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ABSTRACT.Reactor relevant ICRH scenarios have been assessed during D-T experiments on the JET tokamak using H-mode divertor discharges with ITER-like shapes and safety factors. Deuterium minority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, an ICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermal deuterons and thermal tritons. The Q-value of the steady state discharge reached 0.22 for the length of the RF flat top (2.7 s), corresponding to three plasma energy replacement times. The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV which was optimum for fusion and close to the critical energy. Thus strong bulk ion heating was obtained at the same time as high fusion efficiency. Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail. The edge localised modes (ELMs) had low amplitude and high frequency and each ELM transported less plasma energy content

In the JET tokamak, ICRF driven fusion reactivity has been determined using measurements of 16.6 MeV γ-ray emission from d[3He, γ]5Li reactions during central RF heating in the (3He)d minority regime. Up to 1 MJ of fast minority ions in the plasma has been produced with the application of up to 15 MW of RF power. The maximum rate produced by d[3He, p]4He fusion reactions has been estimated as 2 × 10l6 s−1 (equivalent to 60 kW of fusion power in charged particle products). The reactivity increased strongly with coupled RF heating power (proportional to (PRF)5/3), with some evidence of a weakening of the dependence leading to a saturation in the energy gain Q at the highest coupled RF power levels (PRF > 8−12 MW). The experimentally measured anisotropic fast ion energies and fusion reaction rates have been simulated using a radially dependent Stix model for a wide variety of discharges. Analysis of the radial profile of fusion reactivity shows that when the RF power density is maximized on the magnetic axis of the discharge, the fusion reactivity is peaked away from the axis. This effect is caused by the minority ions near the centre of the discharge being driven to energies beyond the maximum in the fusion cross-section.

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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.