance that the characteristic three-Gaussian components are observed in the emittance diagram. The negative ion trajectories are calculated using 3D PIC-MCC simulation in order to clarify the origin of this complicated phase space structure and underlying physical mechanism. The characteristic three-Gaussian components can be reproduced in the emittance diagram from the simulation result. It is verified that the Gaussian components are caused by the negative ions extracted from the different plasma meniscus region, that is, the central region or the region near the edges of the meniscus.

Flows of the charged particles in hydrogen (H) and deuterium (D) plasmas were measured with the single-tip directional Langmuir probe. The flow patterns of positive and negative ions have been constructed from the data by moving the probe near the plasma grid. The positive ion flow from driver to extraction regions showed the different velocity equivalent to the mass difference between H and D. The negative ion flow indicated the surface production of negative ions on the plasma grid, and changed the direction corresponding to the magnitude of the extraction electric field.

A heating source with off-axis electron cyclotron heating (ECH) alone produced a plasma with a quasi-steady-state hollow electron-temperature profile in the Large Helical Device. The clear formation of this quasi-steady-state hollow electron-temperature profile can be explained by adding the outward heat convection term to the diffusion term, as a simple model to describe the electron heat flux, using the energy conservation equation. In addition, we directly observed the non-locality of the non-diffusive (convective) contribution in transient electron thermal transport in the condition that power-modulated on-axis ECH was applied to the plasma sustained by off-axis ECH. The experimentally evaluated flux-gradient relation shows two different positive values of the electron heat flux at zero temperature gradient by going back and forth between positive and negative temperature gradient regions in the transport hysteresis phenomenon.

In order to investigated the dynamics of H− ions and understand the extraction process inside filament-arc-driven plasmas in a Cs-seeded negative ion source, diagnostic experiments using a directional Langmuir probe combined with photodetachment measurement have been conducted. Two-dimensional flow pattern of H− ion flow has been obtained as well as the profile of H− ion density. The result shows that H− ions come from the direction of plasma grid surface, flow to the plasma in the extraction region. During beam extraction, H− ion flow turns to the beam direction. The stagnation point of H− ion flow is at ∼20 mm apart from the plasma grid. Around the stagnation point, the maximum decrement of H− density caused by beam extraction appears. The results indicate that H− ions are extracted from the plasma volume. The extraction process occurs in a region near the plasma grid aperture with boundary of ∼20 mm apart from the plasma gird.

Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m2. We have clearly found the lower limit of electron current ratio depending on the arc discharge power. Certainly, the extracted electron current ratio increased in deuterium operation. However, we have had no trouble on the beam line components.Deuterium beam operations on neutral beam injectors (NBI) have been carried out successfully in the Large Helical Device (LHD) in 2017. We injected a total of 31 MW neutral beam power, which gave us a new operation region for high ion temperature plasma. Upgrading the beam energy from 40 keV to 60 keV and then to 80 keV for deuterium operation in a positive ion based NBI (P-NBI) of BL4 and BL5, respectively, we have achieved 10.6 MW (75 keV) beam injection in the P-NBI. We confirmed a good arc efficiency (extracted beam current/arc power) with 83% high deuteron ratio which is the same as the hydrogen operation in the positive source. We also extracted deuterium ion beam from three negative-ion based NBIs (N-NBI) without increase beam energy, although the injection power decreased by less than half. The averaged arc efficiency of the deuterium negative ion is approximately 66% of the hydrogen case, and the current density of the accelerated beam current reached 190 A/m2. We have clearly found the lower lim...

Recent advances in the high power and steady state heating system and experiment results of the Large Helical Device (LHD) are reviewed in this paper. Plasma performance is extended largely through...

Thermal probe with double function of thermocouples and Langmuir probe has been developed, and the initial data observed in far-SOL in QUEST is obtained. Heat flux of megawatt per square meters related to energetic electrons and sonic plasma flow in far-SOL have been observed in the current rump-up phase although no high power inductive force like ohmic winding is applied. The heat flux and the flow are suppressed after the current is built up. In the quasi-steady state, plasma current starts and keeps sawtooth-like oscillation with 20 Hz frequency. The heat flux and the flow in far-SOL have clear responses to the oscillation.

