Field Emission Electric Microthrusters
The propulsion system used to compensate the disturbances of the Microscope satellite is composed of four independant blocks. Each propulsive block (EPSA : Electric Propulsion Subsystem Assembly) consists in three high accuracy ionic microactuators, two neutralizers (one of them redundant), the power and control electronic unit (PPCU: Power Processing and Control Unit) and the support structure.
The electric microthrusters of the Microscope satellite belong to the category of field emission motors (FEEP: Field Emission Electric Propulsion) with linear geometry and using Cesium as propellant. The Propellant is stored in the tank in solid state. Heated to its fusion point, it is liquefied then moved by capillarity to the emitter. On the slit it stops by forming a meniscus between the two halves of the emitter, which must be perfectly polished, aligned and assembled to avoid the risk of leakage or assymetry in the emission. It is a very tricky technologic operation: Typically the slit of a FEEP motor is 1 µm width, that is to say the hundredth part of a hair width. A strong potential difference (~10 kV) is created between the emitter and an accelerator positioned less than one millimeter away. The tuning of this potential difference enables to choose with a great accuracy the required thrusting level. Each motor can deliver continuously a microthrust comprised between 1µN and 150µN, with a resolution of about 0.1 µN. A neutralizer (electronic source) is activated at the same time as the thruster to avoid an electrostatic imbalance of the satellite.
The linear geometry FEEP motors are under developement at ALTA-Space (Pisa, Italy) under the responsibility of the European Space Agency. This technology, potentially able to push the limits of the accuracy reachable by a spatial propulsion system beyond all that has been realized in the past, will be tested in flight for the first time on board Microscope.
Plaque accélératrice: Accelerator