H. Matsuura

Edge plasma properties in heliotron-E

The magnetic limiter and helical divertor geometry predicted by vacuum magnetic field calculations has been confirmed by measurements with various probe and calorimeter arrays in Heliotron-E. The effect of modification of these configurations with auxiliary magnetic fields or a material limiter on the edge plasma profile is studied. The relation of the edge plasma properties with the main plasma parameters (density, heating power, etc.) are described. The amount of total heat loss to the divertor region is almost the same as that of the heating power. The edge plasma density is approximately proportional to the core plasma density

Plasma application for detoxification of Jatropha phorbol esters

Atmospheric pressure non-thermal dielectric barrier discharge (DBD) plasma generated by helium gas at high voltage and input power of about 50 W was first applied to detoxification of Jatropha curcas phorbol esters (J. PEs) as well as standard phorbol ester (4β-12-O-tetradecanoyl phorbol-13-acetate, TPA) in water and methanol. Plasma irradiation on the solution sample was conducted for 15 min. In aqueous solution, only 16% of TPA was degraded and complete degradation of J. PEs was observed. On the contrary, complete degradation of both TPA and J. PEs in methanol was achieved by the same plasma irradiation condition. Hydroxyl radical (•OH) generated by plasma irradiation of the solution is expected as the main radical inducing the degradation of PEs.

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.

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.

Edge plasma responses to energetic-particle-driven MHD instability in Heliotron J

Two different responses to an energetic-particle-driven magnetohydrodynamic (MHD) instability, modulation of the turbulence amplitude associated with the MHD instability and dynamical changes in the radial electric field (Er) synchronized with bursting MHD activities, are found around the edge plasma in neutral beam injection (NBI) heated plasmas of the Heliotron J device using multiple Langmuir probes. The nonlinear phase relationship between the MHD activity and broadband fluctuation is found from bicoherence and envelope analysis applied to the probe signals. The structural changes of the Er profile appear in perfect synchronization with the periodic MHD activities, and radial transport of fast ions are observed around the last closed flux surface as a radial delay of the ion saturation current signals. Moreover, distortion of the MHD mode structure is clarified in each cycle of the MHD activities using beam emission spectroscopy diagnostics, suggesting that the fast ion distribution in real and/or velocity spaces is distorted in the core plasma, which can modify the radial electric field structure through a redistribution process of the fast ions. These observations suggest that such effects as a nonlinear coupling with turbulence and/or the modification of radial electric field profiles are important and should be incorporated into the study of energetic particle driven instabilities in burning plasma physics.

First Results and Future Research Plan of Divertor Simulation Experiments Using D-Module in the End-Cell of the GAMMA 10/PDX Tandem Mirror

In the new research plan of Plasma Research Center of the University of Tsukuba, a high heat-flux divertor simulator (E-Divertor) was proposed by using an end-mirror exit of a large tandem mirror GAMMA 10/PDX device. Experiments for characterization of end-loss plasma flux have been extensively performed at the end-mirror region of GAMMA 10/PDX and detailed behavior of end-loss particles has been investigated. In standard hot-ion mode plasmas (ne0 ~ 2×1018 m-3, Ti0 ~ 5 keV), the energy analysis of ion flux was performed by using end-loss ion energy analyzer (ELIEA). It was found that the high ion temperature (100 - 400 eV) is generated and has a liner relationship between diamagnetism in the central-cell. The ion temperature determined from the probe and calorimetric measurements gives a good agreement with the ELIEA measurement. Additional ICRF heating in the anchor-cell showed a significant increase of particle flux, which indicated an effectiveness of additional plasma heating in adjacent cells. Superimposing the ECH pulse of 380 kW, 5 ms induces a remarkable enhancement of heat flux and a peak value in the net heat-flux density more than 10 MW/m2 was attained during the ECH injection, which comes up to the heat-load level of the divertor plate of ITER. Recently a large-scale divertor simulation experimental module (D-module) was installed in the west end-cell and the first plasma irradiation experiments onto a new tungsten V-shaped target were successfully performed. A number of interesting results such as neutral compression, enhancement of recycling and impurity radiation during noble gas injection, have been observed.

Application of a Temperature Gradient Type Thermal Probe to the Discharge Plasma

Thermal probes have been proposed to measure the plasma heat flux and to deduced the plasma parameters. Their time response is, however, rather poor, due to the steady-state condition where they work. In this work, a new type thermal probe with three thermocouples is designed and tested for low density discharge plasma. Monitering of heat flux changes due to discharge condition changes is successfully demonstrated. The measurement limit and the error for this probe are also discussed.

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.

Impurity behavior in Heliotron E

The reduction of metallic impurity was found in a carbon limiter configuration in Heliotron E. In order to investigate the impurity penetration into the plasma, the distance between the outermost magnetic surface and the vacuum vessel wall was varied by inserting a carbon rail limiter. The analysis of vacuum ultraviolet impurity spectra and charge exchange neutral fluxes shows that the decrease of the impurity production rates by charge exchange neutral particles is the most probable reason to explain the experimental results. Moreover the electron temperature and density profiles in the limiter shadow provide an ability to screen metallic impurity influxes from the wall.

“Natural” divertor- and limiter-discharges in Heliotron E

A comparative study on edge plasma properties between the original “natural” divertor configuration and the material limiter configuration was performed for Heliotron E currentless plasma. By insertion of a carbon rail limiter, the particle and heat fluxes to the divertor footprint were reduced to 20–50% of that in the divertor configuration. Such a reduction of influx resulted in the decrease of metal impurities sputtered from the wall surface. There were no clear differences in the edge density and heat flux profiles for NBI plasma between the limiter and divertor scrape-off. In the limiter configuration, a highly heat loaded area was observed locally on only one side of the limiter head. A perturbation field affected this local heat load.