Two-dimensional plasma plume density characterisation of the IFM Nano Thruster

The IFM (Indium FEEP Multi Emitter) Nano Thruster, developed at FOTEC and commercialised by Enpulsion, is a 1U sized FEEP (Field Emission Electric Propulsion) system. It can also be used for precise orbit control of satellites and deep space missions. It consists of a porous tungsten crown ion emitter providing thrust in the range of 10μN to 0.4 mN at an Isp of 2000 s to 6000 s. To obtain a better understanding of the thruster efficiencies as well as to determine basic thruster characteristics, a detailed plasma field investigation of this thruster is essential. An extensive measurement campaign was conducted in order to map the beam ion current density at a fixed distance from the thruster at different horizontal and vertical angles from the centre line of the thruster. This two-dimensional mapping was achieved by using an array of 23 Faraday cups on a rotating boom. Measurements were performed with a horizontal spatial resolution of 1◦ and between 4◦ to 10◦ in the vertical direction. At each position the ion current was determined by performing a voltage sweep with each of the Faraday cups. The Faraday cups were specifically designed for this task, taking into account the expected low ion density. Each of those probes have a built-in ring repeller with an orifice of 1 cm. As the estimated Debye length of the plasma is on the order of several cm, this repeller can be used as a single hole grid. When biasing negatively, it prevents thermal electrons from entering the Faraday cup. Furthermore, it retains any secondary electrons created inside the Faraday cup. The resulting beam profiles have densities on the order of μA/cm at 95 cm from the emitter. This paper describes the for these plasmas new developed probes in more detail as well as the first ever 2-D density characterisation of the plume of an IFM Nano thruster.