In this paper the effects of an externally produced magnetic island (MI) on the 3D radiation structure in attached and detached plasmas as predicted by the EMC3-EIRENE code are clearly seen in the imaging bolometer (IRVB) data from two different views of the plasma, experimentally confirming the role that the MI plays in the detachment process. With the addition of the MI the carbon radiation profile from the code in a poloidal cross-section becomes more localized near the helical divertor x-points (HDXs). This is reflected in the focusing of the radiation patterns corresponding to the HDX in both the IRVB and code data in images corresponding to the IRVB field of view (FOV). Detachment results in a more asymmetric radiation profile in the poloidal cross-section code data with localized peaks near the HDX and magnetic island x-points (MIXs). The radiation from the MIXs is reflected in strong radiation from the corresponding location in the IRVB FOV from both code and IRVB data. Also the change in the position of the MI results in a change in the position of the localized radiation peak as predicted by the code. However, the relative increase in the radiation from the MIXs is greater in the code data than in the IRVB data for reasons which are so far unknown. Also similar discharges show detachment with the MI, albeit at a lower density than the discharge without the MI. This work confirms the previous conclusions that the MI enhances the localization of the radiation and is conducive to achieving and sustaining the detachment.
The IR imaging video bolometer (IRVB) provides the power distribution of plasma radiation. The radiation distribution is obtained from the temperature distribution on the bolometer foil. It is necessary to calibrate between the temperature distribution and the incident radiation power on the bolometer foil. This paper describes a new calibration technique for the foil which we have developed. The bolometer foil was irradiated with a He-Ne laser and the temperature distribution was measured by an IR camera while changing the irradiation position. The temperature distribution measured was analyzed by the comparison with the results calculated by FEM. We repeated this comparison while changing the parameters such as effective foil thickness and effective emissivity in the calculation until the calculated distribution converged to the measured one. The temperature distribution calculated by the FEM agreed well with the measured one, so the calibration between the radiation power and the temperature profile can be suitably conducted by this technique.
The progress of physical understanding as well as parameter improvement of net-current-free helical plasma is reported for the Large Helical Device since the last Fusion Energy Conference in Daejeon in 2010. The second low-energy neutral beam line was installed, and the central ion temperature has exceeded 7 keV, which was obtained by carbon pellet injection. Transport analysis of the high-Ti plasmas shows that the ion-thermal conductivity and viscosity decreased after the pellet injection although the improvement does not last long. The effort has been focused on the optimization of plasma edge conditions to extend the operation regime towards higher ion temperature and more stable high density and high beta. For this purpose a portion of the open helical divertors are being modified to the baffle-structured closed ones aimed at active control of the edge plasma. It is compared with the open case that the neutral pressure in the closed helical divertor increased by ten times as predicted by modelling. Studies of physics in a three-dimensional geometry are highlighted in the topics related to the response to a resonant magnetic perturbation at the plasma periphery such as edge-localized-mode mitigation and divertor detachment. Novel approaches of non-local and non-diffusive transport have also been advanced.
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