Heliotron-E Device Research Articles

Effects of trapped particles on distribution of divertor flow in Heliotron-E

The big asymmetry of divertor flows to symmetrically located collectors was observed when divertor plasma flow distributions (DPFD) were measured in Heliotron-E device. Several possible reasons of DPF asymmetry were discussed in previous publications but no one was singled out. In the present work, to clear up the plasma heating effects on DPFD asymmetry, the difference of the DPFDs at plasma heating phase and at an ‘afterglow’ phase of plasma discharge is analyzed for different experimental conditions. During the afterglow phase the plasma is in the regime of Pfirsch–Schlüter diffusion when effects connected with locally trapped particles would be negligible. Besides, the inhomogeneity connected with the locality of plasma heating would disappear and all plasma parameters should be constant along the torus. Therefore, the asymmetry of DPFD observed at this phase of discharge was taken as a base for evaluation of the effects stipulated by the trapped particles.

106 GHz electron cyclotron heating experiment on Heliotron-E

Abstract A new electron cyclotron heating (ECH) system has been installed on the Heliotron-E helical device. The gyrotron output 106 GHz TE 12,2 mode is converted into a gaussian beam by the quasi-optical Vlasov convertor. The gaussian beam is coupled to the HE 11 waveguide mode, then transmitted by the corrugated waveguides. The transmitted beam is well focused and launched to the Heliotron-E device. Plasma has been produced and heated by this 106 GHz ECH. The second harmonic breakdown was investigated and compared with the fundamental harmonic breakdown. The improvement of the particle confinement was observed during 106 GHz second harmonic ECH. Compared with the 53 GHz fundamental ECH phase, the averaged electron density was increased by 20% without gas puffing. The outflux to the divertor plate was reduced.

Hexapole Field Effects on Currentless MHD Equilibrium and Stability ofl=2 Heliotron/Torsatron

Averaged flux surfaces with respect to the toroidal angle in l =2 heliotron/torsatrons are deformed in D-shape or inverse D-shape by the hexapole field. D-shape deformation of vacuum flux surfaces enhances magnetic well or reduces magnetic hill in Heliotron-E and Large Helical Device (LHD) configuration. However, complete elimination of the magnetic hill region is not obtained by the D-shape deformation without substantial reduction of plasma radius. The inverse D-shape deformation degrades MHD stability against ideal interchange modes in the central region of both devices for fairly peaked pressure profiles. In the LHD configuration, a broad pressure profile proportional to (1-ψ 2 ) 2 degrades the MHD stability and the hexapole effect is almost irrelevant, where ψ is a poloidal flux function; however, second stability appears against both the local and non-local interchange modes.

Measurement of Toroidal Plasma Current in RF Heated Helical Plasmas

This report describes the measurement of toroidal plasma current by a semiflexible Rogowski coil in a helical vacuum chamber. A Rogowski coil measures the toroidal plasma current with a resolution of 0.1 kA, frequency range of up to 1 kHz and sensitivity of 6.5×10-9 V·s/A. We measured the spontaneous toroidal plasma current (from -1.2 to +1.2 kA) under electron cyclotron resonance heating at 0.94 T toroidal field in the Heliotron-E device. We found that the measured direction of toroidal plasma current changes its sign as in the predicted behavior of a neoclassical diffusion-driven bootstrap current, depending on the horizontal position of the plasma column. We explain the observed plasma currents in terms of the compound phenomenon of an ohmic current and a neoclassical diffusion-driven current. The magnitude of the neoclassical current component is smaller than the value predicted by a collisionless neoclassical theory.

Reconstruction of Edge-Plasma Density Profiles by Neutral Beam Probe Spectroscopy

Thermal neutral beam probing combined with a spectroscopic technique is numerically developed to reconstruct electron-density profiles of edge plasmas. Although the reconstruction has been performed by assuming all beam atoms to have a thermal velocity, the influence of a beam-velocity distribution has not received as much attention. It is shown that the method which neglects the beam-velocity distribution reconstructs electron densities lower than the true values by a factor of ∼2 in the density range up to ∼1×1013 cm-3, while the procedure which takes into account the velocity distribution yields true density profiles in the same density range. Some results of the beam probe mounted in the Heliotron-E device are also described, and good agreement with Langmuir probe measurements is found.

