Temperature limit in ECH hot electron plasmas

During the last two decades the production of high-beta, hot electron plasmas with electron cyclotron heating (ECH) has been amply demonstrated in open and closed geometries. A wide variety of conditions was present in these experiments with a factor of 2 change in device dimensions and more than an order of magnitude change in magnetic fields (approx. 1 to 10 kG), ECH frequencies (approx. 6 to 55 GHz), and hot electron temperatures (approx. 50 to 1200 keV). An analysis of the data from all the experiments that used single ECH frequency indicates that the hot electron temperatures do increase with magnetic field strength (or, equivalently, ECH frequency) and scale length. In particular, they all obey rho/L approx. = constant (approx. 5 to 6 x 10-2) scaling, where rho and L are the hot electron gyroradius (relativistic) and the magnetic field scale length, respectively. This is roughly the value at which conservationof the adiabatic invariant μ begins to break down and suggests that the hot electron temperatures are probably limited by nonadiabatic particle behavior. Results, primarily from hot electron ring experiments (ELMO, EBT, NBT, etc.), are discussed, and projections for future experiments are given. It is shown that although in all previous experiments the ring temperature is determined by the rho/L criterion EBT-P will be the first experiment unconstrained by this limit