MAGNETRON DISCHARGE APPLICATION PROSPECTS AS AN ELECTRONS EMITTER IN CATHODE-COMPENSATOR FOR ELECTRIC PROPULSION THRUSTERS
The subject of the presented article consists in assessing the prospects of magnetron discharge application as an electrons emitter for electric propulsion thruster cathode-compensator. This theme relevance is associated with the development of new stationary plasma thrusters (SPT) for the spacecraft operating on iodine, as well as low-orbit spacecraft employing outboard air as a working substance. The paper assesses the energy aspect of magnetron cathode-neutralizer application for modern stationary thrusters. The highest operating voltages of the prospective dual-mode SPTs are 500-800 V. If a ten percent sacrifice of the propulsion system efficiency is possible with the view of increasing the service life and chemical resistance of the cathode-neutralizer, then the operating voltage of the magnetron cathode should be reduced to 120-180 V. The article proposes a mathematical model of a magnetron discharge, on which basis a theoretical estimation of the magnetron minimum operation voltage and its dependence on the secondary ion- electron emission coefficient is presented. For a magnetron discharge with a copper cathode in the argon atmosphere, the minimum operating voltage equaled to 126 V. Besides, the minimum magnetic flux necessary for the discharge existence was computed. An experimental study of plasma-forming gas pressure impact on the operating voltage value of the magnetron discharge was conducted for several options of the cathode material-working gas combination. These combinations were copper - argon, stannum - argon, stannum - argon-air mixture and aluminum - argon-air mixture. Minimum discharge voltage of 160-170 V was obtained when operating on an argon- air mixture and employing an aluminum cathode. The performed studies allowed making the following inferences and recommendations: 1. Cathode design should ensure optimal values of both the magnetic flux above the cathode surface and working gas pressure in the discharge area for the effective operation (minimum voltage). 2. One of the ways to the electron cost in the magnetron cathode is the optimal combination of the cathode material and working gas ensuring minimum discharge voltage.