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 Ti were observed. There was no difference in the attained central Ti 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 Ti plasma production under the established magnetic field in the Large Helical Device. The effective RF wall conditioning scenarios are also investigated for high Ti plasma production.

The resistive interchange mode destabilized by the resonant interaction with the trapped energetic ions is fully suppressed when the injected power of electron cyclotron heating exceeds a certain threshold. It is shown for the first time that the complete stabilization of the energetic-particle-driven mode without relaxing the energetic particle (EP) pressure gradient is possible by reducing the radial width of the eigenmodes δ_{w}, especially when δ_{w} narrows to a small enough value relative to the finite orbit width of EP.

Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfven eigenmodes (GAEs) have been made using a second harmonic x-mode electron cyclotron heating and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAEs destabilized by fast ions of neutral beam injection with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase in EC-driven plasma current. These results indicate that magnetic shear plays a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfven spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current leads to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward the core. Steady ECH is also found experimentally to be effective for controlling the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs, decreases with an increase in the ECH power under fixed density conditions.

The properties of cesium (Cs) recycling due to back-streaming ions have been investigated using an optical emission spec-troscopy in the hydrogen negative ion (H−) source with a slot-type grounded grid (GG). The slot-type GG performed well to enhance the beam performance, and to reduce the thermal loading on GG by high transparency. We clearly observed increase of Cs optical emission intensity during beam extraction owing to the increase of the Cs ions sputtered from the back plate of the source due to the back-streaming positive hydrogen ions. Increase of Cs is closely related with the extracted H− current, but it does not depend on the beam energy. Recycling Cs from the back plate is deeply relevant to the perveance condition of the H− beam, and is minimized at the optimum perveance. Strong Cs recycling from the back plate owing to the back-streaming ions is promoted in the high perveance condition with large divergence, which is consistent with the numerical calculation for the distribution of back str...

Total power of 16 MW has been successfully delivered to the plasma confined in the Large Helical Device (LHD) from three Neutral Beam Injectors (NBIs) equipped with negative hydrogen (H−) ion sources. However, the detailed mechanisms from production through extraction of H− ions are still yet to be clarified and a similar size ion source on an independent acceleration test bench called Research and development Negative Ion Source (RNIS) serves as the facility to study physics related to H− production and transport for further improvement of NBI. The production of negative-ion-rich plasma and the H− ions behavior in the beam extraction region in RNIS is being investigated by employing an integrated diagnostic system. Flow patterns of electrons, positive ions and H− ions in the extraction region are described in a two-dimensional map. The measured flow patterns indicate the existence a stagnation region, where the H− flow changes the direction at a distance about 20 mm from the plasma grid. The pattern also suggested the H- flow originated from plasma grid (PG) surface that turned back toward extraction apertures. The turning region seems formed by a layer of combined magnetic field produced by the magnetic filter field and the Electron-Deflection Magnetic (EDM) field created by magnets installed in the extraction electrode.

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)

Recent research results on negative-ion-rich plasmas in a large negative ion source have been reviewed. Spatial density and flow distributions of negative hydrogen ions (H - ) and positive hydrogen ions together with those of electrons are investigated with a 4-pin probe and a photodetachment (PD) signal of a Langmuir probe. The PD signal is converted to local H - density from signal calibration to a scanning cavity ring down PD measurement. Introduction of Cs changes the slope of plasma potential local distribution depending upon the plasma grid bias. A higher electron density H 2 plasma locally shields the bias potential and behaves like a metallic free electron gas. On the other hand, the bias and extraction electric fields penetrate in a Cs-seeded electronegative plasma even when the electron density is similar. Electrons are transported by the penetrated electric fields from the driver region along and across the filter and electron deflection magnetic fields. Plasma ions exhibited a completely di ff erent response against the penetration of electric fields.

