Magnetic Diagnostics of Magnetic Island in LHD

抄録

Characteristics of magnetic islands are investigated by magnetic diagnostics in the Large Helical Device (LHD). The structure of the magnetic island with m/n = 1/1 (where, m and n are poloidal and toroidal mode number, respectively) can be estimated from the perturbed magnetic field appearing when a magnetic island changes. To measure the toroidal profile of the perturbed magnetic field δb₁ originating from the plasma, a toroidal array of magnetic flux loops is set up in the LHD. The toroidal profile of δb₁ is then spatially Fourier decomposed to determine the amplitude of the n = 1 component, δb₁ⁿ⁼¹ and its phase, φ_n=1 which correspond the change of the island width and the toroidal position of the X-point of the island, respectively. Therefore, the information about the magnetic island structure can be obtained from δb₁ⁿ⁼¹ and φ_n=1. In case the island width becomes larger than the seed island, measurements show that δb₁ⁿ⁼¹ is non-zero and φ_n=1 is temporally constant. A non-zero δb₁ⁿ⁼¹ can also be observed when the island width becomes smaller than the seed island. In this case, the angle φ_n=1 shifts by about π[rad] compared with the increasing case and the δb₁ⁿ⁼¹ is limited to a certain value which corresponding to the magnetic field suppressing the seed island.