Fig. 2-3 Time evolution of a current density in an equatorial plane The current density near the central region decays because of a negative electric field as time passes. When the current density at the central part (x~0) has mostly vanished, equilibrium cannot be maintained and the pair vortex motion grows. The current density of the central part of plasma remains almost zero and never cuts it greatly. Within the current hole, a small negative current flows inside of torus (x<0 downstream side).

Fig. 2-4 Driven vortex motion The flow, in an equatorial plane, is directed to the inside of the torus (left side). The vortex rotates, in clockwise in the upper half. In the lower half, not shown in this figure, the vortex rotates counter-clockwise. The current density in the center region is maintained to almost zero regardless of the voltage applied by this whirlpool movement.

Toroidal current is driven by an applied toroidal electric field and we can achieve an equilibrium state, plasma pressure is supported by the magnetic force, in a tokamak system. Furthermore, a bootstrap current will be used to realize a more efficient reactor. A negative voltage appears near the plasma center when a very big bootstrap current flows in the tokamak device to keep the total current constant. Current will flow in a negative direction when a negative voltage is applied in a usual conductor. In the JT-60, however, a negative current was not observed when a negative voltage appeared in the central region. Instead, the current density was found to be almost zero. The central region of the tokamak plasma does not conduct an electric current in the opposite direction though it conducts an electric current in the normal direction. It functions as if it is a huge rectifier. MHD (Magnetohydrodynamics) simulation studies of plasma that applied a negative voltage to the central region of plasma were performed to examine the mechanism that forms the "Current Hole". The outline of the formation and the maintenance mechanism obtained are as follows. The bootstrap current increases at the position of ca. 0.6 of the radius of plasma and a negative electric field appears in the central region of plasma. The area with almost zero current density appears (ca. 2000 Alfvén time in the Fig. 2-3; the Alfvén time, which shows the characteristic time of plasma motion, is used as a time unit). However, plasma equilibrium cannot be supported if the current doesn't flow, and plasma begins to move. At this time, the resultant flow must be confined and have a up-down symmetry, and the plasma on the outside must have equilibrium and remain stationary because the current flows there. Thus, a pair vortices, mutually rotates in the opposite directions, appear as shown in Fig. 2-4. This vortex motion remains driven by the magnetic force due to the slight current that flows in the central region. A negative voltage is canceled by the effective electric field, created by the interaction between the magnetic field and the vortex motion (Fleming's right hand rule), and a negative current barely flows in the center region of plasma. This process can be viewed as a reverse-process of the dynamo effect that the electromotive force is caused by the hydromagnetic movement seen in the generation of terrestrial magnetism, etc., and also is one example of the autonomous formation phenomenon when a system changes its profile by itself under an external influence. This "Current Hole" phenomenon remains stationary without any magnetic fluctuation for a duration longer than the current penetration time. The achievement of a more efficient fusion reactor can be expected by the existence of the "Current Hole" phenomenon that exists stably.

Reference T. Tuda et al., Pair Vortices Formation near Magnetic Axis as an Explanation of the "Current Hole" Sustainment, Proceedings of  20th IAEA Fusion Energy Conference, Nov. 1-6, 2004, Vilamoura, Portugal, IAEA-CN-116/TH/P2-10 (2005).

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