Background β-caryophyllene (BCP) is a common constitute of the essential oils of numerous spice, food plants and major component in Cannabis. The present study investigated the contribution of peripheral cannabinoid (CB) and opioid systems in the antinociception produced by intraplantar (i.pl.) injection of BCP. The interaction between peripheral BCP and morphine was also examined. Methods The antinociceptive effect of i.pl. BCP was assayed by the capsaicin tests in mice. Antagonists for CB and opioid receptors, and antisera against β-endorphin were injected peripherally prior to i.pl. injection of BCP. Morphine in combination with BCP was injected subcutaneously or intrathecally. Results The i.pl. injection of BCP dose-dependently attenuated capsaicin-induced nociceptive response. The antinociceptive effect produced by BCP was prevented by pretreatment with AM630, a selective CB2 receptor antagonist, but not by AM251, a selective CB1 receptor antagonist. Pretreatment with naloxone, an opioid receptor antagonist, and β-funaltrexamine, a selective μ-opioid receptor antagonist, reversed the antinociceptive effect of BCP. Pretreatment with naloxone methiodide, a peripherally acting antagonist for opioid receptors and antisera against β-endorphin, resulted in a significant antagonizing effect on BCP-induced antinociception. Morphine-induced antinociception was increased by a low dose of BCP. The increased effect of morphine in combination with BCP was antagonized significantly by pretreatment with naloxone. Conclusions The present results demonstrate that antinociception produced by i.pl. BCP is mediated by activation of CB2 receptors, which stimulates the local release from keratinocytes of the endogenous opioid β-endorphin. The combined injection of morphine and BCP may be an alternative in treating chemogenic pain.

The neutral beam injection (NBI) system was designed to provide plasma heating and current drive for high performance and long pulse operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) device using two co-current beam injection systems. Each neutral beam injection system was designed to inject three beams using three ion sources and each ion source has been designed to deliver more than 2.0 MW of deuterium neutral beam power for the 100-keV beam energy. Consequently, the final goal of the KSTAR NBI system aims to inject more than 12 MW of deuterium beam power with the two NBI for the long pulse operation of the KSTAR. As an initial step toward the long pulse (∼300 s) KSTAR NBI system development, the first neutral beam injection system equipped with one ion source was constructed for the KSTAR 2010 campaign and successfully commissioned. During the KSTAR 2010 campaign, a MW-deuterium neutral beam was successfully injected to the KSTAR plasma with maximum beam energy of 90 keV and the L-H transition was observed with neutral beam heating. In recent 2011 campaign, the beam power of 1.5 MW is injected with the beam energy of 95 keV. With the beam injection, the ion and electron temperatures increased significantly, and increase of the toroidal rotation speed of the plasma was observed as well. This paper describes the design, construction, commissioning results of the first NBI system leading the successful heating experiments carried in the KSTAR 2010 and 2011 campaign and the trial of 300-s long pulse beam extraction.

We propose Multi Gate Oxide — Dual Work-Function (MGO-DWF)-MOSFET which is suitable for low power AB-class RF power amplifier (RF PA). This was examined for the first time by comparing with a standard Cascode connection circuitry composed of LV- and HV- MOSFETs. Dramatically improved FMAX (150 GHz) with sufficient drain break-down voltage (VBD) was experimentally confirmed in a practical device structure. MGO-DWF-MOSFET has multiple roles in a unit device such as LV-MOSFET in source side regions and HV-MOSFET in drain side regions. This distinctive structure enables the reduction of the device area and a gate capacitance (CG) with a higher transconductance (Gm) and the suppression of drain conductance (GDS). Enhancement of FMAX, in other words, DC operation current reduction is achieved at a given operation point. This indicates that MGO-DWF MOSFET is advantageous for low power amplifier circuitry applications, typically for RF PA in internet of things (IoT) products.

High-power negative and positive ion-based neutral beam injectors (NBIs) are operated with high reliability in the Large Helical Device (LHD). The total injection power is >20 MW, and such high-pow...

To aid understanding of physics related to fast ions in the Large Helical Device (LHD) with a three-dimensional shape, a comprehensive set of fast-particle diagnostics has been developed. Fast ions...

The systems of charge-exchange spectroscopy (CXS) have been improved on the Large Helical Device (LHD). Pitch-controlled fiber bundles give faster readout time, and double-slit fiber bundles increa...

A unique and reliable method of plasma initiation has been established in the Large Helical Device (LHD) by using neutral beam (NB) injection into vacuum. Since LHD is a superconducting machine, th...

