D éveloppement de diagnostics électrostatiques pour le filtrage magn étique et la formation du plasma ion-ion dans le propulseur PEGASES

Jérôme Bredin 1
1 Plasmas Froids
LPP - Laboratoire de Physique des Plasmas
Abstract : PEGASES is a gridded ion thruster that accelerates both positive and negative ions for thrust. This needs the creation of an ion-ion plasma to be able to extract negative ions from the plasma. Efficient ionization as well as attachment requires a plasma with two electron temperatures: one hot to create positive ions and one cold to create negative ions. The ion-ion plasma is created using a magnetic filter that allows electron cooling and slowing down their diffusion. With this method, a high electron temperature region is obtained near the ionization source, upstream of the maximum magnetic field, and a low electron temperature region downstream of the maximum magnetic field. To understand fully this segregation of the PEGASES plasma, this thesis has focused on the development of suitable electric diagnostic. Several problems occur when i) plasma is created with a Radio Frequency (RF) source that might create large RF plasma potential fluctuations, which will lead to difficulties in probe measurements, ii) electrostatic measurement in magnetized plasmas are difficult because of electron anisotropies in the magnetic field, and iii) for the measurements in ion-ion plasma (symmetrical current-voltage characteristics) where few theory exist to extract plasma parameters, for our pressure range. Two electrical diagnostics and a model to extract plasma parameters in electronegative plasmas have been developed during my PhD at the LPP. The first electrical diagnostic is a capacitive floating probe allowing the measurement of RF plasma potential fluctuations. With its measurements, the PEGASES source has been modified to decrease the RF fluctuations. The second diagnostic is a Langmuir probe adapted to make measurements in electronegative magnetized plasmas. High etching rate of our electronegative gas and anisotropy created by the magnetic field impose some constraints on the materials and the dimension chosen to design the probe. The model reproduces Langmuir probe characteristics in electronegative plasmas assuming Boltzmann distributions. From measured I-V curves and with a fitting procedure incorporating the analytical model allows to deduce i) the temperature and density of electrons and both positive and negative ions for high electronegativity ($n_-/n_e>50$), and ii) temperature and density of electrons and density of both positive and negative ion for low electronegativity ($n_-/n_e<50$). These diagnostics allow us to study the effect of various magnetic field configurations on an electropositive and electronegative plasmas. In the former case, we showed that only the positive gradient of the magnetic field reduce the electron temperature, therefore a minimum electron temperature is obtained close to the maximum magnetic field. This minimum temperature can be controlled by the strength of the magnetic field, while the pressure controls the temperature upstream. When an electronegative gas is used, an ion-ion plasma is formed around 1 cm after the maximum magnetic field. The ion density is relatively constant across the magnetic filter while the electron density drops by three orders of magnitude. On the contrary of electropositive plasmas, where the minimum temperature is obtained at the maximum magnetic field, pressure and the percentage of SF6 controls the position of the ion-ion plasma formation. A simple model based on the measured ion densities and temperatures has evaluated the efficiency of PEGASES to be $T=25$~mN.kW$^{-1}$ and $I_{\rm{sp}}=3790$~s, which is ten times less with an exhaust surface four times smaller than the T6 gridded thruster from QinetiQ, which will propel the Bepi-Colombo mission.
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Jérôme Bredin. D éveloppement de diagnostics électrostatiques pour le filtrage magn étique et la formation du plasma ion-ion dans le propulseur PEGASES. Physique des plasmas [physics.plasm-ph]. Ecole Polytechnique X, 2013. Français. ⟨pastel-00993207⟩

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