Temperature limit in ECH hot electron plasmas

Abstract

During the past two decades the production of high-beta, hot electron plasmas with electron cyclotron heating (ECH) has been demonstrated in open and closed geometries. Compilation of data from these experiments shows that in all cases the limiting hot electron temperature is determined by the scaling law ρ/L=const (∼5–6×10−2), where ρ is the hot electron gyroradius (relativistic) and L is the magnetic-field scale length. Results cover a wide variety of conditions in these experiments with a factor of 2 change in device dimensions and more than an order-of-magnitude change in magnetic fields (∼1–10 kG), ECH frequencies (∼2.4–55 GHz), and hot electron temperatures (∼10–1300 keV). The observed scaling law suggests that nonadiabatic effects play an important role in determining the limiting hot electron temperature. It is shown, however, that although in all previous experiments the limiting temperature is determined by the ρ/L criterion future experiments may not reach this limit.