Energetic ion-driven magnetohydrodynamic instabilities such as Alfven eigenmodes (AEs), energetic particle modes (EPMs), and their impacts on energetic ion confinement are being studied on the Larg...

The internal transport barrier (ITB) appears in ion root plasmas with NBI heating, when the Te/Ti ratio is held below unity in LHD. The improvement of transport is observed only in the ion heat transport, not in the electron heat transport nor in the particle transport. During the formation of ion ITB, the region of low ion thermal diffusivity expands toward the plasma center and results in a wide weak ITB, which is in contrast to the narrow strong ITB typically observed in tokamaks (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfven eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic-particle driven magnetohydrodynamic (MHD) instabilities such as global Alfven eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast-ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co-directed probe but also by the counter-directed one. A linear correlation between the response of the co-directed ion flux and the mode amplitude has been found. The radial profile of the response of the co-directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast-ion response is due to a resonant convective oscillation. The ion flux response of the counter-directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co-directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter-directed probe have been discussed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper describes 1) representative results on excitation of energetic-particle mode (EPM) and toroidicity-induced Alfven eigenmode (TAE) and consequent beam-ion losses in CHS, and 2) recent results on beam-ion transport and/or losses while EPMs are destabilized in LHD. Bursting EPMs and TAEs are often excited by co-injected beam ions in the high-beam ion pressure environment and give a significant effect on co-going beam ions in both experiments. It seems that in CHS, resonant beam ions are lost within a relatively short-time scale once they are anomalously transported due to energetic-ion driven MHD modes, whereas unlike CHS, redistribution of beam ions due to energetic-ion driven MHD modes is seen in LHD, suggesting that not all anomalously transported beam ions escape from the plasma.

The neoclassical (NC) transport analyses have been performed to elucidate the plausible approaches towards higher ion-temperature heliotron plasmas. Avoidance of the ripple transport is the key issue, for which the neoclassical ambipolar radial electric field (Er) can be utilized. The ion-root scenario and the electron-root scenario are expected to be effective according to the experimental situation (especially, the temperature ratio between ions and electrons). The impact of the ion mass on the neoclassical ambipolar Er is also investigated to reveal the easier realization of electron-root Er in heavier ion plasmas (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ion heating experiments have been intensively carried out in high- and low-Zeff conditions of Large Helical Device plasmas. In high-Zeff plasmas utilizing neon or argon gus puffing, the ion heating...

In the large helical device (LHD) having three dimensional configuration, Alfven eigenmodes (AEs) destabilized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-monotonic rotational transform (ι/2π) profiles. In a plasma with monotonic ι/2π-profile, core-localized toroidicity-induced Alfven eigenmode (TAE) as well as global one are often observed. With the increase in the averaged toroidal beta value, defined as the ratio of total plasma pressure to toroidal magnetic pressure, core-localized TAE with low toroidal mode number becomes global. In a relatively high beta plasma with monotonic ι/2π-profile, two TAEs with different toroidal mode number often interact nonlinearly and generate another modes through three wave coupling. In a plasma with non-monotonic ι/2π-profile generated by intense counter neutral beam current drive, reversed shear Alfven eigenmode (RSAE) and geodesic acoustic mode (GAM) excited by energetic ions were observed for the first time in a helical plasma. Nonlinear coupling was also observed between RSAE and GAM.

This article reviews 10 years of engineering and physics achievements by the Large Helical Device (LHD) project with emphasis on the latest results. The LHD is the largest magnetic confinement device among diversified helical systems and employs the world's largest superconducting coils. The cryogenic system has been operated for 50,000 h in total without any serious trouble and routinely provides a confining magnetic field up to 2.96 T in steady state. The heating capability to date is 23 MW of NBI, 2.9 MW of ICRF and 2.1 MW of ECH. Negative-ion-based ion sources with the accelerating voltage of 180 keV are used for a tangential NBI with the power of 16 MW. The ICRF system has full steady-state operational capability with 1.6 MW. In these 10 years, operational experience as well as a physics database have been accumulated and the advantages of stable and steady-state features have been demonstrated by the combination of advanced engineering and the intrinsic physical advantage of helical systems in LHD. Highlighted physical achievements are high beta (5% at the magnetic field of 0.425 T), high density (1.1 × 1021 m−3 at the central temperature of 0.4 keV), high ion temperature (Ti of 5.2 keV at 1.5 × 1019 m−3), and steady-state operation (3200 s with 490 kW). These physical parameters have elucidated the potential of net-current free helical plasmas for an attractive fusion reactor. It also should be pointed out that a major part of these engineering and physics achievements is complementary to the tokamak approach and even contributes directly to ITER.

