Achieving high Q(sub DD) operation in high poloidal beta discharges in TFTR

摘要

Tokamak operation at high poloidal beta ((var epsilon)(beta)(sub p) (approximately) 1, (var epsilon)(quadruple bond) a/R) offers several important advantages to future fusion reactors: low plasma current, high bootstrap current fraction, and the possibility of improved confinement due to equilibrium modification. However, since the plasma current is low at high (var epsilon)(beta)(sub p), the achievement of high fusion reactivity generally requires operation at a high Troyon-normalized beta, (beta)(sub N) (quadruple bond) 10(sup 8) <(beta)> aB(sub t)/I(sub p), and a high L-mode normalized confinement, H (quadruple bond) (tau)E /(tau)E,ITER(sub p). The TFTR high poloidal beta experiments have explored this advanced operating regime in reactor-like conditions, and we have demonstrated techniques that can be used to achieve high Q(sub DD) at high (var epsilon)(beta)(sub p). In this paper, we present new results showing that Q(sub DD) as high as 1.3 (times) 10(sup (minus)3), can be achieved at (var epsilon)(beta)(sub p) (approximately) 0.85 using transient current profile modification techniques, which nevertheless, maintain high Q(sub DD) for an alpha slowingdown time. Future DT experiments in TFTR will be able to study collective alpha-particle physics in discharges having high (var epsilon)(beta)(sub p), high (beta)(sub N), and evolving current profiles. This paper also illustrates the need for steady-state current profile control to sustain high Q(sub DD) at high (var epsilon)(beta)(sub p) by presenting recent observations of long-pulse, high (beta)(sub p) discharges in which the current density is allowed to relax naturally to a broad, bootstrap-dominated profile.