I2HET: Development of an Iodine-Fed Hall Effect Thruster

This paper deals with the results of an ongoing activity carried out by SITAEL and UniPi aimed at the development of technologies for iodine-fed Hall effect thrusters. Current applications of Hall effect thrusters use xenon as propellant. This element has several properties that make it an optimal choice for ion and Hall effect thrusters, such as a high atomic mass, low first ionization energy and a large cross section, which imply an easier generation of the plasma with respect to other options. However, given the increasing applications of xenon in several high-technology industries, its availability and price can fluctuate substantially, with negative consequences on the development of space projects. An additional drawback is that its storage conditions at supercritical state require a high-pressure tank (of about 150-200 bar), a pressure regulation system and a distribution system to supply the gas at low pressure into the thruster discharge chamber. Being stored in a supercritical state complicates the load operations and poses security issues, hindering the applicability of this thruster technology in missions where restrictions on pressurized systems are important. Therefore, alternative propellants, among which iodine, have been investigated. A successful alternative propellant will allow for both reducing the utilization cost and increasing the application envelope of EP technologies. Iodine presents several conditions that make it a good alternative to xenon. Its atomic weight of 126.9 amu is close to the 131.3 of xenon and it has similar ionization properties on a monoatomic gaseous state. Iodine allows for solid storage at ambient pressure and below 100 ºC at a density of 4.9 g/cm3 (three times the density of supercritical xenon at 200 bar), with consequent simpler loading and handling procedures, longer shelf-life, and higher specific impulse density. Additional advantages of iodine with respect to xenon are a cost ten times lower and almost an unlimited availability in the high purity required. The major drawback of iodine is its chemical reactivity. Iodine is a halogen and, despite being the least reactive of this group, it still can interact with certain materials. It is possible to cope with this characteristic working at spacecraft/subsystem material selection level. This paper starts by illustrating the development and characterization activities of an iodine feeding system for 100 W-class Hall thruster. The feeding architecture is described in detail and then the developed prototypes are introduced, and the results of the mass flow generation characteristics are presented. The coupling of the feeding system to a modified Sitael HT100 Hall thruster is envisaged, using a xenon-fed cathode. A test facility has been set up and a verification test of the thruster running on xenon as a worst-case-scenario has been carried out to assure the pumping capacity is enough for performing the test on iodine, with positive results. The vacuum chamber is equipped with an iodine extraction line to perform post-test cleaning operations. Additionally, during the tests, the thrust measurements will be made using a thrust balance.