Enhancement of electro dynamic tether electron current collection using radio frequency power: Numerical modeling and measurements

Tether electron current collection in the Orbital Motion Limited regime is one of the limiting factors in power and thrust generation applications of electro dynamic tethers. Injection of radio frequency power along tethers is considered in order to enhance electron current collection. As a basic assessment tool, Particle-In-Cell modeling of the tether system is performed using a 1-d cylindrical code. Comparison of test electron trajectories shows that the time periodic field distribution created by the RF excitation results in electrons being scattered off their usual OML trajectories, which under some conditions increases their probability of being collected by the tether. Analysis of simulation results reveals that large current enhancements can occur at resonance frequencies of the input reactance (where X_in = 0), but at the expense of high RF power. Current enhancement is best measured in terms of the relative current variation per unit of RF power dissipated for every 1-meter section of the tether. Optimum enhancements of about 9% per RF watt per meter were obtained by simulation at low frequencies (75 MHz). Similar enhancements were observed during Phase 1 of the experimental measurements on tether samples. Initial experimental results from Phase 2 of the tests are presented in which the RF power was kept constant at a reference plane along the transmission line connecting to a tether sample. These results show some frequency dependence of the current enhancement for a fixed power level. Further measurements are needed which will control the RF power sent at the tether sample itself, rather than at an upstream position on the transmission line.