Physics of advanced tokamaks

Significant reductions in the size and cost of a fusion power plant core can be realized if simultaneous improvements in the energy replacement time, {tau}{sub E}, and the plasma pressure or beta, {beta}{sub T} = 2 {micro}{sub 0} <P>/B{sup 2} 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 (RI) mode, the high internal inductance or high {ell}{sub i} scenario, and the negative central magnetic shear, NCS (or reversed shear, RS) 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 E x B flow. There is strong temporal and spatial correlation between the increased sheared E x B 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 {beta}{sub T}/(I/aB) {equivalent_to} {beta}{sub N} {approximately} 4.5%-m-T/MA simultaneously with confinement enhancement over L-mode scaling, H = {tau}/{tau}{sub ITER-89P} {approximately} 4, have been achieved in the DIII-D tokamak. In the JT-60U tokamak, deuterium discharges with negative central magnetic shear, NCS, have reached equivalent break-even conditions, Q{sub DT} (equiv) = 1