Suppression and control of magnetic islands in toroidal plasmas

Magnetic islands play an important role in determining the stability and confinement properties of toroidal plasmas. The nonlinear growth of magnetic islands can lead to major disruptions in tokamaks. In stellarators, which do not have a continuous symmetry, the formation of magnetic islands can set equilibrium beta limits. In the first part of this paper, a novel method is proposed for reducing drastically the size of m=2, n=1 islands in tokamaks by introducing a beamlet of energetic ions just outside the q=2 surface via parallel neutral-beam injection. A simple physical picture is given of the nonlinear stability of a tearing mode in the Rutherford regime in the presence of energetic ions. This physical picture is supported by a kinetic calculation in the long mean-free-path regime in which the effect of bootstrap currents and resistive interchanges are retained. Estimates show that the energy of the beamlet required for the suppression is a small fraction of that used for Ohmic or beam-heating of the background plasma. In the second part of the paper, an equilibrium beta limit is obtained for stellarators by calculating the island widths induced by finite plasma pressure; the widths are then constrained by the criterion of island overlap. The theory is applied to the Heliotron-E device [Nakamura et al., Phys. Fluids B 2, 2528 (1990)]. It is shown that some aspects of the experimental observations on internal disruptions in Heliotron-E can be interpreted as signatures of the equilibrium beta limit.

Control of the magnetic configuration in the Heliotron-E device

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.

Study on edge plasma physics and particle control in the Heliotron-E device

The edge plasma physics and the particle control under the intrinsic magnetic limiter configuration of a helical system have been studied with the Heliotron-E device, where currentless plasmas of T e ≤ 1–2 keV, T i ≤ 1 keV and n ̄ e ≤ 2 × 10 20/ m 3 are produced by a combination of ECRH, NBI and/or ICRH. It is indicated that the separatrix region of the heliotron device is able to act as a divertor magnetic field. According to calculations of the magnetic field line in the edge region, the separatrix region has some different characteristics from the scrape-off layer in tokamak devices; the existence of a fine structure in the separatrix region and asymmetry of the region in toroidal and poloidal directions are observed. A localized pattern of the heat load on the first wall is experimentally observed. This agrees with the heat-load profile expected from the magnetic configuration and the distribution of the plasma in the edge region. A carbonization of the first wall is successfully applied to the Heliotron E device for reduction of metallic impurity contents. The heat load at the divertor trace decreased and that on the other part of the first wall increased in the high recycling conditions after the carbonization.

Study on carbon coating films produced in Heliotron-E

In the Heliotron-E device, carbonization experiments were successfully performed by using a DC glow discharge in a mixture gas of hydrogen and methane. The properties of carbon films produced on surface probes were analyzed by Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy (RS). Film thickness varied between 17 and 40 nm, and almost no metal impurities were found in the film layer. Both the radial and the toroidal distributions of the film thickness were analyzed. The toroidal uniformity of the carbon film was within a factor of two. The structure of carbon film produced in Heliotron-E was amorphous carbon, while that of TEXTOR film was a mixture of amorphous and graphite. According to the depth profile analysis, the carbon film could hardly be eroded by main discharges. But D 2 or He discharge cleaning effectively removed the film. In addition, after the main discharge with Ti-flashing in the carbonized chamber, the formation of TiC in the film region was observed.

