Electric and magnetic structure of tokamak edge plasma with static and rotating helical magnetic limiter

Abstract Modification of tokamak edge plasma by an ergodic magnetic limiter is studied. Field line structures disturbed by a series of local helical coils are precisely investigated to make clear the presence of a distinct structure with the periodicity of the local helical coil near the wall as well as the presence of a chaotic region somewhat away from the wall. The enhancement of recycling due to the increase in plasma transport at the edge was confirmed with a two-dimensional profile of Ly α emission obtained by computed tomography. Poloidal modulations of the edge plasma parameters were detected with a Langmuir probe, and compared with the field line structure. It was found that such a nonuniform wall loading could be avoided by using a rotating helical field with a pair of local helical coils driven by an orthogonal ac current.