Stable existence of central Current Hole in the JT-60U tokamak

An advanced tokamak scenario by a reversed magnetic shear configuration has a hollow current profile and shows very high confinement with an internal transport barrier (ITB) inside the position of the minimum safety factor. In an extreme situation of the hollow current profile, it was found for the first time in JT-60U that there is no plasma current in the central region (called Current Hole). Though the current was believed to be necessary for tokamak plasma equilibrium. Moreover, it was observed for the first time that the Current Hole region extends to 40% of the plasma minor radius and it exists stably for several seconds. The Current Hole is formed by the growth of the bootstrap current and it is impossible to drive current in either positive or negative direction by ECH or N-NB in the Current Hole. In that region, there is almost no gradient of density, temperature and toroidal rotation velocity. The density fluctuation in the flat density region is about one order of magnitude smaller than that in the ITB. These results suggest that there may be very little free energy source, such as current or pressure gradient to drive fluctuation in the stable Current Hole. (author)