A phenomenological probe model was developed for quantitative fast-ion-measurement using a hybrid directional probe (HDP), which is a directional probe to measure ion current and heat flux at the same position. The secondary electrons due to fast-ion-injection onto the probe surface enhances the ion-saturation-current, thus a thermal probe method was required for quantitative measurement of fast ions. The model gives a simple expression of the secondary electron yield as a function of ion currents and heat fluxes measured by the HDP, and can determine the net fast-ion-current without the calibrations of the thermal probe method. The model was applied to the experimental results of fast-ion-measurements using a HDP in CHS, and the secondary electron yield obtained by the model agrees with the calibration results of the HDP. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Second-harmonic electron cyclotron (EC) current drive experiments have been performed in the Compact Helical System (CHS). The driven current changes its direction according to the change of the EC...

The internal structural measurements of electric field and density using twin heavy ion beam probes have been performed to elucidate the nonlinear evolution of the magneto-hydro-dynamic (MHD) bursty phenomenon driven by the interaction with high-energy particles in a toroidal plasma. The results have given the finest observation of the internal structure of plasma quantities, such as electric field, density and magnetic field distortion, which nonlinearly develop during the MHD phenomenon. In particular, the finding of a new kind of oscillating zonal flow driven by interaction between energetic particles and MHD modes should be emphasized for burning state plasmas.

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. High temperatures of approximately 1–2 keV, and steady-state plasma sustainment operations have been reported. Recently the steady-state operation regime was greatly extended in the Large Helical Device (LHD). A high-temperature plasma was created and maintained for 54 min with 1.6 GJ in the 2005FY experimental programme. The three-dimensional heat-deposition profile of the LHD helical divertor was modified, and during long-pulse discharges it effectively dispersed the heat load using a magnetic axis swing technique developed at the LHD. A sweep of only 3 cm in the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies and partially by electron cyclotron of fundamental resonance frequency. By accumulating the small flux of charge-exchanged neutral particles during the long-pulse operation, a high energy ion tail which extended up to 1.6 MeV was observed. This is the first experimental evidence of high energetic ion confinement of MeV range in helical devices. The long-pulse operations lasted until a sudden increase in radiation loss occurred, presumably because of metal wall flakes dropping into the plasma. The sustained line-averaged electron density and temperature were approximately 0.8 × 1019 m−3and 2 keV, respectively, at a 1.3 GJ discharge (#53776) and 0.4 × 1019 m−3and 1 keV at a 1.6 GJ discharge (#66053). The average input power was 680 kW and 490 kW, and the plasma duration was 32 min and 54 min, respectively. These successful long operations show that the heliotron configuration has a high potential as a steady-state fusion reactor.

An increase in the ion temperature due to transport improvement has been observed in plasmas heated with high-energy negative-ion-based neutral beam injection (NBI), in which the electrons are dominantly heated, in the Large Helical Device. When centrally focused electron-cyclotron-resonance heating is superposed on the NBI plasma, the ion temperature is observed to rise, accompanied by the formation of the electron internal transport barrier (electron-ITB). In the electron-ITB plasmas, an increase in a positive radial electric field is observed, and the transport analysis indicates that the ion transport in the half-radius region is improved with a reduction of the anomalous transport. Thus, this ion temperature rise is ascribed to the ion transport improvement by the transition to the neoclassical electron root. In high-Z plasmas, the ion temperature is increased with an increase in the ion heating power and reaches 13.5 keV. The central ion temperature increases with an increase in a gradient of the electron temperature in an outer plasma region of ρ = 0.8, suggesting the ion transport improvement in the outer plasma region induced by the neoclassical electron root. These results indicate the effectiveness of the electron-root scenario for obtaining high-ion temperature plasmas in helical systems.