Project results

Abstract

In this contribution, we examine the plasma (inside the discharge chamber) and the plume (outside of the chamber) of two high-power Hall thrusters in terms of electron and ion properties, namely the PPSâ5000-ML and the PPS20k-ML. The Hall thrusters were fired with xenon, krypton and argon as fuel. The applied voltage was varied from 300 V up to 600 V and the input power ranged from 1 kW up to 20 kW. In addition to review and comment data about thrust, specific impulse and efficiency, we propose to detail results on electron parameter and ion current obtained in the thruster plasma and plume.

Time-averaged Langmuir probe measurements have been performed downstream the thruster ceramic channel in the PIVOINE-2G facility in Orléans, France, with the PPSâ5000-ML. The probe was mounted on a rotating arm in order to explore a large range of angles from -180 to +180 degrees. Our analysis shows that the electron energy distribution function departs from the equilibrium distribution, which means a Maxwellian distribution. A Druyvesteyn distribution appears to fit better the experimental data, especially for large electron energies. We applied and compared several methods to determine the electron density and the electron temperature using the probe current-voltage curve and the EEDF. Angular profiles of the electrons parameters are presented for the three atomic gases for many operating conditions. Our study indicates the electron density is larger for xenon while the electron temperature is larger for argon and krypton, as a direct consequence of the ionization energy difference. Moreover, the electron density increases with the propellant mass flow rate and the electron temperature increases with the discharge voltage. Finally, combining density and temperature data, the electron isentropic exponent has been computed. In the plume far-field, the value of the exponent is below the value of an atomic gas (5/3) whatever the power level, in agreement with previous works. The exponent is larger for argon and krypton. Unexpectedly, in the plasma plume the value of the exponent does not depend upon the position despite the existence of large gradients in electron density and temperature.

Langmuir probe measurements have also been performed inside the thruster ceramic channel in the large vacuum chamber of the AeroSpazio Tecnologie company in Italy with the PPS20k-ML. Two series of planar probes were installed inside the wall at various axial positions in the ionization zone (back section of the channel). Measurements have been performed with the three fuels for a large number of operating points. I-V characteristics curves are still under treatment. Preliminary analysis indicates n_e = 10¹⁷ m^-3 and T_e = 3 eV for xenon with changes according to position and firing parameters. Electron data will be incorporated in a plasma sheath model to extract the ion current at the walls from the ion branch of the I-V curve. This quantity will be used to estimate the sputtering yield and erosion level of the chamber walls for the three gases.

Authors

• S. Mazouffre,
• A. Lecervoisier,
• V. Delbosq