Fig. 2-14 Equilibrium configuration of a tokamak plasma with three axially symmetric magnetic islands Assume a magnetic structure having three magnetic islands located along the R direction with an interval of r1 (the distance between adjacent magnetic axes) and two X-points (a crossing point of magnetic lines of force) along the Z direction at Z = Z1 and Z = - Z1 in the central plasma region (upper below). The central magnetic island has a small toroidal current flowing in the counter-direction with respect to the tokamak electric field (shown in blue). The adjacent two islands also have co-directed currents. The magnitude of the currents is much smaller than the total plasma current. A large bootstrap current (a self-generated current in high-temperature plasmas) exists in the region surrounding the three magnetic islands (shown in red, encircling the three magnetic islands, upper below). A hollow current distribution or a current hole is formed in the region of the three magnetic islands in the center of the tokamak plasma as seen in the current distribution (shown in upper below). The proposed model describes stable equilibrium configuration of the JT-60 tokamak with a current hole. The equilibrium configuration and the current distribution discussed above are in marked contrast with those of a standard tokamak, as shown schematically in the figure, upper above.

Recent experiments with the JT-60 tokamak being operated in a reverse-shear configuration have revealed that a stable confinement of high-temperature plasmas can be obtained in a tokamak that has almost no toroidal plasma current in the central plasma region, contrary to the usual tokamak concept. We have made a theoretical investigation to elucidate how such a particular tokamak configuration, which has been unknown to tokamak physics, could stably exist. The experimental results will be well interpreted theoretically if we assume a particular complex magnetic structure consisting of three axially symmetric magnetic islands in the central plasma region, in contrast with the magnetic field structure of a standard tokamak having a set of coaxially nested, doughnut-shaped magnetic surfaces woven by twisted magnetic lines of force (Fig. 2-14). The magnetic islands are spatially localized, closed magnetic field structures formed through turbulent magnetohydrodynamic interactions between the local magnetic field and high-temperature plasmas in the tokamak. The positional balance (equilibrium) of this new magnetic structure was found to be maintained by the resultant effect of electromagnetic forces acting among small currents flowing in the three magnetic islands, the currents being much smaller than the total plasma current, and forces exerted on the currents in the magnetic islands by the external control field coils. This equilibrium configuration was also found to be stable, for the currents in the three magnetic island regions remain very small, and the derived theoretical stability condition is quite consistent with the experimental results. An experimental confirmation of the above-proposed equilibrium configuration in JT-60 will be pursued by observing the non-flat structures of the distribution of plasma density and temperature, and other physics phenomena in the three central magnetic island regions, the current hole region.

Reference T. Takizuka, Axisymmetric Tri-Magnetic-Islands Equilibrium of Strongly-Reversed-Shear Tokamak Plasma: An Idea for the Current Hole, Purazuma, Kaku Yugo Gakkai-Shi, 78(12), 1282 (2002).

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