Tests of local transport theory and reduced wall impurity influx with highly radiative plasmas in the tokamak fusion test reactor

The electron temperature (T-e) profile in neutral beam-heated supershot pla smas (T-e0 similar to 6-7 keV ion temperature T-i0 similar to 15-20 keV, be am power P-b similar to 16 MW) was remarkably invariant when radiative loss es were increased significantly through gas puffing of krypton and xenon in the Tokamak Fusion Test Reactor [McGuire et al., Phys. Plasmas 2, 2176 (19 95)]. Trace impurity concentrations (n(z) /n(e) similar to 10(-3)) generate d almost flat and centrally peaked radiation profiles, respectively, and in creased the radiative losses to 45%-90% of the input power (from the normal similar to 25% ). Energy confinement was not degraded at radiated power fr actions up to 80%. A 20%-30% increase in T-i, in spite of an increase in io n-electron power loss, implies a factor of similar to 3 drop in the local i on thermal diffusivity. These experiments form the basis for a nearly ideal test of transport theory, since the change in the beam heating power profi le is modest, while the distribution of power flow between (1) radiation an d (2) conduction plus convection changes radically and is locally measurabl e. The decrease in T-e was significantly less than predicted by two transpo rt models and may provide important tests of more complete transport models . At input power levels of 30 MW, the increased radiation eliminated the ca tastrophic carbon influx (carbon "bloom'') and performance (energy confinem ent and neutron production) was improved significantly relative to that of matched shots without impurity gas puffing. (C) 1999 American Institute of Physics. [S1070-664X(99)02903-1].