Modelling of the current hole in JT-60U

Magnetic equilibria of central regions of the plasma in a regime with the current hole with a large radius in JT-60U have been analysed. The analysis was based on solutions of the Grad-Shafranov equation with prescribed dependence of the poloidal flux function Psi on minor radius at the outer midplane, corresponding to experimentally measured current density (j(parallel to)) profile. The code separates total toroidal current density into Ohmic plus externally driven (j(ex)), bootstrap (j(bs)) and Pfirsch-Schluter (j(ps)) contributions. Under experimental conditions of JT-60U, the latter was found to dominate at the edge of the current hole. The experimentally observed sharp j(parallel to) rise at the edge of the current hole is mostly attributed to jps. This is caused by very high safety factor q at the edge of the current hole (similar to100 or larger), which both increases Pfirsch-Schluter current and decreases relative contribution of the bootstrap current (the latter-due to high dimensionless electron collisionality v(e)(*), with the plasma being in the plateau collisionality regime). The results also reveal an important role played by Ohmic and/or externally driven current (e.g. by electron cyclotron current drive) at the edge of the current hole, for existence of closed magnetic surfaces. A certain amount of j(ex) is necessary to compensate for negative Pfirsch-Schluter current at the inboard side of the current hole edge, which may otherwise cause a local drop in poloidal magnetic field and the formation of an internal X-point.