Tesi di dottorato di ricerca
DE TATA, MARCO
Multi-Channel Hollow Cathode for High Power Electric Thrusters: Experimental Characterization and Theoretical Analysis
Settore scientifico disciplinare
Corso di studi
tutor Prof. Andrenucci, Mariano
Data inizio appello
This dissertation deals with theoretical and experimental studies on an argon-fed multichannel hollow cathode. In particular, it investigates the physical processes which take place inside the hollow cavity. These include the cathode voltage drop, the surface temperature and the plasma penetration depth. An insight into the ignition mechanisms is outlined as well. Moreover, the erosion process is studied by measuring the erosion rate of the cathode walls after continuous operations at fixed mass flow rate and discharge current. Thus, this research is divided into<br>two main activities: the experimental campaign and the theoretical study.<br>For the experimental study, a multichannel hollow cathode with a main tube inner diameter of 10 mm and 36 rods was designed. It was tested at discharge currents in the range 20-160 A and at mass flow rates between 1 and 5 mg/s. It was propelled with argon gas. In order to identify the nominal operative points, the cathode underwent a characterization test which was followed by an endurance test at continuous operations for 100 hours. This test aimed at understanding the erosion mechanism of the cathode. Then, a second characterization test was performed, which permitted to assess the effect of the erosion on the cathode performance. The cathode surface temperature was measured at different values of discharge current and mass flow rate. The measurements were gathered by using an optical pyrometer that was mounted outside the vacuum chamber. Temperature profiles with respect to the cathode axis are presented. It was found that the surface temperature of the multichannel hollow cathode is different from that in the single channel hollow cathode, since the maximum value is placed at the cathode exit cross section for all operative conditions. The ignition process of the cathode was investigated. The cathode was capable to sustain a discharge current of 20 A with a few channels ignited. In addition, it was noted that a<br>specic discharge current value exists to ignite all channels. This value is considered as the transition point between the multi-step ionization process and the single-step ionization process. However, since a plasma diagnostic system was not available during the experimental campaign, this hypothesis cannot be proved in this study.<br>In addition, an insight into the design and manufacture of the multichannel hollow cathode is given. The design procedure is outlined, as well as the method for scaling-down the cathode dimensions. The assembly process is described with a focus on the technical issues that were solved. The second part of this research deals with two theoretical models which were developed and validated with the experimental data. The first model is the product of a joint research programme between the RIAME-MAI and Alta. It can be adapted to multichannel hollow cathodes with an indefinite number of channels. Cathode operative parameters such as the cathode voltage drop and the surface temperature are calculated. In addition, the model is capable to predict the main plasma parameters: the electron and ion temperatures, the particles number density and the plasma potential. The results are provided as functions of the cathode axial coordinate. Furthermore, the erosion rate of the cathode is computed for each cross section, thus providing the effective channel erosion along its length. The second numerical model was developed during the experimental campaign and represents a valid basis for future improvements. It is a mono-dimensional code, therefore the multichannel hollow conguration is studied by adapting the plasma penetration length in case of many channels to a single channel hollow conguration. The model is capable to predict the discharge voltage and the surface temperature as functions of the cathode axis. However, it requires the discharge current profile as an input. Moreover, it calculates the electron temperature and particles number density for each cathode cross sections.