H-MODE CONFINEMENT AT LOW Q AND HIGH-BETA IN JET

Good confinement at low values of q0.95 and at high values of total beta is crucial for the economical development of a fusion reactor. It is shown that an extension of the domain of H-mode operation in JET to values of q0.95 ≥ 2.0 (i.e. a plasma current of 3 MA and a toroidal field of 1.2 T), with additional heating at a power of up to 12 MW, does not lead to an appreciable confinement deterioration when q0.95 is decreased, for total beta values of up to 4.2%, corresponding to 90% of the Troyon limit. Pressure profiles with a peaking factor p0/〈p〉 ∼ 3 can be achieved for such low values of q0.95. Analysis of the MHD equilibrium and stability shows that the conditions of marginal ballooning instability near the plasma edge are reached in a discharge with β ∼ 4.2%. © 1990 IOP Publishing Ltd.