The characteristics of neutral-beam-heated ASDEX discharges exhibiting either low (L)or high @)-confinement are described. H-mode discharges, which are by now observed over a wide range of operating conditions, show a spontaneous imnroverrlent in particle and energy confinerent after a short i-phase at the beginning of neutral injection. Y-discharges yield high R values comparable to the aspect ratio A ( R p 5 2.65 -0.65 A). The most important parameter for transition to the H-node seems to be 2 high edge electron temperature: Te values of -600 e17 only a few centimeters inside the separatrix with radial gradients of -300 eV/cm are measured. This requirement of high edge temperatures explains the lack of success in obtaining the H-regime in limiter discharges. Numerical simulation of the broad ne and Te profiles typical of H-mode plasmas indicates a reduction in electron thermal diffusivity by a factor of typically 2 over the entire plasma. H-mode energy confinement times are found to scale linearly with current, but to have little dependence on ?lama density and absorbed beam power (P~~<3.4Mld). The confinement is degraded by a fast growing mode localized at the plasma edge that may be identified as a kink or tearing mode driven unstable by the high current densities at the edge. P 1.

High fusion power experiments using DT mixtures in ELM-free H mode and optimized shear regimes in JET are reported. A fusion power of 16.1 MW has been produced in an ELM-free H mode at 4.2 MA/3.6 T. The transient value of the fusion amplification factor was 0.95±0.17, consistent with the high value of nDT(0)τEdiaTi(0) = 8.7 × 1020±20% m-3 s keV, and was maintained for about half an energy confinement time until excessive edge pressure gradients resulted in discharge termination by MHD instabilities. The ratio of DD to DT fusion powers (from separate but otherwise similar discharges) showed the expected factor of 210, validating DD projections of DT performance for similar pressure profiles and good plasma mixture control, which was achieved by loading the vessel walls with the appropriate DT mix. Magnetic fluctuation spectra showed no evidence of Alfvénic instabilities driven by alpha particles, in agreement with theoretical model calculations. Alpha particle heating has been unambiguously observed, its effect being separated successfully from possible isotope effects on energy confinement by varying the tritium concentration in otherwise similar discharges. The scan showed that there was no, or at most a very weak, isotope effect on the energy confinement time. The highest electron temperature was clearly correlated with the maximum alpha particle heating power and the optimum DT mixture; the maximum increase was 1.3±0.23 keV with 1.3 MW of alpha particle heating power, consistent with classical expectations for alpha particle confinement and heating. In the optimized shear regime, clear internal transport barriers were established for the first time in DT, with a power similar to that required in DD. The ion thermal conductivity in the plasma core approached neoclassical levels. Real time power control maintained the plasma core close to limits set by pressure gradient driven MHD instabilities, allowing 8.2 MW of DT fusion power with nDT(0)τEdiaTi(0) ≈ 1021 m-3 s keV, even though full optimization was not possible within the imposed neutron budget. In addition, quasi-steady-state discharges with simultaneous internal and edge transport barriers have been produced with high confinement and a fusion power of up to 7 MW; these double barrier discharges show a great potential for steady state operation. © 1999, Euratom

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