No abstract

The energy confinement properties of low density, high ion temperature L-and H-mode plasmas are investigated. For L-mode plasmas it is shown that, although the global confinement is independent of density, the energy confinement in the central region is significantly better at low densities than at higher densities. The improved confinement appears to be associated with the steepness of the density gradient. For the H-mode phase, although the confinement at the edge is dramatically improved, which is once again associated with the steep density gradient in the edge region, the central confinement properties are essentially the same as for the standard L-mode. The results are compared in a qualitative manner with the predictions of the ion temperature gradient instability theory and appear to be in disagreement with some aspects of this theory.

Flux-averaged ICRF heating of the JET tokamak is considered, utilizing an anisotropic, time-dependent Fokker-Planck code. Alternative ICRF heating scenarios are considered, and fundamental minority deuterium and second-harmonic tritium and deuterium cases are selected for detailed analysis of wave accessibility, strong single-pass damping and enhanced ion-tail-produced fusions. It is found that enhanced fusions, resulting from heated minority deuterium ion tails, and second-harmonic deuterium heating, yielding fusion power gains near Q = 1, can be achieved near the plasma core with moderate absorbed power levels, pulse widths and energy confinement times. The corresponding case for second-harmonic tritium heating located on the major axis, with the associated fusion power gain, does not appear as attractive.

Two antennae have been installed in JET and operated to the maximum design capability of the generators. 4.5 MW, 10 MJ have been coupled to the plasma which heated up to a maximum stored energy of 3 NJ with central temperatures of TeO = 5 keV and Ti0 = 4 keY without increase of the relative impurity concentration. Degradation of energy confinement is observed according to an L mode scaling. Hydrogen and Helium 3 minority heating regimes give similar results.The effect of k/ shaping is discussed using a quadrupole antenna.

The D-D fusion reactivity from JET plasmas has been optimized by using combined neutral beam injection and radiofrequency heating. Reaction rates of up to 2.9 × 1016 reactions per second (equivalent to neutron emission rates of 1.45 × 10l6 neutrons per second) have been achieved. The best results were obtained in the doublenull magnetic configuration or with plasmas limited by the inner wall. By far the largest fraction of the neutron production originates from beam induced reactions, although, in some cases, about 25% of the production can be attributed to the acceleration of deuterium ions by the radiofrequency. The instantaneous neutron yield was limited by a strong influx of carbon impurity ions due to excessive, but local, wall heating.