The Hall effect thruster (HET) was designed for long-duration operation with gaseous iodine as the propellant. Iodine is an alternative to the state-of-the-art propellant xenon. Compared to xenon, iodine stores as a solid at much higher density and at a much lower pressure. Because iodine is a halogen, it is reactive with some of the materials with which a Hall thruster is typically constructed. Through research and testing, the new method allows for the HET to be used with iodine propellant for long periods of time.
The thruster is distinguished from the nominal commercial thruster by the materials of construction, the geometry of the anode, and the presence of iodine-resistant coatings. The anode and gas flow lines are made from a non-magnetic, iodine-resistant alloy. The propellant voltage isolator is made from iodine-resistant metals and brazes. The gas distributor was also completely redesigned to allow the use of multiple materials, and for it to be disassembled.
A HET uses crossed electric and magnetic fields to generate and accelerate ions. The overall structure is defined by a magnetic circuit that produces a steady magnetic field across a typically annular channel. The upstream portion of the channel includes a gas distributor that also typically functions as an anode. In the HET described in this work, the downstream portion of the channel is dielectric. A potential difference or discharge voltage is applied between the anode and an external cathode. The resulting electric field is predominantly axial, and is concentrated near the channel exit by interactions between the magnetic field and the plasma. In the channel, electrons are strongly magnetized and their transport is predominantly azimuthal due to the Hall effect. The extended electron path enables an efficient, impact-driven ionization cascade. Ions are weakly magnetized, and most are accelerated directly out of the channel, forming the ion beam.
The physical shape and dimensions of the thruster are very similar to a xenon thruster. However, because iodine stores as a low-pressure solid, the reservoir may be irregular in shape, conforming to available space. The gas pressure only needs to be several psi — a factor of 1,000 lower than that of xenon. The feed system also needs to be heated to prevent condensation.