Background of the invention The present invention relates to the field of space propulsion. In this field, electric thrusters are becoming more and more frequent, in particular for controlling the attitude and the orbit of spacecraft. Specifically, the various types of electric thruster available provide specific impulse that is generally greater than that of conventional chemical or cold gas thrusters, thus making it possible to reduce the consumption of propellant fluid for the same maneuvers, thereby increasing the lifetime and/or the payload of spacecraft. Among the various types of electric thruster, two categories are known in particular: so-called thermoelectric thrusters in which the propellant fluid is heated electrically prior to expanding in a thrust nozzle, and so-called electrostatic thrusters in which the propellant fluid is ionized and accelerated directly by an electric field. Among thermoelectric thrusters, there are in particular those known as ''resistojets'', in which heat is transmitted to the propellant fluid by at least one resistor heated by the Joule effect. 25 Furthermore, among electrostatic thrusters, there are in particular so-called ''Hall effect'' thrusters. In such thrusters, also known as close electron drift plasma engines or as stationary plasma engines, electrons emitted by an emitter cathode are captured by a magnetic 30 field generated by coils situated around and in the center of a discharge channel of annular section, thus forming a virtual cathode grid at the end of the discharge channel. The propellant fluid (typically xenon in the gaseous s~ate) is injected into the end of the 35 discharge channel and electrons escaping from the virtual t Translation of the title as established ex officio. 2 cathode grid towards the anode situated at the end of the discharge channel impact molecules of the propellant fluid, thereby ionizing it, so that it is consequently accelerated towards the virtual cathode grid by the 5 electric field that exists between the grid and the cathode, prior to being neutralized by other electrons emitted by the emitter cathode. Typically, in order to ensure that electrons are emitted from the cathode, the cathode is heated electrically. 10 Furthermore, Hall effect thrusters are not the only thrusters that include similar emitter cathodes. Another example of an electrostatic thruster with an analogous cathode is the high efficiency multistage plasma thruster (HEMP) as described for example by H.-P. Harmann, 15 N. Koch, and G. Kornfeld in "L01v complexity and low cost electric propulsion system for telecom satellites based on HEMP thruster assembly", IEPC-2007-114, 30th International Electric Propulsion Conference, Florence, Italy, September 17-20, 2007. In such a HEMP thruster, 20 the ionized propellant fluid is accelerated by an electric field formed bet