T. Hamada

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.

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.

Behavior of pellet injected Li into Heliotron E plasmas

Abstract Li pellet injection has provided a complex plasma with a large fraction of Li ions, which is characterized by intense emissions from Li I and III. The spatial profiles of the fully ionized Li 3+ ions are measured by charge exchange recombination spectroscopy with a resolution of 13 mm and the local decay time of the injected Li ion has been estimated. The spectral profile of the charge exchange recombination line of Li III from n = 5 to n = 4 shows a complicated structure, which depends on Li 3+ density. The effects on other intrinsic impurities and recycled Li are also discussed.

Effect of radial electric field and bulk plasma velocity shear on ion thermal transport in Heliotron-E

The effect of radial electric field and velocity shear on thermal transport is studied in Heliotron-E. When neutral beams are injected into the target plasma produced by electron cyclotron heating (high mode), or an ice pellet is injected into an NBI plasma (pellet injection mode), the central ion temperature increases in time up to 0.7 - 0.8 keV. These high plasmas are characterized by a peaked ion temperature profile and are associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling, or by pellet fuelling. The global energy confinement is improved compared with L-mode plasmas by a factor of 1.4 for the same line-averaged electron density. The observed improvement in ion heat transport is related to the radial electric field shear rather than to the rotation velocity shear in the bulk plasma.

Enhancement and suppression of density fluctuations around electron drift frequency in Heliotron E plasmas measured using CO2 laser phase contrast method

Enhancement and suppression of electron density fluctuations around the electron drift frequency was observed in Heliotron E. This behavior corresponded to changes in the density profiles. Density fluctuations around this frequency were enhanced when the density profile was peaked in the inner region, and suppression was associated with the flattening of the density profile. An examination of the frequency, wavenumber, and collisionality regimes suggested that the most likely candidate for the instability responsible for driving these fluctuations is the collisionless circulating electron mode.

ECE diagnostic using multi-channel radiometer in Heliotron-E

Abstract Electron cyclotron emission (ECE) is measured with a multi-channel radiometer system in Heliotron-E. The system detects the second harmonic emission from 67 to 115 GHz at the magnetic field B = 1.9 T, covering the plasma confinement region up to the last closed flux surface. The radial and temporal evolution of the electron temperature has been obtained for electron cyclotron heated (ECH) and neutral beam injected (NBI) plasmas. Nonthermal electrons are observed at the plasma breakdown, and the nonthermal electron energy is estimated from the frequency downshift. We also discuss the effect of the magnetic shear on the second harmonic electron cyclotron wave propagating through the plasma with a strong magnetic shear such as heliotron/torsatron configurations.

A new approach for vacuum surface mapping in Helitron E

Abstract Wall boronization was applied in Heliotron E with special reference to the study of impurity and recycling control in the helical system. However this technique was also found to work as an advantageous tool to enhance the diagnostic sensitivity of the beam impedance method for measuring the edge and divertor field line structure. In this paper we describe a new approach to the beam impedance method that utilizes the wall boronization.

A physical model of the beam impedance method within ideal nested magnetic surfaces

The beam impedance method is used to measure the vacuum magnetic structure of heliotron/torsatron systems. It measures the current that flows from an electron gun inserted into the vacuum chamber to the chamber wall. To interpret experimental results, we have been developing a physical model of the method and a numerical code based on the model. For simplicity, we have focused on a limited model in which only ideal nested magnetic surfaces exist. As a new approach to reproduce the real physical condition, we treat the current that flows across magnetic surfaces and that which is emitted from the electron gun separately. Radial electron distribution forms for the two currents to match each other. The characteristic of electron emission from the gun is obtained experimentally and included in the code. The diffusion process is assumed to obey the neoclassical diffusion theory. The numerically obtained current profile agrees well with the measurement if the suppression of diffusion due to a radial electric fi...

Field line measurements in the divertor of Heliotron-E under boronized conditions

Abstract Vacuum magnetic field line structure of Heliotron-E has been studied experimentally with special reference to the edge plasma physics of the helical system (heliotron/torsatron). A new approach for measuring the vacuum impedance between the magnetic surface and the chamber wall under the boronized conditions was attempted in order to extract the detailed field line topology efficiently. Measurements have revealed the position of the experimental last closed surface and the nesting and folding structure of the divertor field lines. In conclusion, an advanced 2D visualization of Heliotron-E divertor field lines was obtained as compared with the previous one using the stainless-steel wall conditions. In addition, the relationship between the edge plasma distribution and the relevant field line topology is discussed.

Spectroscopic study of ECH and neutral beam injection heating of the Heliotron E plasma

Abstract VUV, UV and visible spectroscopy of fusion plasmas enables the main plasma contaminants to be identified, and time-resolved measurements allow a study of the development of the major impurity emission lines to be undertaken. In this work, Heliotron E VUV emission is shown to be dominated by oxygen, carbon, iron and nickel lines, and a scan of the emission in the UV-visible range reveals that the most intense impurity lines in this range are from boron, carbon and oxygen. P, study of Ha and Da emission from the plasma is carried out, and it is found that the release of hydrogen from the vessel wall is the dominant source of hydrogen in the discharge plasma. Preliminary spectroscopic results of lithium injection into the plasma are also presented.