Tests of local transport theory and reduced wall impurity influx with highly radiative plasmas in the Tokamak Fusion Test Reactor

The electron temperature ${\mathrm{(T}}_e)$ profile in neutral beam-heated supershot plasmas ${\mathrm{(T}}_{\text{e0}}\text{∼6–7\hspace{0.05em}}\mathrm{keV}$ ion temperature $T_{\text{i0}}\text{∼15–20\hspace{0.05em}}\mathrm{keV},$ beam power $P_b\text{∼16\hspace{0.05em}}\mathrm{MW})$ was remarkably invariant when radiative losses were increased significantly through gas puffing of krypton and xenon in the Tokamak Fusion Test Reactor [McGuire et al., Phys. Plasmas 2, 2176 (1995)]. Trace impurity concentrations ${\mathrm{(n}}_z{\mathrm{/n}}_e{\text{∼10}}^{\text{−3}})$ generated almost flat and centrally peaked radiation profiles, respectively, and increased the radiative losses to 45%–90% of the input power (from the normal ∼25%). Energy confinement was not degraded at radiated power fractions up to 80%. A 20%–30% increase in $T_i,$ in spite of an increase in ion–electron power loss, implies a factor of ∼3 drop in the local ion thermal diffusivity. These experiments form the basis for a nearly ideal test of transport theory, since the change in the beam heating power profile is modest, while the distribution of power flow between (1) radiation and (2) conduction plus convection changes radically and is locally measurable. The decrease in $T_e$ was significantly less than predicted by two transport models and may provide important tests of more complete transport models. At input power levels of 30 MW, the increased radiation eliminated the catastrophic carbon influx (carbon “bloom”) and performance (energy confinement and neutron production) was improved significantly relative to that of matched shots without impurity gas puffing.