A negatively-sheared magnetic field is produced by making the plasma current distribution hollow. The safety factor then decreases from the center to a minimum near the plasma edge.

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Fig. 2-2 Simulation results of plasma turbulence and magnetic shear Dependence of magnitude of plasma turbulence on magnetic shear is calculated by particle simulation technique. In the plasma cross sections shown, less turbulence is observed in the negative shear case (c) compared to the positive shear cases (a) and (b).

In tokamak plasmas, plasma turbulence (unstable waves) often extends over a wide region of the confined plasma, eventually deteriorating confinement. An effective way to suppress the turbulence and therefore to improve the confinement is to produce a confining magnetic field that has a particular structure called "negative shear." Such a field is generated by controlling the plasma current distribution so that it is hollow, as shown in Fig. 2-2. In a plasma confined in a negative-shear magnetic field, a virtual confinement wall called an "internal transport barrier (ITB)" appears, that prevents turbulence-induced thermal flow out from the core plasma. We have confirmed experimentally that, once the ITB is formed in the plasma, the confinement improves remarkably inside the ITB. We have shown by theoretical and simulation studies that the negative magnetic shear has a general stabilizing effect on the turbulence, and that the growth of the turbulence is suppressed globally in the negative shear region. In addition to this overall effect, the growth rate of the turbulence becomes extremely small at the zero or very close to zero magnetic shear region, so that the structure of the turbulence is broken up, or a gap is formed at this boundary, and no global structure can exist in the plasma. The existence of this zero or weak-shear boundary is considered to play a crucial role in the formation of the internal transport barrier which has been observed in JT-60 experiments.

Reference Y. Kishimoto et al., Effect of Weak/Negative Magnetic Shear and Plasma Shear Rotation of Self-organized Critical Gradient Transport in Toroidal Plasmas, 16th IAEA Fusion Energy Conference, Montreal, Canada, IAEA-CN-64/DP-10 (1996).

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