Session 7IB - Alternative Concepts.
INVITED session, Thursday morning, November 14
Grand Ballroom, Adam's Mark
Current profile control is employed in the MST reversed field pinch to reduce the magnetic fluctuation amplitude. Global energy confinement is observed to improve by as much as five times. The dominant transport mechanism in the RFP is magnetic flutter transport. Large amplitude magnetic fluctuations arise due to resistive tearing instabilities and lead to a stochastic magnetic field topology; parallel particle and heat transport along the stochastic field result in rapid radial losses. On MST our strategy for improving confinement is to reduce or eliminate the free energy source for the tearing modes, i.e. the gradient in the parallel current density. The current profile is flattened by application of an inductive poloidal electric field. This method of current profile control, although transient, demonstrates the expected reduction in fluctuation amplitude (as much as 40%), and improvement in global confinement; a MST record low fluctuation amplitude and record high electron temperature (600 eV) were observed simultaneously during current drive experiments. The energy confinement time improves by as much as five times with application of the poloidal current drive pulse. Plasma beta increases by as much as 50% and the Ohmic input power drops by a factor of three or more. Particle confinement improves and plasma impurity contamination is reduced. The results of the transient current drive experiments provide motivation for development of steady-state current drive schemes. Experiments are underway on MST to drive parallel current with electrostatic sources (plasma source electron guns), and experiments using RF current drive in the lower hybrid frequency range are in the development stage. Work supported by USDOE grant No. DE-FG02-96ER54345. \ddag In collaboration with J.S. Sarff, S.C. Prager, N.E. Lanier, and the MST Group.