A simple annulus power balance in EBT experiments

Abstract

An essential feature of the ELMO Bumpy Torus (EBT) concept is the presence of a relativistic electron annulus in each of the toroidal mirror sectors. These high-beta annuli are formed and sustained by microwave heating and are of sufficient density and temperature that diamagnetic currents produce the necessary minimum in the magnetic field required for MHD stability of the toroidal core plasma. Since electron rings play an important role in confinement characteristics and performance of EBT, the trade off between the quality of the confinement afforded by the rings and the power required to sustain them represents an important consideration in a fusion reactor. A power balance for the rings that includes energy transfer between the hot annulus electrons and the core electrons via Coulomb collisions (drag cooling), in addition to radiation (synchrotron and bremsstrahlung) and pitch-angle scattering of the ring electrons, indicates that drag dominates the annulus energy balance in EBT-I. Drag cooling of the relativistic annulus electrons on the toroidal core plasma appears to provide a reasonable explanation for the decrease in the annulus electron temperature in going from ELMO to EBT-I. Theoretical estimates of the microwave power required to sustain the annulus are found to be within a factor of two of the experimentally determined value. Scaling projections for both EBT-I and EBT-S are shown, enabling one to examine the sensitivity of the annulus electron temperature as a function of core plasma density for various microwave power levels. The results indicate that a range of annular beta values between 15–30% are achievable for projected EBT-S toroidal plasma parameters if the microwave power levels are in the range of ∼ 10–15 kW. Further extrapolations to future planned experiments are also included. The sensitivity of these results to the details of the hot-electron distribution function and geometric and scaling parameters are also discussed.