Significant reductions in the size and cost of a fusion power plant core can be realized if simultaneous improvements in the energy replacement time, , and the plasma pressure or beta, can be achieved in steady-state conditions with high self-driven, bootstrap current fraction. Significant recent progress has been made in experimentally achieving these high performance regimes and in developing a theoretical understanding of the underlying physics. Three operational scenarios have demonstrated potential for steady-state high performance, the radiative improved mode, the high internal inductance or high scenario, and the negative central magnetic shear, NCS (or reversed shear) scenario. In a large number of tokamaks, reduced ion thermal transport to near neoclassical values, and reduced particle transport have been observed in the region of negative or very low magnetic shear: the transport reduction is consistent with stabilization of microturbulence by sheared flow. There is strong temporal and spatial correlation between the increased sheared flow, the reduction in the measured turbulence, and the reduction in transport. The DIII-D tokamak, the JET tokamak and the JT-60U tokamak have all observed significant increases in plasma performance in the NCS operational regime. Strong plasma shaping and broad pressure profiles, provided by the H-mode edge, allow high beta operation, consistent with theoretical predictions; and normalized beta values up to simultaneously with confinement enhancement over L-mode scaling, , have been achieved in the DIII-D tokamak. In the JT-60U tokamak, deuterium discharges with negative central magnetic shear have reached equivalent breakeven conditions, .
Local magnetic shear in tokamak plasmas