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

Energy confinement is studied in lower hybrid current driven (LHCD) plasmas in Alcator C in the density range _e = (1–8) × 10¹³ cm⁻³. In LHCD plasmas, the stored energy in the electron tail, W^tail, can be a significant fraction of the total stored energy, W^tot, especially at lower densities. At sufficiently low densities, the confinement time of the high energy electrons is expected to become shorter than their collisional slowing down time, and direct energy losses from the electron tail can become important in the overall power balance. The global energy confinement time, defined as is found to be comparable to or exceed that in ohmically heated (OH) plasmas at low densities _e ⪅ 3 × 10¹³ cm⁻³, where a steady state current can be maintained with relatively low RF power. However, at higher densities where substantially more RF power is needed (relative to the Ohmic power required to maintain a similar plasma), a deterioration of relative to Ohmic confinement, similar in magnitude to that predicted by the neutral beam heated L-mode scaling, is observed. Theoretical modelling with the aid of a ray tracing Fokker-Planck transport code suggests that the deteriorated confinement in this high density, high power regime may be attributed to an enhanced bulk electron thermal diffusivity. In a combined OH-LHCD plasma, a value of greater than the Ohmic value is obtained as long as the applied RF power does not significantly exceed the Ohmic power.