The thermal-barrier concept for tandem magnetic mirrors has greatly enhanced the attractiveness of this approach to confining high-temperature plasmas for fusion power. Transient existence of thermal barriers has already been demonstrated in the TMX experiment at Livermore and in the gamma 10 experiment in Japan, using electron cyclotron heating (ECH) techniques. If the thermal barrier electrostatic potential well is to be maintained for significant periods of time in collisionally equilibrated plasmas, it is essential to develop effective, economical techniques for preventing the accumulation of positively-charged ions in the potential well. That is, electrostatically-trapped ions that have diffused into the well from the central cell must be pumped out of the well. No entirely satisfactory concept for accomplishing this pumping has yet been developed, although it seems likely that RF electric fields parallel to the confining magnetic field could be useful for this purpose if their space and time dependences could be synchronous with the ballistic motion of the marginally-trapped ions. Here we propose a novel concept for creating suitable electric fields deep in the interior of the thermal barrier plasma by modulating the frequency of the fundamental-resonance ECH power. The concept is based on the well-known accumulation of the turning points of mildly-relativistic electrons about the resonant surface where their gyrofrequency matches the frequency of ECH microwave power. These electrons, whose energies are typically some tens of kev, are confined within a cylinder terminated by the (symmetric) resonant surfaces. Their density can apparently be varied by modulating the ECH frequency. We will seek to establish the dynamical limits on this concept and, in particular, adapt it to provide the desired synchronism with trapped ions. A demonstration of the concept will be planned for Phase II.
