Activity of DLR’s STG-ET Facility Toward the Standardization for Plume Qualification of Electric Propulsion

Abstract

Electric Propulsion (EP) is a promising technology used for spacecraft maneuvering. EP utilizes the electricity to heat up or ionize materials and further accelerate them directionally to produce vectoring thrust via gas expansion or applied electric/magnetic field. The latter concept allows EP to generate a much higher exhaust velocity of the plume and attractively high specific impulse than chemical propulsion with tiny amount of propellants. This characteristics is especially beneficial for mission requiring high delta-V maneuvering, such as deep-space exploration or station-keeping of satellites. The ground testing facility plays a crucial role in EP performance and lifetime verification, such as thrust vector, Isp, beam divergence, composition, thruster efficiency, contamination to spacecraft, etc. However, setting up an accurate measurement for EP performance verification is much more complicated. First, the facility effect can significantly affect the plume behavior and distort plasma properties. A ground facility needs a sufficiently large dimension and high-vacuum condition to provide a relatively low impact from the facility effect. However, this kind of facility is limited or inaccessible for most EP developers. Second, despite EP plume measurement being established for decades, most plasma plume diagnostics used by academics, scientific institutes, or industries are incomparable in design, implementation, and analysis. This fact potentially hinders the accuracy of the plume measurement, hence incomparable results for EP validation. Third, ensuring EP performance, system reliability, and yet remain low cost in the development and validation become urgently needed due to the soaring demands of EP from the satellite industry. To reduce the testing cost, the processes of EP validation have to be automatized; and the verification result must be representable in a shorter time frame. Conclusively, established standardized facilities and verification methodologies are required. The STG-ET in DLR Göttingen dedicates to simulating the space environment for EP verification and plume investigation for users from research organizations and industries. The STG-ET infrastructure comprises a vacuum chamber of 12 m in length and 5 m in diameter with a pumping capability of up to 276,000 L/s in Xenon. The ultimate pressure is 1×10^(-7) mbar. Theses ensures that STG-ET offers an acceptable condition to mitigate the facility effect on the EP device. For EP performance verification and plume diagnostics, miscellaneous infrastructure is equipped in STG-ET, e.g., thrust pendulum and multiple plasma beam scanners for the EP near-field and far-field plume measurement. The beam scanners are flexible for adapting scientific instruments, which makes it an attractive candidate for establishing standization of EP diagnostics. One recent activity in DLR Göttingen focuses on establishing the standard approaches and quality assurance for the EP plume measurement. A modulated system for the plasma diagnostic tools is developed in the STG-ET facility to achieve this objective. The main goal of the modulated diagnostic system is to provide a platform that offers a comparable condition, interface, and approach for the different kinds of scientific equipment used in plasma measurement. The system has several features from the perspective of basic infrastructure, e.g., mechanical and electrical interfaces: 1. Provide miscellaneous functions for diagnostics tools, such as cooling interface with precise temperature control, temperature monitoring on diagnostics system, mechanical shutter, and electrical floating for diagnostic tools. 2. Minimize the physical disturbance from the tool to the plasma plume. 3. Offer unified mechanical structure and electrical interface for adapting different diagnostic tools. 4. The diagnostic module is pre-aligned to the EP device, which enables plug-and-play when implementing new equipment or different tools. 5. The system is easy to expend due to the modulation. The concept enables a comparable measurement condition and offers high adaptability for different diagnostic tools. This minimizes the impact of various diagnostic tools to the thruster and reduces the preparation time for the implementation of diagnostic tools. Indeed, the measurement quality also depends on the equipment and analysis approach. These two aspects are strongly related to the design of each diagnostic tool; and must be determined by the tool developer. Nevertheless, DLR Göttingen continuously dedicates this aspect by establishing a functional demonstration with the electrostatic probes, i.e., retarding potential analyzer and ExB probe. Details of the implementation and other activity for EP test standardization in DLR Göttingen will be included in the presentation.