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Field emission electric propulsion is the technological application of the principle of liquid metal ion sources as thrusters in electric space propulsion. Research work sponsored by the European Space Agency (ESA) on a slit-type field ion thruster is reported and discussed. The most significant new features of its emission performance are as follows: For the first time, a slit emitter with a closed propellant supply system was fired in any optional direction, requiring, in principle, no gravitational forces. Quantitative data relating the constituents of the residual gas atmosphere to the wetting behavior of the liquid metal propellant and the emission site distribution were obtained. A homogeneous distribution of equally spaced emission sites was observed; the measured spacing is in good agreement with a simple hydrostatic model of wavelike instabilities on electrically stressed surfaces of fluids.