K. Hanatani

First plasmas in Heliotron J

Results obtained in the initial experimental phase of Heliotron J are reported. Electron beam mapping of the magnetic surfaces at a reduced DC magnetic field has revealed that the observed surfaces are in basic agreement with the ones calculated on the basis of the measured ambient field around the device. For 53.2 GHz second harmonic ECH hydrogen plasmas, a fairly wide resonance range for breakdown and heating by the TE02 mode has been observed in Heliotron J as compared with that in Heliotron E. With ECH injection powers up to ≈ 400 kW, diamagnetic stored energies up to ≈ 0.7 kJ were obtained without optimized density control.

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.

Recent Heliotron E physics study activities and engineering developments

Recent studies of transport, magnetohydrodynamic stability, and divertor action on Heliotron E are summarized. A pellet injector and a new diagnostic system are developed. Moreover, the Heliotron groups is conducting research and development on heating and other new systems for the Large Helical Device.

Study of a helical axis heliotron

Optimization studies have been done for the helical axis heliotron configuration. One purpose is to find a configuration suitable for experimental studies of the basic properties of a helical axis heliotron. In the present study, the role of the bumpy field component (toroidal mirror ratio) in MHD stability and neoclassical confinement for this type of configuration is examined. The physical mechanism of the improvement of the neoclassical transport through control of the bumpy field component is clarified. The physics design and current status of the new helical axis heliotron device, Heliotron J, are also described.

Shafranov shift in the low aspect ratio heliotron/torsatron Compact Helical System

The MHD equilibrium properties of neutral beam heated plasmas have been experimentally investigated in the Compact Helical System (CHS)-a low aspect ratio (Ap ~ 5) heliotron/torsatron. This configuration is characterized by a strong breaking of helical symmetry. The radial profiles measured by various diagnostics have shown a significant Shafranov shift due to the plasma pressure. The deviation of the magnetic axis from is vacuum position has become as large as 50% of the minor radius. When the three-dimensional equilibrium code VMEC is used to reconstruct the equilibrium from the experimental data, the result is in good agreement with the experimentally observed Shafranov shift as well as with the diamagnetic pressure in plasmas with β ≤ 1.2% and β0 ≤ 3.3%. This beta values corresponds to half of the conventional equilibrium β limit defined by the Shafranov shift reaching a value of half of the minor radius. Although tangential neutral beam injection causes pressure anisotropies, p||/p⊥ ≤ 3, the description of the equilibrium assuming isotropic pressure is consistent with the experiment

Velocity-space loss regions in toroidal helical systems

Velocity-space loss regions are studied systematically on the basis of the adiabatic invariants J and in a large-aspect-ratio toroidal helical system with an l = 2 helical winding. The adiabatic invariant J is related to bounce motions in a helical ripple and used to study drift surfaces of localized particles, while the adiabatic invariant is related to bounce motions in a toroidal ripple and used to find drift surfaces of blocked particles or transit particles. The velocity-space loss region is determined from the condition whether the drift surfaces cross a limiter radius or not. The transition probability between localized particles and blocked particles is assumed to be unity. – For a configuration with large rotational transform and high shear, or a helical system with a short-pitch helical winding such as Heliotron E, localized particles trapped in helical ripples and with small parallel velocities, v|| 0, are confined in a plasma column by a drift motion due to a strong magnetic-field inhomogeneity. Velocity-space loss regions appear for both v|| > 0 and v|| < 0. On the other hand, for a configuration with weak shear such as the W VII A stellarator, localized particles are lost from the confinement region completely. Then the velocity-space loss region appears in the neighbourhood of v|| 0. – Velocity-space los is strongly dependent on the type of helical system.

Studies of currentless, high-beta plasma in the Heliotron E device

A currentless plasma with a volume-averaged beta value of 2% has been produced with neutral beam heating. Target plasmas were created by second harmonic resonance heating with electron cyclotron waves (150–350 kW and 53.2 GHz) at a magnetic field strength of 0.94 T. Neutral beam injection (23–30 keV and 1.3−2.6 MW) was used to heat the plasma further. MHD stable and unstable high-beta plasmas were observed. The Q-mode plasmas were produced with the help of intense neutral gas puffing. Properties of the MHD activity and confinement of high-beta plasmas are discussed and compared with theoretical studies.

Resonant superbanana and resonant banana losses of injected fast ions in Heliotron E and Wendelstein VII-A: effects of the radial electric field

The role of the radial electric field Er in energetic trapped particle confinement was investigated in nonaxisymmetric toroidal devices with orbit following Monte Carlo beam ion thermalization codes. A comparative study was performed in Wendelstein VII-A (Max-Planck-Institut fur Plasmaphysik, Garching) and in Heliotron E (Kyoto University) to explain the efficient heating achieved with perpendicular neutral beam injection. Re-entering of fast ions was examined in the presence of the Er field with multiple assumptions on the loss boundary of particles. Monte Carlo simulations showed that an inward Er field improves the heating efficiency in W VII-A but tends to deteriorate it in Heliotron E. This difference was interpreted in terms of fast ion loss regions induced by two different branches of the E × B drift resonance. The efficient heating in W VII-A was explained by a counter-side shift of the trapping boundary of resonant banana orbits due to the E × B resonance. The increased fast ion loss in Heliotron E was explained by the resonance occurring between the E × B drift and the poloidal drift of helically trapped ions. However, the resulting resonant superbanana loss was found to be small when the vacuum vessel wall was used as the boundary. The effects of re-entering of ions on the ion heating rate and the charge exchange loss were also clarified. It is concluded that the efficient heating observed in Heliotron E experiments is compatible only with Monte Carlo simulations that take re-entering of fast ions into account

Neutral beam injection benchmark studies for stellarators/heliotrons

Neutral beam injection in stellarators/heliotrons is studied with Monte Carlo codes that treat the initial beam deposition and the fast-ion thermalization process. The birth deposition model carefully treats the geometry of the vacuum vessel and includes beam divergence, focusing, and aperture losses. The thermalization process is determined by integrating the guiding centre equations of the fast ions and simulating collisions with the plasma by Monte Carlo collision operators. This process may include charge exchange and neutral reabsorption. For the purposes of this benchmark, we review the different formulations of the guiding centre equations and the Monte Carlo collision operators. We studied perpendicular injection into Heliotron-E, which is located at the Plasma Physics Laboratory at Kyoto University. The magnetic fields of Heliotron-E are computed using the Biot-Savart law with realistic filament models. The sensitivity of the computed heating efficiency to the modelling of the particle loss boundary and to the numerical procedures is examined. The results of three different codes were compared. When the codes solve the same problem, the answers agree quite well. However, changing some of the modelling assumptions (such as the loss boundary location) can create significant differences in the results.

Effect of the loss cone on confinement in toroidal helical devices

The paper present an analysis of the loss cone boundary in phase space for toroidal helical devices. The effects of the radial electric field and the modulation of the helical field ripple are taken into account. The minimum energy of particles entering the loss cone is calculated. Modulation of the helical ripple is not always effective in reducing the loss when a radial electric field is present. The particle loss due to the loss cone is estimated in the collisionless limit. The impact of the radial electric field on the loss cone is discussed. It is more difficult to find a solution in the case of a positive radial electric field, which is required for the high ion temperature mode. This difficulty is great for systems with a small helical field ripple.