Thorium Migrational Effects on Cathode Temperature in Electric Thrusters

Abstract

Time-dependent charge-coupled device camera cathode axial temperature measurements obtained from solid tungsten and thoriated-tungsten cathodes in a 30–60 A arc discharge with mass-flow rates varying from 450 to 640 standard cubic centimeters per minute are presented. Recorded thoriated-tungsten temperature measurements adhere to the same trends observed in previous optical pyrometric studies with temperatures ranging from 2300–2500 K at the cathode tip, a peak temperature between 2500–2800 K some short distance upstream, and a corresponding minimum temperature of 2150 K at a location nearly 20 mm from the tip. The cathode axial temperature profiles were compared against those of pure tungsten to illustrate the large temperature-reducing effect lowered work function imposes by increasing thermionic electron emission from the cathode surface. In addition, the surface microstructure and elemental composition of the cathodes were analyzed using a scanning electron microscope in conjunction with energy-dispersive X-ray spectroscopy. Multiple runs varying in length and current intensity on thoriated-tungsten cathodes revealed typical pathways of thorium migration as well as changes in surface composition.