Stiffness of central current and temperature profiles in JT-60U current hole plasmas

Full text: In an extreme situation of the hollow current profile, a region with nearly zero plasma current ('current hole') is formed and the temperature profile becomes nearly flat in the central part of tokamak plasma. Our concern is whether these profiles, namely small current density and temperature gradients, are the results of small driving force (current drive and heating) or are maintained by some damping mechanism against the driving force. To address this issue, we applied local current drive by EC waves and inductive electric field and applied local heating by EC waves in the current hole to enhance the driving force, and investigated responses of profiles in JT-60U. We found that no toroidal current was driven in the current hole, both in co- and counter-directions to the plasma current, in spite of high electric conductivity. On the other hand, an electron temperature gradient was formed with heating localized on the low-field-side equatorial plane. These results, namely stiff current profile and non-stiff temperature profile, suggest that there are finite poloidal fields to confine electrons and a mechanism to clamp the current density nearly zero in the current hole. (author)