A comparison of energy confinement in tokamaks and stellarators

We overview and compare the energy confinement behaviour of tokamaks andstellarators at high heating powers. Tokamaks show in all parameter ranges ananomalous enhancement of electron energy losses. At high heating powers abifurcation is observed in the plasma energy vs. heating power diagram, withthe low energy (L-regime) branch showing a clear deterioration of confinementtime with power. Divergent results exist concerning the corresponding behaviorof the high energy (H-regime) branch. Experiments with ECR-heating ofnet-current-free stellarator plasmas can be to a large extent explained byneoclassical electron heat conduction losses, including ripple effects. Inthese experiments anomalous transport enhancement has been identified only inthe outer plasma layers and at low temperatures and densities, which in thisform should not affect reactor aspects. At a given density, neoclassicalelectron heat conduction losses scale however in a similar manner with heatingpower as the L-regime losses observed in tokamaks. For a better discrimination- important due to the differing variation with size expected for the twotypes of losses - density scans over a range not accessible with present dayECRH-systems are needed. Corresponding experiments with NBI-heating on thehigh shear Heliotron-E device indeed gave evidence for a confinement anomalyincreasing with heating power, although not well fitted by tokamak L-regimeexpressions. For low shear systems, conclusive evidence is expected fromNBI-heating experiments on W VII-AS.

Pump limiter studies on the Heliotron-E device

Particle control with pump limiters has been successfully demonstrated in a variety of tokamaks. This experiment has obtained, for the first time, experimental data on pump limiter operation in a heliotron configuration. A movable pump limiter module was installed on a horizontal midplane port of Heliotron-E. The limiter module was installed on a horizontal midplane port of Heliotron-E. The limiter assembly consists of a TiC-coated graphite head with single-sided particle collection and active pumping. The location of the limiter is varied from near the vacuum vessel wall or up to 8 cm inside the last closed magnetic flux surface. This flexibility permits the study of a heliotron plasma that is limited either by the magnetic separatrix or by a material limiter. In the configuration investigated, only very low pressures are observed in the pump limiter when it is located outside the last closed flux surface, indicating that the scrape-off layer density is very low (< 5 × 10 11 cm −3). For limiter positions inside the last closed flux surface, pressures of 2–6 mTorr are observed (similar to comparable tokamak operation). Erosion patterns on the vacuum vessel, as well as hot spots on the limiter head, indicate that the particle flow out of the confined plasma exhibits complicated patterns. This suggests that for efficient particle collection in a stellarator, pump limiters must be matched to the particle flow patterns.

In-situ surface probe analysis for impurity flux in the heliotron-E device

The impurity problem is one of the most important areas of study for improving Heliotron-E currentless plasmas. Impurity reduction by 2.45 GHz ECH discharge cleaning and the impurity flux in the edge region of the NBI currentless plasmas were studied by using a newly installed surface analysis station with an “in-situ” AES system. It was confirmed that the light impurities such as C and O were well reduced by the discharge cleaning. The amount of metal impurities on the α-Si probe surface exposed to the plasma increased as the NBI power increased. The impurity level observed in one side of the probe inserted into the edge region of the NBI plasma was larger than that in the opposite side.

Ballooning modes in 3-D Stellarators with shear

Pressure-driven ballooning and interchange instabilities in three-dimensional (3-D) vacuum Stellarator equilibria similar to those of the Heliotron-E device are investigated. The toroidal curvature dominates the mode structure, which concentrates where the field line bending is weakest. The Mercier criterion yields a more stringent condition for stability than the incompressible ballooning mode equation. The growth rates vary significantly with the radial wave number only on those field lines located on high-shear flux surfaces close to the edge of the plasma. They remain almost constant from field line to field line of a flux surface. An outward shift of the plasma induced with a vertical magnetic field increases the vacuum magnetic well, which improves the conditions for ballooning stability. An inward shift decreases the vacuum magnetic well, which deteriorates the conditions for ballooning stability.

Ion energy containment in Heliotron-E

First measurements of the charge-exchange ion temperature in the Heliotron-E device are described. The limiting temperatures calculated from a simple energy balance equation are presented and compared with the measured temperatures. In low-current plasma, the measured temperatures agree well with the thermal conduction limit based on neoclassical theory, which includes an effect of the electron-to-ion temperature ratio Te/Ti. In high-current plasma, however, the ion temperature is about one-third of the limiting temperature. This discrepancy may be due to anomalous heat and particle transport driven by the plasma current.