Submitted for Publication in: Nuclear Fusion ENERGY CONFINEMENT STUDIES OF LOWER HYBRID CURRENT DRIVEN PLASMAS

ABSTRACT. Energy confinement is studied in lower hybrid current driven (LHCD) plasmas in Alcator C in the density range ii = (1-8) x 1013 cm- 3. In LHCD plasmas, the stored energy in the electron tail Wail can be a significant fraction of the total stored energy W"*, especially at lower densities. At sufficiently low densities, the energy confinement time of the high energy electrons is expected to become shorter than their colli-sional energy 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 rE* * = W**t/Ptot, is found to be compara-ble to or exceed that in ohmically heated (OH) plasmas at low densities n, < 3 x 1013 cm- 3, where a steady state current can be maintained with * present address: University of Texas, Austin, Texas, U.S.A. 1 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 rtot relative to ohmic confinement, similar to that predicted by the neutral beam heated L-mode scaling, is observed. Theoretical modeling 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 dif-fusivity. In a combined OH-LHCD plasma, a value of r ' greater than the ohmic value is obtained as long as the applied rf power does not significantly exceed the ohmic power. 2 1