M. Yoshikawa

Radial density profile measurement by using the multichannel microwave interferometer in GAMMA 10

Plasma density radial profile measurements are an important study for fusion plasma researches. We reconstructed a multichannel microwave interferometer for radial plasma electron density and density fluctuation measurements with both changing the transmission horn position and using the Teflon lens by only using this system in a single plasma shot. By using this system, we can successfully measure the radial density and density fluctuation spectra in a single plasma shot.

Progress in microwave diagnostics and physics issues in magnetically confined plasmas

Microwave/millimeter-wave techniques such as interferometry, reflectometry, scattering, and radiometry have been powerful tools for diagnosing magnetically confined high-temperature plasmas. Important plasma parameters were measured to clarify the physics issues such as stability, wave phenomena, and fluctuation-induced transport. Recent advances in microwave and millimeter-wave technology together with computer technology have enabled the development of new generation of diagnostics for visualization of 2D and 3D structures of plasmas. Microwave/Millimeter-wave imaging is expected to be one of the most promising diagnostic methods for this purpose. We report here on the recent progress in microwave diagnostics and the results obtained in magnetically confined plasmas.

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.

Calibrations of the LHD Thomson scattering system

The Thomson scattering diagnostic systems are widely used for the measurements of absolute local electron temperatures and densities of fusion plasmas. In order to obtain accurate and reliable temperature and density data, careful calibrations of the system are required. We have tried several calibration methods since the second LHD experiment campaign in 1998. We summarize the current status of the calibration methods for the electron temperature and density measurements by the LHD Thomson scattering diagnostic system. Future plans are briefly discussed.

Electron temperature and density distributions of detached plasma in divertor simulation experiments in GAMMA 10/PDX

Spatial characterization of plasma detachment attributed to molecular activated recombination (MAR) has been done by measuring distributions of electron temperature, electron density, space potential and floating potential of divertor simulation plasma in GAMMA 10/PDX tandem mirror with Langmuir probes. As the hydrogen pressure in the divertor simulation experimental module was increased by supplying additional hydrogen gas, the electron temperature near the target decreased from 30 eV to about 2 eV and the electron density first increased to an order of nearly 1017 m−3, and then decreased in contrast to the density measured at the upstream of the target which became saturated. The density decrement was higher near the corner of the V-shaped target although temperature distribution was almost flat near the target. In addition, space potential decreased, floating potential increased, and then the potential difference between those became small. The potential difference decreased to almost zero near the tar...

Overview of recent progress and future in GAMMA 10/PDX project

This paper presents an overview of the recent progress of GAMMA 10/PDX project. In the GAMMA 10/PDX project, development of fusion reactor relevant research related to magnetic mirror devices has been conducted. One of the main objectives is divertor simulation studies under the similar condition of actual fusion devices, which contributes to comprehensive development strategy towards the divertor plasma physics and plasma wall interaction (PWI) studies. The recent progress of this research is that an additional plasma heating (ICRF and ECH) significantly increased the ion flux up to 3.3 × 1023 /m2 s at the end-mirror exit, which proved an effectiveness of additional plasma heating for generating high ion flux from the mirror end. Superimposing a short pulse of ECH produces the maximum heat flux of 21 MW/m2 by improving the west plug ECH antenna system, which exceeds the heat-load of ITER divertor plates. In detached plasma formation experiments using the divertor simulation experimental module (D-module)...

Time Evolving Stucture of Alfvén Ion-Cyclotron Waves in GAMMA 10

A two-channel reflectometer has been developed and successfully applied to the GAMMA10 central cell to investigate the spatial structure of spontaneously excited Alfven ion-cyclotron (AIC) waves. At the frequencies of externally applied ICRF waves and AIC waves, good correlations of well above the statistical noise level are obtained between two density fluctuations at axially separated positions in the central cell. Density fluctuations at various radial positions and two axially separated positions show that the power distribution among the AIC waves changes much both in radial and axial directions especially in the earlier period just after the excitation and in the core region of r/a < 0.3. Phase differences of the AIC waves at axially separated positions show features of the transformation of the AIC waves from propagating to standing waves. The pass of the node of the standing wave is also observed. The time of the pass is different by AIC waves, which clearly indicates difference of the axial wavelengths of the simultaneously excited AIC waves.

Overview of Recent Progress in the GAMMA 10 Tandem Mirror

(1) Four-time progress in ion-confining potentials Φc to 3.0 kV in comparison to Φc attained 1992-2002 is achieved in the hot-ion mode (Ti=several keV). A scaling of Φc, which favorably increases with plug electron-cyclotron heating (ECH) powers (PPECH), is obtained. (2) The advance in Φc leads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φc with increasing nc to ˜1019 m-3 with the recovery of Φc with increasing PPECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Φc, generates a strong Er shear and peaked vorticity with the direction reversal of Err × B sheared flow near the Φc peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in Te and Ti rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises Te0 from 70 to 300 eV together with Ti[perpendicular]0 from 4.5 to 6.1 keV, and Ti//0 from 0.5 to 1.2 keV with τp0=95 ms for Φc (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of τp0/τE0 is observed to be 1.7 for Φc trapped ions (consistent with Pastukhov’s theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (ηICH˜0.3; nlc=4.5×1017 m-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (Te=600 eV and Ti=6.6 keV) with azimuthal Er×B sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.

Analysis of Neutral Transport in the GAMMA10 Anchor-Cell Using H{alpha}-Emission Detectors

The neutral transport was studied in the anchor cell. The Hα line intensities were measured by using axially aligned Hα detectors. It was found that the intensity is considerably dependent on ECRH and GP#3,4. A 5ch Hα detector was newly installed in the outer-transition region of the anchor-cell. From the measurement of the spatial distributions, the vertical intensity profile is estimated to be about 2.5 cm on the half width half maximum, while the horizontal distribution shows roughly flat around Z=-670 cm. The above characteristics were discussed with aid of neutral transport simulation using DEGAS Monte-Carlo Code.