G. Motojima

H-mode confinement of Heliotron J

The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.

Steady-state operation using a dipole mode ion cyclotron heating antenna and 77 GHz electron cyclotron heating in the Large Helical Device

The steady-state operation of high-performance plasmas in the Large Helical Device (LHD) has progressed since the 2010 IAEA Conference in Korea by means of a newly installed ion cyclotron heating (ICH) antenna (HAS antenna) and an improved electron cyclotron heating (ECH) system. The HAS antenna can control the launched parallel wave number and heat the core plasma efficiently in the case of dipole mode operation. Understanding of the physics and technology of wave heating, particle and heat flow balances, and plasma?wall interactions in LHD has also improved. The heating power of steady-state ICH and ECH exceeded 1?MW and 500?kW, respectively, and a higher density helium plasma with minority hydrogen ions was maintained using the HAS antenna and new 77?GHz gyrotrons. As a result, plasma performance improved, e.g. electron temperature of more than 2?keV at a density of more than 2???1019?m?3 became possible for more than 1?min. Heat flow balance and particle flux balance of steady-state operation were evaluated. Particle balance analysis indicated that externally fed helium and hydrogen particles were mainly absorbed by the chamber wall and divertor plates, even after the 54?min operation.

Initial experiments towards edge plasma control with a closed helical divertor in LHD

A baffle-structured closed helical divertor (CHD) is being constructed in the Large Helical Device to actively control the edge plasma, which consists of ten discrete modules installed on the inboard side of the torus. At this stage, two of the ten modules have been constructed. In the initial experiments, the performance of the CHD was experimentally investigated, comparing with numerical expectations. During the continuous gas puffing discharge, it was observed that the neutral pressure in the CHD was more than ten times higher than that in the open helical divertor, which agrees well with the numerical simulation. In the high-density regime, an indication of divertor detachment was observed in the CHD, which was caused by the high recycling and the high-density state in the CHD. With a Penning discharge diagnostics, the neutral particle behaviour in the mixture gas discharge was investigated, measuring hydrogen and helium pressures simultaneously. Similar compression properties were observed between two gases in the CHD, although larger recycling character was seen in helium.

Control of non-inductive current in Heliotron J

Non-inductive currents of electron cyclotron heated plasmas have been examined in the helical-axis heliotron device, Heliotron J. The bootstrap and EC currents were separated by comparing experiments with positive and negative magnetic field. The estimated bootstrap current was found to be affected by the magnetic field configuration. It increases with an increase in the bumpy component of the magnetic field spectrum, which agrees well with a neoclassical prediction calculated using the SPBSC code. The EC current driven by oblique launch with respect to the magnetic field strongly depends on the field configuration and the location of the EC power deposition. The EC current is enhanced when the EC power is deposited on the magnetic axis. The maximum EC current is IEC = ?4.6?kA and the current drive efficiency is ? = neRIp/PEC = 8.4 ? 1016?A?W?1?m?2. The flow direction of the EC current depends on the magnetic field ripple structure where the EC power is deposited.

Development of impurity seeding and radiation enhancement in the helical divertor of LHD

Impurity seeding to reduce the divertor heat load was conducted in the large helical device (LHD) using neon (Ne) and krypton (Kr) puffing. Radiation enhancement and reduction of the divertor heat load were observed. In the LHD, the ratio between the total radiated power and the heating power, f rad = Prad/Pheating, is limited up to around 30% in hydrogen plasmas even for high density plasma just below the radiative collapse (ne, bar > 1 × 1020 m−3), where ne, bar is the line averaged density. With Ne seeding, the ratio could be raised to 52% at ne, bar ~ 1.3 × 1019 m−3, albeit with a slight reduction in confinement. f rad ~ 30% could be sustained for 3.4 s using multi-pulse Ne seeding at ne, bar ~ 4 × 1019 m−3. The localized supplemental radiation was observed along the helical divertor X-points (HDXs) which is similar to the estimated structure by the EMC3-EIRENE code. Kr seeding was also conducted at ne, bar ~ 3.1 × 1019 m−3. f rad ~ 25% was obtained without a significant change in stored energy. The radiation enhancement had a slower time constant. The supplemental radiation area of the Kr seeded plasma moved from the HDXs to the core plasma. Highly charged states of Kr ions are considered to be the dominant radiators from the plasma core region.

