Nanobalance: the European balance for micro-propulsion

Within a contract of the European Space Agency, Thales Alenia Space Italia in co-operation with the Italian National Metrology Institute and Polytechnic of Turin, has designed and implemented the Nanobalance Facility, a device for the direct measurements of the force of micro-thrusters (of electrical and cold-gas type). The Nanobalance is aimed to support the development of micro-propulsion technologies for major European space programs as LISA-Pathfinder, Microscope and GAIA. The Nanobalance thrust stand is essentially composed by two vertical “tilting plates” connected by a flexible joint to a rigid block made of Zerodur, in a pendulum-like arrangement. The micro-thruster to be characterized is installed on one of the tilting plates. A counterweight is installed on the second tilting plate to ensure the same dynamics behavior of the first one and therefore the rejection of the common-mode environmental vibrations acting on the thrust stand. When the micro-thruster is switched on, it produces a displacement of one plate relative to the other, which is measured by means of a Fabry-Perot laser interferometer, which reference spherical mirrors are mounted on the tilting plates. The Fabry-Perot interferometer is fed by an Nd:YAG source working at the wavelength λ = 532 nm (2nd harmonic). The laser frequency is regulated so as to maintain it locked to the F-P resonator as the relative distance between the two mirrors changes under the action of the MT (the laser frequency tracks the distance variation). The frequency of this laser (measurement laser) is measured against the frequency on an identical Nd:YAG laser (reference laser) stabilized on a Iodine molecular transition using the Pound-Drever technique. This frequency measurement is converted in a force measurement using a calibration relationship, verified by means of a voice coil actuator permanently installed on the tilting plate and operable even during the test campaign on the micro-thruster. The Nanobalance thrust stand is operated inside a vacuum chamber capable to maintain the pressure below 1E-4 mbar while a cold-gas thruster is firing, and below 1E-6 mbar while an electrical thruster is firing. Inside the chamber, the thrust stand is mounted on a horizontal basement which inclination is actively controlled to zero. The vacuum chamber is installed on six pneumatic isolators, rested on an anti-seismic block. The Nanobalance has been fully commissioned without thruster and with representative thrusters (pressurized micro-valve and ion source): it has a force resolution = 0.1 μN and an intrinsic force measurement noise spectral density ≤ 0.1 μN/sqrt(Hz)2 between 0.01 and 1 Hz (≤ 1μN/sqrt(Hz) down to 1 mHz), over a measurement range of 1 mN. The Nanobalance has been already utilized for test campaigns on a cold-gas thruster and on an electrical thruster (FEEP type).