Determination of Hollow Cathode Plasma Contactor System Requirements using an Electrodynamic Tether System Simulation Tool

No. # 121 Determination of Hollow Cathode Plasma Contactor System Requirements using an Electrodynamic Tether System Simulation Tool One application of bare electrodynamic tether systems is to deorbit spent satellites and debris from Low-Earth Orbit. In these systems a hollow cathode plasma contactor sub-system is used to form a plasma bridge between one end of the tether and the space plasma in the ionosphere. The electrical contact function is vital to the operation of a bare electrodynamic tether system and poor system performance results when the bias voltage between the contactor and space plasma is a significant fraction of the tether electromotive force. Specifically, the plasma contactor bias voltage can affect deorbit time, overall system mass, and impose requirements that complicate the operation of the system. A design-decision-support tool, SimBETS, is used to perform studies by varying plasma cathode bias voltage, satellite mass, orbital inclination, tether length, and tether width. The results are used to define plasma contactor system requirements specific to the characteristics of the debris and tether. Insights gained from the studies show longer deorbit missions force the use of larger plasma contactor sub-systems that must be operated for longer durations, which is especially true for low tether emf conditions that are encountered in orbits with large inclination. Fluctuations in operating parameters are predicted by SimBETS due to changes in plasma density and magnetic field that occur on every orbit and as a function of altitude during a mission. For example, an equatorial orbit using a 10-km long, 2-cm wide, 50micron thick tether generates an average current of ~7A with a peak-to-peak variation of ~4 A at an altitude of 300 km. The peak tether current determines the design of the hollow cathode device, and the minimum tether current and hollow cathode design in combination determine the auxiliary power requirements of the plasma contactor. In turn, the auxiliary power requirements can affect system complexity by requiring the use of batteries and energy harvesting subsystems. One example is the need to power externally the plasma contactor during periods when the tether current is below 1 A. Finally, the overall mission time determines the amount of expellant that is needed, and a model is presented that predicts expellant mass and tank size for a given mission. Keywords—hollow cathode, plasma contactor, electrodynamic tether, system study, space debris, deorbit, simulation, and