Energy confinement studies of lower hybrid current driven discharges in the Alcator C tokamak

The energy confinement properties of purely rf-driven plasmas on Alcator C are being investigated by experimental measurements of the bulk electron and ion temperature profiles, and by numerical modelling of the lower hybrid wave propagation and thermal energy transport. Power balance studies are performed on plasmas with parameters n bar/sub e/ = 3 - 7 x 1013 cm-3, B = 7 - 11 T, I/sub p/ = 100 - 200 kA, and q (a) greater than or equal to 8, and with rf powers up to 1 MW at 4.6 GHz. The temperature measurements from rf-driven discharges are compared with those from similar ohmic discharges (identical current and density). The gross energy confinement time is lower in the rf-driven discharges than in the ohmic ones by a factor of 1.5 to 4, depending on the plasma conditions and rf power. While tau/sub E/ in ohmic discharges increases with density and is independent of the toroidal field, the confinement time in the rf-driven discharges decreases with rf power, is independent of density in the range n bar/sub e/ = 3 - 7 x 1013 cm-3, and increases with the toroidal magnetic field. The confinement degrades slightly with increasing current in both rf-driven and ohmic discharges. The code used to simulate these results employs a lower hybrid ray tracing package, a Fokker-Planck code for the evolution of the fast electron tail, and enhanced thermal transport models for auxiliary-heated plasmas. In the code simulations, more than 80% of the injected rf power is absorbed by electron Landau damping in the inner half of the plasma column, the remainder being dissipated by collisions near the plasma edge. To correctly simulate the experimentally observed temperatures, the electron thermal diffusivity must be increased with rf power and/or density relative to its ohmic values