29th European Physical Society Conference

Modelling of the Current Hole in a Tokamak

A.V.Chankin¹, V.S.Mukhovatov², T.Fujita¹, and Y.Miura^1
1Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, Japan
²ITER EDA, Naka Joint Work Site, Mukoyama, Naka-machi, Naka-gun, Ibaraki-ken, Japan

Abstract.
A high confinement equilibrium with nearly zero toroidal current in the central region (a "current hole") has been recently observed in JT-60U [1] and JET [2]. In the case of JT-60U, the current hole was sustained for several seconds. This observation indicates the possibility of a stable non-inductive tokamak operation, which could be a promising scenario for the tokamak-reactor. The formation of the central region with very low current density, j_||, has been successfully explained as a result of the increase of the off-axis non-inductive current which reduces inductive toroidal electric field and toroidal current density in the centre [1,2]. At the same time, the stability of the current hole (shrinks in time, but maintains nearly zero j_|| in the centre with sharp gradients of j_|| at its edge) remained an unresolved issue.
To address this issue, a novel approach based on the solution of the Grad-Shafranov equation with boundary conditions set on the inside, near the magnetic axis, was developed. Such approach proved useful for testing theoretical concepts for the formation and sustainment of the current hole. It is shown that the stability of the current hole can be explained by assuming that the plasma confinement requires a certain minimum of the poloidal field B_p, which could be related to the level of error fields. This critical B_p is reached at the edge of the current hole. The code separates current density into Ohmic (j_oh) + externally driven (j_ext), Bootstrap (j_bs) and Pfirsh-Schluer (j_ps) currents. The latter was found to dominate at the edge of the current hole. Large j_ps can force a negative net j_|| on the inboard side of the edge of the current hole, resulting in an unrealistic solution with B_p falling below B_p,crit just outside of the current hole on the inboard. Our findings, therefore, indicate the need for an externally driven current either inside or at the edge of the current hole, for such a scenario to be realized in a tokamak-reactor without inductive current.

[1] Fujita T et al, Phys. Rev. Lett. 87 (2001) 245001.
[2] Hawkes N C et al., Phys. Rev. Lett. 87 (2001) 115001.