K. Oki

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.

A Novel Frequency-Multiplied Interferometer System with 1-D Horn-Antenna Mixer Array in the GAMMA 10/PDX End Divertor Module

Abstract In GAMMA 10/PDX, divertor simulation studies have been started as a new research plan by using end-loss plasma flux at the end-mirror region. A divertor simulation experimental module (D-module) was installed in the west end-cell. Here we introduce a novel microwave interferometer system with a phase imaging method, and all the crucial components are designed and tested. Finally, we tested the interferometer system using a Teflon plate in substitution for plasma. In order to improve the system accuracy, the phase detection circuit was modified and the phase difference of each channel was verified.

High-Density plasma production in the Gamma 10 central cell with ICRF heating on both anchor cells

Abstract In the GAMMA 10 tandem mirror, divertor simulation experiments that utilize particle flux toward the west end region (called end-loss flux) have been implemented. Since a positive correlation has been reported between the end-loss flux and the central-cell density, an increase of the central-cell density is important for obtaining a higher end-loss flux on the divertor simulation experiments. By arranging the ion cyclotron range of frequency (ICRF) systems so as to excite strong ICRF waves in both anchor cells simultaneously, we have succeeded in producing high-density plasmas (line density of 1.2 × 1014 cm−2) in both anchor cells. As a result, a higher central-cell density of 4.4 × 1012 cm−3 and a higher end-loss flux of more than 1023 m−2s−1 have been obtained. One of the possible mechanisms of the high density production is a formation of positive potentials on both anchor cells. Plasmas in the central cell are confined due to those potentials.

Asymmetric toroidal flux generation due to phase locking of internally resonant tearing modes in the RELAX reversed-field pinch

Self-generation and sustainment of toroidal flux via a dynamo is one of the unique characteristics of the reversed-field pinch (RFP) configuration. In a small aspect ratio RFP machine RELAX, it has been observed that a sudden dynamo usually occurs at a localized toroidal location. This localized dynamo evolves in time, causing a toroidally asymmetric flux distribution. The degree of asymmetry depends on the location and propagation of the localized dynamo. It has been observed that the position where the localized dynamo occurs coincides with the position where the phases of the internally resonant tearing modes align. The phase alignment plays an important role in triggering a localized dynamo activity and in the formation of asymmetric toroidal flux distribution especially in deep-reversal plasmas in RELAX.

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.

GAMMA 10/PDX Project Status and Future

GAMMA 10/PDX project is the development of the mirror devices to aim the fusion-reactor relevant research using the potential control and the end loss as the particle and heat fluxes for transport, divertor plasma physics and Plasma Wall Interaction (PWI) studies. It was obtained that more than 10 MW/m2 heat flux density for PWI studies. The closed divertor-simulator module has been newly installed. The first studies of this module have been done and obtained the expected plasma flow. We will strengthen the module functions and diagnostic capability and heating systems to enable to be more reactor-relevant simulator. In the core transport control by the ECH, correlation studies of the radial electric field profile and the fluctuations are made and the high coherency is confirmed by using the new two-point potential detector of the Gold Neutral Beam Probe(GNBP). The maximum electron temperature obtained so far is - 150 eV during the ECH by Thomson Scattering, but we need more studies for the high power ECH optimization. As one of the key tools for the new GAMMA 10/PDX project, 2 MW gyrotron development at 28 GHz has been started, based on the success of 77 GHz-1.9 MW output. The maximum outputs of 1 MW at 28 GHz in short pulse and 540kW for 2 s are obtained.

Dependence of Properties of Quasi-Single-Helicity States on Field Reversal Parameter in a Low-Aspect-Ratio Reversed Field Pinch

Dependence of several properties of Quasi-Single-Helicity (QSH) states on field reversal parameter F is investigated in low-aspect-ratio reversed-field-pinch (RFP) machine RELAX. QSH probability with dominant n = 5 or 6 (n is toroidal mode number) is higher in low F region than high F. QSH probability and duration with dominant n = 3 or 4 are higher in high F region. These suggest change of innermost resonant n with F as shown in an equilibrium reconstruction. QSH probability, duration and dominant mode amplitude with the dominant mode of possible innermost resonant n = 3, 4 or 5 are higher than other n, which is desirable for improved confinement. Averages of the spectral index NS for flat-topped plasma current phases are similar to that predicted by a simulation which has shown NS is lower as aspect ratio is lower. This agreement and the higher QSH probability of 2 - 3 % than another high aspect ratio RFP machine may show advantage of the low aspect ratio RFP in achieving QSH.

