Nathaniel Wirgau
University of Michigan
Hollow Cathode Assemblies (HCA) are the heart of gridded ion and Hall effect thrusters. The HCA serves as the electron source that sustains the plasma in these devices. Electrons are thermionically emitted from the surface of low work function material comprising the cathode.These surfaces are typically self-heated by ion impact. These devices have demonstrated remarkably long operational lifetimes in excess of 10,000 hours both in the experimental and space-flight environments with nominal discharge currents up to 30 Amps. However, future missions will require similar or longer lifetimes with discharge currents over 100 Amps. While HCAs capable of producing these high currents have been built, there is still much uncertainty of their lifetime. Long duration life tests are ideal, but they are both expensive and allow for only limited statistical insight due to practical limitations on the number of concurrent long duration cathode operation and testing. It is possible for models to provide operational cathode characteristics and ultimate life time. Before such a model can be practically implemented, thorough validation with experiment is necessary. The research proposed here aims to interrogate the plasma and emission insert conditions as a function of operating conditions at high current for two purposes. First, understanding operation of the device at elevated emission current. Second, to establish an operational database including plasma properties as well as thermal conditions to provide a basis for model validation. The validated model can then be used predict ultimate cathode operational life as well as inform design solutions that could potentially extend life. This effort will use a combination of high speed scanning Langmuir and thermal probes as well as laser induced fluorescence to fully map the parameter space that ultimately will be used as inputs and validation output. In particular, cavity ring down will be used to detect metal vapor products emitted through the orifice. This effort will also characterize cathode sensitivity to conditioning and handling protocols, which will also be used for model validation. The goal here is to both explore the physical basis of the conditioning protocols and perhaps provide insight into relaxing them. Such relaxation would result in reduced cost associated with the potential elimination of environmental handling steps which are both economically costly and time-consuming. This research directly supports Technology Area Breakdown Structure (TABS): TAO2 2.2.1 Electric Propulsion. In particular, the Level 3 2.2.1 objective is the explicit goal of this research, increasing the performance and lifetime of electric propulsion devices by enabling reliable, robust, high current hollow cathode assemblies for these devices.