Multiple-frequency electron-cyclotron heating of hot-electron rings in a bumpy torus

The use of multiple-frequency microwave power for electron-cyclotron heating significantly increased the ring stored energy in the SM-1 simple mirror device. Multiple-frequency electron-cyclotron heating (MFECH) was used on the ELMO Bumpy torus (EBT) in an effort to increase its hot-electron beta. No substantial improvement in the ring parameters was observed in a series of two-frequency ECH experiments, with frequency separations up to 90 MHz, in contrast to the dramatic improvement found in the axisymmetric SM-1 experiment. The toroidal canting of the EBT mirror sectors introduces asymmetries that destroy the superadiabatic behavior of the energetic electrons, reduce microwave heating efficiency, and produce additional ring losses. These effects qualitatively explain the different multiple-frequency heating results obtained in EBT and SM-1