Formularization of the confinement enhancement factor as a function of the heating profile for FFHR-d1 core plasma design

A quantitative estimation of the confinement enhancement due to the heating profile effect is introduced to the helical fusion DEMO reactor design of FFHR-d1, based on the experimental results of the Large Helical Device. By applying this to the direct profile extrapolation (DPE) method, radial profiles in the reactor are extrapolated from experimental results. In reactor plasmas, the heat deposition profile of alpha heating is expected to be peaked in the core region as in the case of tangential neutral beam (NB) injection on low-density plasmas. The height of the pressure profile normalized by the gyro-Bohm-type parameter dependence increases with the power (~0.6) of the peaking factor of the heat deposition profile, as long as the core confinement degradation observed in low-density plasmas is ignored. According to this observation, the confinement enhancement factor expected under the self-ignition condition ranges from ~1.1 to ~1.7, for example, depending on the used data. Degradation of the global energy confinement observed in high-density NB-heated plasmas is mitigated and the gyro-Bohm-type parameter dependence reappears after introducing the confinement enhancement due to the heating profile effect. Finally, typical example profiles in FFHR-d1 are provided by the DPE method for future analyses.

Effect of magnetic field ripple on electron cyclotron current drive in Heliotron J

Electron cyclotron current drive (ECCD) experiments have been conducted in the helical heliotron device, Heliotron J. A wide configuration scan shows that the electron cyclotron (EC) driven current is strongly dependent on the magnetic ripple structure where the EC power is deposited. As the EC power is deposited on the deeper ripple bottom, the EC current flowing in the Fisch?Boozer direction decreases, and the reversal of directly measured EC driven current is observed. Measurement results using electron cyclotron emission and soft-x ray spectrum diagnostics imply that high-energy electrons are generated for ripple top heating while they are suppressed for ripple bottom heating, indicating that the generation and confinement of trapped electrons have an important role on ECCD. For ripple top heating, the typical ECCD efficiency is estimated as ? = neIECR/PEC = 0.8 ? 1017?A?W?1?m?2 and , where ne is in 1020?m?3, IEC in A, R in m, PEC in W and Te in keV. The normalized ECCD efficiency is found to be independent of the absorbed EC power for both ripple top and bottom heating cases.

Enhancement of helium exhaust by resonant magnetic perturbation fields at LHD and TEXTOR

Sufficient exhaust of helium as a fusion born plasma impurity is a critical requirement for future burning plasmas. We demonstrate in this paper that resonant magnetic perturbation (RMP) fields can be used to actively improve helium exhaust features. We present results from the TEXTOR tokamak with a pumped limiter and from the LHD heliotron with the closed helical divertor. In both devices RMP fields are applied to generate a magnetic island located in the very plasma edge and this magnetic island has a noticeable impact on the helium exhaust. Also, the effect of the intrinsic stochasticity at the X-point of the LHD plasma on helium exhaust is investigated. Reduced helium fueling efficiency accompanied by enhanced outward transport is shown to facilitate enhanced helium exhaust from the system under RMP application. 3-D fluid plasma edge transport and kinetic neutral gas modeling with the EMC3-EIRENE code generally support these experimental findings.

Twenty barrel in situ pipe gun type solid hydrogen pellet injector for the Large Helical Device

A 20 barrel solid hydrogen pellet injector, which is able to inject 20 cylindrical pellets with a diameter and length of between 3.0 and 3.8 mm at the velocity of 1200 m/s, has been developed for the purpose of direct core fueling in LHD (Large Helical Device). The in situ pipe gun concept with the use of compact cryo-coolers enables stable operation as a fundamental facility in plasma experiments. The combination of the two types of pellet injection timing control modes, i.e., pre-programing mode and real-time control mode, allows the build-up and sustainment of high density plasma around the density limit. The pellet injector has demonstrated stable operation characteristics during the past three years of LHD experiments.

Observation of Magnetohydrodynamic Instabilities in Heliotron J Plasmas

Abstract Two kinds of magnetohydrodynamics instability are observed in electron cyclotron heating and neutral beam injection-heated Heliotron J plasmas. One is the pressure-driven interchange modes with m = 2/n = 1 and m = 5/n = 3 and low frequency (fexp < 20 kHz), which are observed in the plasma with a rational surface of ι/2π = 0.5 or 0.65. The other is the energetic ion-driven global Alfvén eigenmodes (GAEs) in the Alfvén frequency range. To identify the observed GAEs, the frequency of the observed mode with shear Alfvén spectra calculated by CAS3D3 is compared. The interchange mode with m = 5/n = 3 and bursting GAE with intense magnetic fluctuations may affect the bulk plasma confinement and energetic ion transport, respectively.