<RESEARCH REPORT>Soft X-Ray Imaging Study on Disruptions in the JIPP T-II Tokamak
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概要
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A multi-channel soft X-ray imaging system (29 channels for viewing the plasma cross section, 3 channels for identifying poloidal and toroidal mode numbers, m and n, respectively) was installed on the JIPP T-II device to investigate external disruptions. A high-performance digital data acquisition system (12-bit resolution, 40 channels, 100 kHz/channel) was newly developed for data processing. A weak m=1/n=1 mode which is locked with an m=2/n=1 mode is observed preceding an external disruption. Once the disruption occured, the m=2/n=1 mode is converted into a strong m=1/n=1 mode. The internal thermal energy is swept out accompanied by vertically asymmetric collapse of soft X-ray profile due to the m=1/n=1 mode. A localized flash on the limiter is always observed due to a major disruption (a hard external disruption). These phenomena can be explained by the theory of asymmetric reconnections due to the interaction of the m=2/n=1 and m=1/n=1 modes proposed by B. B. Kadomtsev. If the amplitude of the m=2/n=1 mode becomes sufficiently large, the resulting islands begin to come closer and to produce a m=1/n=1 shift at the center. And eventually asymmetric reconnections will occur at the place where the shifted inner region due to the m=1/n=1 mode touches the X-point of the separatrix of the m=2/n=1 magnetic island. Since the modes with different helicities can not exist at the same place, the m=2/n=1 islands approach with each other and merge into an m=1/n=1 island. During this process the internal thermal energy is expelled in the same way as the usual internal disruption. If the reconnections extend to the plasma boundary, they may lead to a major diruption. Although the overlapping of the m=2/n=1 and m=3/n=2 islands is experimentally observed, the resulting ergodization of the magnetic surfaces only flatten the soft X-ray profile in the region between q=2 and q=3/2 surfaces.
- 核融合科学研究所の論文