• probe
• plasma
• Hall thruser
• bayesian analysis
• triple

Plasma characterization in Hall thruster by triple Langmuir probe
This thesis is in the eld of plasma electric propulsion for space applications. Plasma propulsion systems work by electrically expelling an ionized propellant at high velocity. Electric thrusters typically use much less propellant than chemical rockets because they have a higher exhaust speed. Due to the limited electric power available on-board of satellites, the thrust is much weaker compared to chemical rockets, but it can be provided for a longer time and thus electric thrusters are more suitable for deep space missions. Amongst the several dierent electric propulsion systems, in the present work we focus on the Hall thruster (HT). The HT is an electric thruster that uses both electric (E) and magnetic (B) elds to generate plasma inside the thruster channel and to extract from the plasma the ions which provide the thrust. The name of the thruster originates from the fact that the E×B force (Hall term) in the plasma causes the electrons to drift in the azimuthal direction. These thrusters are able to accelerate ions to exhaust speeds between 10 and 80 km/s, but most of them operate between 15 and 30 km/s. The thrust produced varies depending on the power level at which the thruster works. The one considered in the present work is a high power HT designed for operating in the 2.5−7.5 kW power range, and the exhaust velocity reached by the ions is of the order of 20 km/s. The applications of Hall thrusters include control of the orientation and position of orbiting satellites and they are also used for medium size robotic space vehicles. In order to optimise the thruster operations, it is important to characterize the plasma generated in them. In the present case, we use triple Langmuir probes. Langmuir probes are essentially electrodes inserted into the plasma: from the current owing into the electrodes and the potential applied to them, the local plasma potential, density and temperature can be determined under suitable assumptions. The triple probe we use consists of three electrodes, between two of them a bias voltage is imposed and the owing current is measured, while the third electrode is held oating (i.e. no current ows through it) and its potential is measured. Given the harsh plasma environment present inside the thruster, the triple probe was inserted for a short time (a few ms), to avoid any damage to the probe. Several measurements were made for the same thruster operative conditions using dierent electrode congurations and bias voltages. In order to calculate the plasma properties from the measured current and potential data, it is necessary to consider how the probe interacts with the plasma, so a model of the plasma sheath around the electrodes is needed. In our work we use an empirical parametrization of numerical simulations based on a steady state Vlasov-Poisson model. Initial measurements showed an unexpected dependence of the results on the electrodes conguration, as if the plasma parameters were varying in space on a scale comparable to the distance between the electrodes, contrary to assumption of a locally homogeneous plasmas. The need to analyse data coming from dierent electrode congurations, as well as the addition of parameters to model inhomogeneities in the plasma, make it dicult to reliably calculate the plasma properties. In the present work, we use a Bayesian probabilistic approach to combine data from dierent congurations and to obtain a "most likely" plasma state, while keeping track of the uncertainty in the estimations of the plasma properties. The results suggests that the drift of the electrons in the thruster causes a dierence in the potential measured by the electrodes due to the "screening" of the ow by the upstream electrode with respect to the downstream ones. This eect can be modelled by extending the plasma probe theory to a drifting electron distribution.