Physics of high performance JET plasmas in DT
The Joint European Torus (JET) has recently operated with deuterium-tritium (DT) mixtures, carried out an International Thermonuclear Experimental Reactor (ITER) physics campaign in hydrogen, deuterium, DT and tritium, installed the Mark IIGB 'Gas Box' divertor fully by remote handling and started physics experiments with this more closed divertor. The DT experiments set records for fusion power (16.1 MW), ratio of fusion power to plasma input power (0.62, and 0.95 +/- 0.17 if a similar plasma could be obtained in steady state) and fusion duration (4 MW for 4 s). A large scale tritium supply and processing plant, the first of its kind, allowed the repeated use of the 20 g of tritium on-site to supply 99.3 g of tritium to the machine. The H mode threshold power is significantly lower in DT, but the global energy confinement time is practically unchanged (no isotope effect). Dimensionless scaling 'wind tunnel' experiments in DT extrapolate to ignition with ITER parameters. The scaling is close to gyro-Bohm, but the mass dependence is not correct. Separating the thermal plasma energy into core and pedestal contributions could resolve this discrepancy (leading to proper gyro-Bohm scaling for the core) and also account for confinement degradation at high density and at high radiated power. Several radiofrequency heating schemes have been tested successfully in DT, shearing good agreement with calculations. Alpha particle heating has been clearly observed and is consistent with classical expectations. Internal transport barriers have been established in optimized magnetic shear discharges in DT and steady state conditions have been approached with simultaneous internal and edge transport barriers. First results with the newly installed Mark IIGB divertor show that the in-out symmetry of the divertor plasma can be modified using differential gas fuelling, that optimized shear discharges can be produced and that krypton gas puffing is effective in restoring L mode edge conditions and establishing an internal transport barrier in such discharges.