A field effect spacecraft neutraliser for fundamental physics missions

Tests of fundamental physics in space make use of the gravitationally quiet environment of a free falling spacecraft. However non-gravitational disturbances can still influence measurements at the highest precision. These disturbances, which include drag from the residual atmosphere in low Earth orbit or solar radiation pressure for heliospheric missions, may be compensated, or nulled by means of a system of sensors and thrusters. The disturbance compensation system, often described as drag free control, achieves the pico-gravity environment required for fundamental physics missions. In drag free systems, disturbances are sensed by the position of a free-floating test mass with respect to the spacecraft. Micropropulsion thrusters, providing thrust in the micro to milli Newton range, are then used to counteract the disturbance and return the test mass to its reference position. In the European context the preferred microthrusters are field effect electric propulsion devices or FEEPs. FEEPs use the field effect process to extract ions from a liquid metal, and then eject them at high velocity using electrostatics. The act of ejecting ions causes the spacecraft to quickly become charged to very high potentials, and thus a neutralising source of electrons is required to maintain spacecraft neutrality. We describe the design, construction and testing of a field effect emitter array, which provides a low power source of electrons providing up to 6mA, for spacecraft neutralisation. The emitter array has been designed and engineered to be suitable for integration into the micro propulsion system of fundamental physics mission such as LISA Pathfinder. The Field Emitter Array uses silicon emitters constructed using microfabrication techniques. Electron beam lithography is used to ensure uniformity and precise control of emitter location on each array. Process control of the remaining techniques including plasma enhanced vapour deposition, wet and dry plasma etch, and various coatings, has been important in achieving robust reproducible devices suitable for space use.