K. Hosoi

Recent results of divertor simulation experiments using D-Module in the GAMMA 10/PDX tandem mirror

Abstract This paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.76 × 1023 particles/·m2 at the end-mirror exit. Massive gas injection (H2 and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Hα emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is also found that Xe gas is much more effective in achieving detached plasma than Ar gas. Numerical simulation studies also have been performed toward the understanding of the cooling mechanism of divertor plasma. The above results will contribute to establishment of detached plasma control and clarification of the radiation cooling mechanism toward the development of future divertor systems.

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.

Plasma Characterization in Divertor Simulation Experiments with a V-Shaped Target on GAMMA 10/PDX

The divertor simulation experimental module (Dmodule) has been installed in the west end region in GAMMA 10/PDX. By use of Langmuir probes and spectroscopic measurement of intensity ratios of He I lines, temporal evolution of electron temperature and that of electron density of the plasma in the D-module with the V-shaped tungsten target are obtained. When the additional ICRF heating is applied to the anchor cell, the electron temperature evaluated with He I intensity ratios decreases from ~60 eV to ~25 eV and that from the probe measurement decreases from ~27 eV to ~14 eV. The difference between both measurements seems to be attributed to the difference of their measurement positions. The electron density measured by the Langmuir probe increases 2.3 times due to the RF3 power but it is rather low (< 1017 m-3). The electron density at the end region is expected to be increased by enhancement of ICRF heating and additional gas puffing at the plug/barrier cell which is the upstream cell of the end region.

High-Speed Imaging of Edge Plasma in the GAMMA 10 Tandem Mirror

Abstract The dependence of the resonance position of the central-ECH (c-ECH) on plasma behavior was investigated using a fast camera. When the resonance positions were higher, low frequency fluctuation of ~6kHz was observed and at the same time the diamagnetic signal decreased. Also, the same frequency fluctuation was observed in the potential signal by Au beam probe. On the other hand this low frequency fluctuation was not appeared when the resonance position was seemed to be on the axis of the plasma. These results indicate that the convective ExB drift instabilities may be induced due to the off-axis heating by c-ECH.

Research Plan for Divertor Simulation Studies and Its Recent Results Using the GAMMA 10 Tandem Mirror

Abstract As the new research plan of Plasma Research Center of the University of Tsukuba, we are planning to start a study of divertor simulation under the closely resemble to actual fusion plasmas environment making an advantage of the GAMMA 10 tandem mirror and to contribute the solution for realizing the divertor in future toroidal systems. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with separatrix (A-Div.) and the other is a high heat flux divertor simulator by using an end-mirror exit of the large tandem mirror device (E-Div.). Preparative experiments have been successfully started at the end-mirror region of GAMMA 10 and detailed behavior of end-loss particles has been investigated by using newly developed diagnostic instruments. In standard hot-ion mode plasmas (ne0 ~ 2 × 1018 m-3, Ti0 ~ 5 keV), the heat flux density of 0.8 MW/m2 and the particle flux density of 4 × 1022/s·m2 were observed at 30 cm downstream of the end-mirror exit on the machine axis. It is confirmed that the heat flux density increases in proportion to the applied RF power. Superimposing the ECH pulse induces a remarkable enhancement of heat flux and a peak value in the net heat flux density of 8 MW/m2 was attained during the ECH injection, which almost comes up to the heat load level of the divertor plate of ITER. Two-dimensional visible image measurement of newly installed target plates using high-speed camera revealed a significant difference in the behavior of visible emission from plasma-material interaction. The above results give a clear prospect of generating the required performance and providing useful information for divertor studies in GAMMA 10.

Status and Plan of GAMMA 10 Tandem Mirror Program

Abstract Recent progress and near future plan of GAMMA 10 efforts are presented. With high power plug electron cyclotron heating (ECH) up to ~400 kW, the ion confining potential of more than 2 kV was confirmed. The drift type low frequency fluctuations were suppressed by the positive radial electric field produced by plug ECH. It is found that the efficient EC heating on mirror devices from a strong B field side requires the minimization of the stray microwave in addition to the 100% X-mode excitation to avoid the enhancement of the ion loss. The development of a gyrotron, the key tool of these ECH experiments, has been made in collaboration with NIFS (National Institute for Fusion Science), More than 1.5 MW for more than 1s has been demonstrated at 77GHz. The plan of the boundary plasma research program with modification of GAMMA 10 is in progress. The new program includes the physics and technology studies of the divertor and SOL plasmas and PWI relevant to torus plasmas like ITER. The high heat flux experiments using the open end mirror throat has been started and we have obtained successful preliminary data, which include the heat flux of 8 MW/m2.

Characteristics of SMBI fueling with laval nozzle in GAMMA 10 based on experimental and simulation results

Results of supersonic molecular bean injection (SMBI) with Laval nozzle has been carried out in the GAMMA 10 tandem mirror. The neutral transport during SMBI was investigated by the two-dimensional image of the light emission captured by the high-speed camera. We used the value of full width at half maximum (FWHM) of the distribution of emission intensity as an index of neutral transport. The FWHM value decreases with the increase of the plenum pressure which indicates that the particle diffusion was more convergent at high plenum pressure. Fully three-dimensional neutral transport simulation using a Monte-Carlo code DEGAS is applied to SMBI experiment and simulation results qualitatively explained the experimental results.

Study on the Optimization of Fueling by Using the SMBI Method in GAMMA 10

We investigate the effect of nozzle on the particle transport in the SMBI experiments for optimization of fueling in the GAMMA 10 tandem mirror. The diffusion particle during SMBI was evaluated from the emission intensity captured by the fast camera. Compared with the results without nozzle, the particle diffusion was more convergent in the case using the straight nozzle. Simulation results by using Monte-Carlo simulation code (DEGAS) roughly reproduce the 2-D image captured by the fast camera.

Numerical Simulation Study for Background Plasma in the GAMMA 10 End-Mirror Cell

We have started a simulation study of background plasma in the GAMMA 10 west end-cell using a fluid-code in order to understand divertor simulation experiments. In this fluid-code, the configuration of magnetic field lines in the west end-cell of GAMMA 10 is adopted with a mesh structure and basic physical processes such as reflection coefficient and recombination are considered. An initial computation result reveals that in the case of the plasma density range: ne ~ 1019 m-3, ion and electron temperature could not be reduced significantly to the suitable temperature for achieving the detachment. The effect of heat transfer coefficient on the target plasma is also discussed.

Electron Temperature and Density Measurements by Using Thomson Scattering System in GAMMA 10

Thomson scattering (TS) is the most reliable diagnostic to measure the electron temperature and electron density radial profiles. In GAMMA 10, the yttrium-aluminium-garnet (YAG)-TS system was constructed with the large solid angle of TS collection optics. We carried out the Rayleigh and Raman scattering experiments for system settings and applied the YAG-TS system to the GAMMA 10 plasma. We can successfully measure the electron temperature and density radial profiles in the central cell of GAMMA 10 by using YAG-TS system. The supersonic molecular beam injection (SMBI) experiments have been started for plasma fueling study in GAMMA 10. In SMBI experiments, we observed the electron temperature decrease and electron density increase in the plasma center.