Development of an Iodine Feeding System for Low Power Ion and Hall Effect Thrusters

Iodine is considered as a feasible alternative to xenon to be used as a propellant for electric propulsion systems, thanks to its high density, solid state density and the good propulsive performance. The paper illustrates the results of a development and experimental activity, aimed at producing a controlled iodine feeding system for low power (200 W class) ion and Hall effect thrusters. A prototype has been designed, manufactured and tested. The feeding system mainly consists of a slender, cylindrical reservoir, capable of storing up to 250 g of solid iodine in form of a cartridge, a sublimation assembly, an on/off valve and a thermal throttle. Both the sublimation assembly and the thermal throttle are equipped with heating electrical resistors and Pt100 thermal sensors. The coarse regulation of iodine mass flow rate is made by controlling the sublimation pressure through the temperature of the sublimation assembly; the fine regulation by controlling the thermal throttle temperature. A pair of Gefran 1350 PID is used for temperature control. Seven thermocouples are placed in various positions of the feeding system for a thermal characterization during transient as well as steady state operation. The prototype has been tested in a vacuum chamber at a back-pressure of about 1 Pa. Temperature measurements in seven locations of the system have been made during a complete operation cycle lasting about three hours. Mass flow rates at different sublimation assembly and thermal throttle temperatures have been measured by means of a method based on charcoal chemical traps. The system has proved to be capable of delivering an iodine mass flow rate in a range between 0.5 to 1.5 mg/s with a resolution better than 0.05 mg/s, being the sublimation assembly temperature regulated between 90 and 106 ◦C and the thermal throttle temperature between 100 and 112 ◦C. Electrical power delivered to the feeding system (sublimation assembly, valve, thermal throttle) has been up to 24 W during the heating up of the system starting from room temperature and up to 3 W during the steady state operation.