This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3 × 1023 m2 s−1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ~30 MW m−2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m−2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ~1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors.
This research investigated the radiation cooling mechanism and formation of detached plasma in the case of gas injection in the D-module of GAMMA 10/PDX. In GAMMA 10/PDX, divertor simulation experiments have been started by using a divertor simulation experimental module (D-module). A V-shaped target made of tungsten has been installed in this module. In order to understand the effect of impurity injection into divertor simulation experimental module, we injected H 2 and Ar gases to the D-module and measured the heat flux and ion flux. According to the increase of gas injection, reduction of ion and heat fluxes have been observed. In the Ar injection experiments, H 2 gas has been injected simultaneously to examine the effect of molecular process on detached plasma formation. In this case, both the heat flux and ion flux are drastically reduced. These results indicate radiation cooling and formation of detached plasma due to gas injection. Simultaneous injection of noble gas and hydrogen gas showed the most effective results on detached plasma generation.
We have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to scrape-off layer plasma of tokamaks in the GAMMA 10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N 2 , Ne, Ar, Kr, Xe) in combination with a puff of H 2 gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained. (i) A combination of N 2 and H 2 puffs showed a clear decrease of electron density and ion flux; (ii) N 2 and H 2 puffs form a strong density gradient along the axial direction; and (iii) other noble impurity gases showed an insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of a deeper understanding of the H 2 and N 2 assisted recombination process.
High-speed camera measurement was performed for analyses of detached plasma fluctuation in the GAMMA 10/PDX tandem mirror device. Significant spectral peaks at frequencies of 1.5 and 3 kHz were observed to have several local maxima along the vertical, near the entrance of the V-shaped target. Wavelet analysis indicated that 1.5-and 3-kHz fluctuations became non-simultaneously strong. Their phase relationships demonstrated inversion-symmetric and symmetric features with respect to the midplane. Such spectral patterns were interpreted as the line-integral of odd-and even-mode structures. It is suspected that the vertical edge part was due to radially elongated structures with an azimuthal phase shift.
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