ECH plasma potential structures were measured by Langmuir probes in the Tohoku University HELIAC. By adjusting the coil current ratio of the center conductor and the toroidal coils, 2 characteristic configurations were selected, the 1st case with the n ̸ m = 3/2 rational surface, the 2nd case without the rational surface. A perturbation field was added in the 1st case in order to produce m=2 magnetic islands. Before ECH turn-off the potential structure near the ECH antenna strongly depends on MOD B surfaces; on the other hand, away from antenna the potential structure agrees well with magnetic surfaces, but peculiar regions resembling m=2 islands appear along a rational surface and disappear in the case without a rational surface. After ECH turn-off both potential structures, near and away from the antenna, not only agree very well with magnetic surfaces, but also preserve the magnetic island structures.
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