| Popis: |
An oxygen-fuelled Hall Effect Thruster is investigated at the Imperial Plasma Propulsion Laboratory vacuum chamber facilities over a different range of discharge channel axial lengths, ceramic walls and anode materials. The purpose of using oxygen as a propellant is to better understand the principles of water electrolysis Hall Effect Thrusters, which are envisaged to use oxygen to propel the thruster. These studies aimed to answer whether if for molecular plasmas, a larger channel length would benefit the overall performance of the thruster by increasing the length of the ionization region, or if a shorter channel would be more beneficial due to a reduction in the energy losses associated with the plasma-wall interactions. Experimentally, it is found that channel lengths of 13.1 mm performed the best amongst the lengths tested in terms of thrust, specific impulse and thrust efficiency. Larger channels (59.8 mm, 44.8 mm and 34.8 mm) showed a reduction in thruster performance with increasing channel length. A very short channel length of 5.5 mm is found to be less efficient than the best performing case (13.1 mm), possibly indicating that the ions are being formed within or downstream of the peak acceleration region due to the constrained length of the channel. These behaviours appear to be more evident the higher the discharge power. The impact of the walls material is also investigated. In the past, changing the thruster walls from Alumina (Al2O3) to Boron Nitride (BN) made a significant improvement on the performance, generally because of the lower Secondary Electron Emission of the BN walls. In this study, two different grades of BN walls are used: 99% purity BN (grade AX05) and a BNSiO2 compound (grade M26). Although BNSiO2 walls are said to have slightly lower Secondary Electron Emission than BN, the thrust measurements obtained using these walls are very similar. Finally, anodes made out of different materials are also tested. The main goal is to identify a suitable anode material that prevents a non-conductive oxidation layer to be formed when in contact with the oxygen plasma. Tungsten anodes are found to develop, after just a few hours of testing, a non-conductive layer that prevents the long-term operation of the thruster. Stainless steel anodes however, generate an oxidation layer which is still conductive and did not compromise the functionality of the thruster throughout the duration of the experimental campaigns. The highest thrust measurement obtained was |
