DTM-POA-MT · Direct thrust measurement and plume optical analysis of micro-thrusters

Final Activity Report Summary - DTM-POA-MT (Direct thrust measurement and plume optical analysis of micro-thrusters)

Several forthcoming space missions like Microscope (Microsatellite with drag control for the observation of the equivalence principle), LISA (Laser interferometer space antenna), LISA Pathfinder (a mission intended to demonstrate the key technologies for LISA), and Darwin (an infrared space interferometry mission) require very accurate satellite position and attitude control. For example, the mission LISA aims to detect gravitational waves in the frequency range 10-4-2 Hz. In such a mission, three identical spacecrafts form an equilateral triangle. Each side is 5 million km long and acts as the arm of an interferometer used to detect any spacecrafts' relative displacement generated by a gravitational wave. The spacecraft position must be controlled to a fraction of laser wavelength, which means nanometres. This requirement translates into thruster specifications of 0-100 µN of thrust range, 0.1 µN of resolution, and a noise figure lower than 0.1 µN/Hz0.5 in the frequency range 10-4-2 Hz. The types of thruster deemed to satisfy these requirements are Field emission electric propulsion (FEEP) thrusters and cold gas thrusters. Both technologies are still under development, and one of the aspects to be verified is their thrust performance and their plume characteristics, because their physics is very complex and the numerical simulation is challenging. Diagnostics development for micronewton thrusters is a difficult task, but is a cornerstone of the activities at Onera, the French aerospace lab. The fellow, Simone Rocca, participated in world-leading studies and development on new diagnostics: - Development of a microNewton balance at Onera. Although the work on the balance started in 1999, major achievements and improvement were obtained with the help of Simone Rocca, especially regarding data processing in the frequency domain, and noise cancellation. In fact, the micronewton balance eventually achieved performances higher than expected, and these have been demonstrated with an actual space micronewton thruster (cold gas thruster) to be among the best in Europe. Currently the noise performance of the balance with a cold gas thruster is 0.1µN/vHz in the frequency range [10-2 - 2 Hz] and 1µN/vHz in the larger frequency range [10-3 - 10 Hz]. Test campaign on a Cesium FEEP thruster: emission spectroscopy and Laser induced fluorescence (LIF) were performed point-by-point (1 mm spatial resolution) with three laser axes and in a 3D volume in order to map the field of absolute number density and velocity vector of neutral Cesium in the plume of the thruster. The thruster is a flight model thruster provided by the Italian company ALTA in the framework of an ESA contract. Simone Rocca helped with the data acquisition of the different fluorescence signal. The work performed on the FEEP thruster has provided world-class results including, for the first time, an optical measurement of the mass flow rate, which provided a measurement of the mass efficiency of the thruster (about 50 %). Emission spectroscopy measurements have also shown the presence of Cs2 in the plume. Development of a new optical diagnostics method for cold gas thrusters. The technique is a combination of two methods: Electron beam fluorescence (EBF) measures the number density, and Laser induced fluorescence (LIF) combined with EBF measures the velocity. Simone Rocca was instrumental in the application of the technique. He elaborated an image processing routine for the analysis of the EBF density images obtained, including their absolute calibration. He also developed a code for the wavelength calibration during data acquisition. For the first time, to our knowledge, number density images were obtained in a cold gas plume, and a velocity measurement was done in an argon plume. This will allow us to obtain insight in the physics of the plume of these thrusters, and therefore will help for their modelling and optimisation.

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