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JET experiments of significance to a Next Step tokamak are reported in the areas of plasma transport, steady-state operation and the divertor and impurity control. Non-dimensional scaling experiments, varying only the normalised Larmor radius, favour long wavelength turbulence as the basis for transport, but uncertainties make it impossible to exclude short wavelength turbulence. On- and off-axis heating experiments show that the plasma temperature responds to the heating profile and that this can be described by a local, diffusive transport model. Impurity transport experiments show anomalously poor particle confinement outside q = 1, depending, like anomalous heat transport, on input power and plasma density. Experiments with sixteen, rather than all thirty-two JET toroidal magnetic field coils, show poorer plasma performance and increased fast particle losses. In steady-state operation, bootstrap current fractions as high as 70 % (in 1-1.5 MA discharges) and a bootstrap current of 1 MA (in high performance, hot ion H-modes at 3 MA) have been obtained. These discharges also have high confinement. Current drive efficiencies with LHCD and ICRH have reached 0.45 x 1.0 E20/m2.AW, comparable with the highest achieved elsewhere but at least a factor of 3 lower than required to sustain the full current in an ITER-type reactor. Reliable AC operation at 2 MA has also been demonstrated. Divertor studies stress the benefits of high edge density in a high-recycling divertor for energy exhaust, impurity control and helium pumping. Experiments with strong gas puffing show the formation of a strongly radiating cold gas target in the divertor region and the virtual elimination of conducted power to the target plates. This is apparently easier to achieve with beryllium target plates. The benefits of optimising the ionisation distribution of recycled neutrals through geometry changes is demonstrated computationally.

Additional information

Authors: KEILHACKER M ET AL., JET Joint Undertaking, Abingdon, Oxon. (GB)
Bibliographic Reference: Report: JET-P(92)84 EN (1992) 141 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
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