Phase Locking and Unlocking Associated with Transition to Quasi-Single Helicity State in the RELAX Reversed-Field Pinch

In a reversed-field pinch configuration, many m 1⁄4 1 internally resonant tearing modes grow. The phase relationship among the modes is usually random, while there are cases where the phases tend to align each other. This phaselocking structure causes local enhancement of the heat flux to the wall. In the low-aspect-ratio reversed-field pinch (RFP) machine RELAX (major radius R 1⁄4 0:508m, minor radius a 1⁄4 0:250m, aspect ratio A 1⁄4 2:032), it has been observed that the phase-locking is released when a dominant mode grows to a large extent with suppression of the remaining modes, which state is often referred to as the quasi-single helicity (QSH) state. It has been found that the experimental phase behavior can be described as a probability distribution in ð min; NsÞ space, where min is the minimum of the phase dispersion, and Ns the spectral index. RFP is a relatively high-beta magnetic confinement configuration with much higher toroidal current than in Tokamaks with comparable size. Due to its high toroidal current, magnetic field lines skew in the poloidal direction and at the edge the direction of the toroidal field reverses. The safety factor (q) profile in RELAX is a monotonically decreasing function of radius with on-axis value of 0:3. Therefore, m 1⁄4 1 internally resonant tearing modes with toroidal mode numbers n satisfying 1=n . 1=3 can be simultaneously unstable in RELAX. The operational regime of the RELAX RFP is characterized by a wide range of field-reversal parameter F , which is the ratio of edge toroidal field Bta to the averaged toroidal field hBti. There are two characteristic regimes, possibly characteristic to low-A RFP. One is a shallow-reversal regime where F > 0:1, characterized by frequent transitions to QSH states, which may be attributable to larger separations between mode rational surfaces in the core region and power distribution of the m 1⁄4 1 modes to fewer toroidal mode numbers; some equilibrium analyses and numerical simulations predicted those effect of low aspect ratio on the RFP configuration. The helical structure coinciding with the magnetic island of the dominant mode in QSH state has been clearly observed in RELAX. The other is a deep-reversal regime where F 6) aspect-ratio RFPs. Although the reason of the attainment of the deep-reversal regime is not clear so far, this regime seems attractive in terms of optimization of the aspect ratio in the RFP since the tearing mode amplitudes are lowered in this regime. In a wide range of operational regimes of RELAX, phaselocking phenomena have been observed. Phase-locking structure called slinky mode tends to be locked to the wall due to electromagnetic interaction with error fields, and causes locally enhanced heat flux on the wall. In RFXmod, by an elaborate feed-back control of tearing modes’ dynamics with 192 saddle coils covering all over the wall, they have successfully released the wall-locking and, recently, higher plasma current operation of 2MA was performed. In the following, we show a spontaneous release of wall-locking and/or phase-locking due to transition to QSH state observed in RELAX. Figure 1(a) shows time evolution of spectral index Ns defined as

Electron Temperature and Density Distributions in a Divertor Simulation Experimental Module with Gas Injection in GAMMA 10/PDX

Abstract In tandem mirror GAMMA 10/PDX, H2 gas has been injected to a divertor simulation experimental module (D-module) for radiation cooling and detachment study. Electron temperature (Te) and density (ne) distributions have been measured by Langmuir probes in the D-module. With increase of H2 gas amount, Te has decreased especially at the V-shaped target plate from 30-40 eV to 3-5 eV. Moreover, ne and ion saturation current Iis have increased at the inlet but decreased at the target and had a steeper distribution toward the corner. These results suggest detachment. The movement of the ionization front to the upstream side with increase of the gas is suggested from the change in the plasma parameters profiles along the magnetic field line.