A. Komori

S. Masuzaki

S. Sakakibara

T. Morisaki

National Institute for Fusion Science (NIFS-Repository)

§27. Cancellation of Magnetic Islands by an LID Coil System Komori, A., Masllzaki, S., Morisaki, T., Sakakibara, S. The LID is a divertor that uses an min = 1/1 island fonned at the edge region, so the LID experiment is perfonned by combining a coil system for controlling magnetic islanm and an LID head system for receiving heat and particle fluxes from the core. The LID coil system consists of 20 island control coils, installed around the upper and lower ports of the LIID cryostat. When a resonant perturbation field, generated by a couple of island control coils located above and below the cryostat, is added to the . standard LHD magnetic configuration, an min = 1/1 island appears at the ll2Jt = 1 surface, together with min = 2/1 islanm which appear owing to the toroidal coupling at the ll2Jt = 0.5 surface. However, since the min = 2/1 islanm can be generated by another couple of island control coils, the min = 2/1 islanm are eliminated by a proper arrangement of the coil currents, so that a clean min = 1/1 island is generated [1]. In other worm, the min = 1/1 and 2/1 islanm can be controlled indepemEntly by the LID coil system. Thus the LID coil system is also used to eliminate natural islanm, which may be produced by error field, for example, due to ferromagnetic material located near LHD and small misarrangement of helical and poloidal coils. Magnetic flux mapping was done under steady-state conditions, using the fluorescent method, after the first and second campaigns. The mapping perfonned at reduced magnetic fielm ranging up to 0.25 T just after the first campaign, showed clearly that there are closed, nested magnetic surfaces in good agreement with the theoretical calculation. Although clear min = 1/1 and 2/1 islanm were also observed, they were considered to be fonned by the terrestrial magnetism, because the poloidal phases and maximum widths of the islanm, calculated taking into account the terrestrial magnetism, agreed well with those of the experimental results. After the second campaign, the mapping was carried out at Bt = 2.75 T and Rax = 3.6 m. Surprisingly, there exist the clear min = 1/1 island with a maximum width of about 8 em and 2/1 islanm with a maximum width of about 5 em, as shown in Fig. 1. An error magnetic field arising from the misalignment of the coils, limited to 2 mm in the calculation of the plan, is expected to fonn the min = 1/1 island with a maximum width of only about 4 cm. The cause of such large islands is not clear at this stage, and there are a lot of possibilities, as mentioned above. The size of the islanm can be, however, reduced by the LID coil system. Figure 2 shows that both 1/1 and 2/1 islanm are almost eliminated by the LID coil system at the same time, and more accurate magnetic surfaces are realized, than those fonned with the 2-mm misalignment of the coils, pennitted in the calculation of the plan. The plasma perfonnance will be examined with the reduced magnetic islands in the third campaign. Fig. 1. Magnetic surface mapping at 2.75 T before the cancellation. Fig. 2. Magnetic surface mapping at 2.75 T after the cancellation. References [1]Komori, A., et al., in Plasma Physics and Controlled Nuclear Fusion Research 1994, Seville (IAEA, Vienna, 1996), Vol. 2, p. 773. 31 

Cancellation of Magnetic Islands by an LID Coil System

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