M. Nakasuga

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.

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.

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.

Asymmetric divertor plasma distribution observed in Heliotron J ECH discharges

Abstract An asymmetric divertor plasma distribution observed in the standard configuration of Heliotron J is reported. The divertor plasma profiles were investigated with two Langmuir probe arrays, which were installed at the geometrically up–down symmetric positions, for three different heating schemes of ECH with 53.2 or 70 GHz microwaves. Although the position of the divertor plasma flux was almost consistent with the footprint position of the divertor field lines, the existence of two types of up–down asymmetry was revealed in the divertor plasma density and floating potential profiles. The first type of asymmetry was mainly observed near the boundary to the ‘private region’. This asymmetry seems to be independent of the heating schemes of the toroidal position of the heating source. As the direction of the confinement field was reversed, the feature of the plasma profile on the top array came to appear on the bottom array, and vice versa. This field-direction dependence indicates that the asymmetric B ×∇B drift motion of charged particles might cause this type of asymmetry. The second type of asymmetry was observed in the region away from the boundary and seemed to depend on the heating schemes.

Recent results on heliotron E

Abstract Recent results of the heating experiment on the Heliotron E plasma are reported. Two heating methods are applied; electron cyclotron resonance heating (ECRH) and neutral injection (NBI) heating. In the ECRH experiment, a currentless high-temperature plasma is produced with the use of a gyrotron capable of delivering an output power of 200 kW for 40-ms duration at 28 GHz. The electron and the ion temperatures at the center −Te(0) and Ti(0) − 1.1 keV and 120 eV, respectively, and the average density n e of 5 × 1012 cm−3 is obtained. The NBI heating has produced the plasma which has the temperatures of Te(0) ∼ Te(0) ∼ 660 eV and the density n e of 3 × 1013 by the NBI power of about 1.6 MW. The heating efficiency has been found to be about 2 ( eV ·P abs ( kW ) n e (10 13 cm −3 ), where P abs is the absorbed neutral beam power, for bot perpendicular and 28° co- and counter-injection angles from the normal direction of the torus. The central beta value reaches about 1%.

Influence of magnetic configuration and heating methods on distribution of diverted plasmas in Heliotron E

Abstract A study on the distribution of the amount of diverted plasma along torus in Heliotron E was performed for NBI and ECH plasmas under different experimental conditions. A strong up–down asymmetry of the diverted plasma flux was observed contrary to what should be expected from the vacuum magnetic configuration. The degree of this asymmetry depends on the discharge conditions. This result indicates that the knowledge of only vacuum field traces in a divertor region is not enough to predict how much ratio of the total diverted plasma comes to a concerning divertor section in the heliotron/torsatron devices.

Complete integral suppression of Pfirsch–Schlüter current in a stellarator plasma in Heliotron E

The poloidal magnetic field was measured to detect the plasma boundary position. It was found that the pressure-induced plasma shift, an observable characteristic of the Pfirsch–Schluter current, depends strongly on the initial position of the magnetic axis. When the axis was moved by the vertical field inside the torus, the finite-β shift became smaller. Complete suppression of the finite-β plasma shift was achieved in a deeply inward shifted configuration: 7 cm from the standard position Raxis=2.20 m. This effect is explained by magnetohydrodynamic (MHD) equilibrium theory for stellarator toroidal plasmas with a large magnetic hill and deep inward shift.

Behaviour of electrons of an ECRH plasma in Heliotron E

In Heliotron E, a plasma is produced and heated only by an electron cyclotron resonance wave (f = 28 GHz). The output power and pulse width are up to 90 kW and 40 ms, respectively. The parameters of the plasma are in a wide range (Te = 0.3−1.1 keV and e = (1.2−9.5) × 1018m×3). The dependence of the electron temperature on the microwave output power indicates no saturation of the electron temperature. A preliminary analysis is also presented which indicates that the absorption efficiency may be rather good. The anomaly factor, which is the ratio of the experimental particle diffusion to the neoclassical value in the plateau regime, is 2 to 6 for the case of medium density (about 5 × 1018m−3).

Control of the magnetic configuration in the Heliotron-E device

Abstract The magnetic configuration of the Heliotron-E device was controlled by adding auxiliary toroidal and/or vertical fields. The changes in the plasma edge and the “divertor trace” were experimentally studied by using a double probe, calorimeters, a thermal Li-beam probe and a laser Thomson scattering system. The variation of the vacuum configuration was also confirmed by the “stellarator diode method” with a small hot cathode. It was found that the observed change in the edge region basically agreed with that expected from the line-tracing calculation.