Molecular propellants for ion thrusters

Abstract

There is no ideal atomic propellant for ion thrusters. Xenon commonly used as propellant becomes resource-critical in light of electric propulsion commercialization. Combining these considerations leads to seeking alternatives to xenon as propellant. In this review, we summarize the current literature on molecular propellants. We define two classes of molecules, group I and II, comprising diatomic molecules and more complex molecules, respectively. We identify basic properties which a candidate molecule belonging to either group, I or II, should possess in order to be suitable as molecular propellant. We discuss the pits and traps in testing such candidate molecules inside a thruster on the basis of our experiences with iodine (a member of group I) and adamantane (a member of group II). The thruster system needs to be individually adopted for each propellant candidate in order to enable a thorough testing inside the thruster. The same holds for optimizing the thruster’s performance when fed with a new propellant because the microscopic processes occurring inside the plasma will differ from molecule to molecule. These circumstances make such testing time-consuming and costly. To accelerate systematic screening of the vast number of molecular species in terms of suitability as propellant, we propose a screening and evolution procedure based on combining chemical engineering and fundamental physical measurements.

Secondary electron emission due to multi-species iodine ion bombardment of different target materials

“…It consists of a radiofrequency gridded ion source that uses an inductively coupled plasma (ICP) discharge with iodine gas to produce ions that are accelerated by a set of two biased grids, referred to as the screen grid and accel grids respectively. Due to the molecular nature of iodine gas, the plasma in the ICP, and the ion beam propagating downstream, is in general composed of three main ion species: I + , I + 2 and I 2+ , as shown theoretically 8 and experimentally 7 . Because of iodine's electron affinity, negative ions Ican also be formed in the ICP, however their creation is not favored in the discharge conditions used here 8 and, because of the polarity used and the potential profile of the plasma sheath, the grid set does not extract or accelerate them into the beam.…”

“…Despite numerous experimental 7 and theoretical 8,9 investigations of iodine-based discharges, there is still a lack of fundamental data for many important properties of iodine. Specifically, data for secondary electron emission (SEE) from a material due to iodine ion bombardment is notably scarce.…”

Electric Propulsion Methods for Small Satellites: A Review

“…The most common type of propellant is xenon (Xe) though earlier versions of this thruster used metallic propellants such as mercury or cesium which have high atomic masses, ionize easily but have very high boiling points and are toxic chemicals. Xenon, in comparison to cesium and mercury, ionises more easily, has a high atomic mass and critically, and it has a low boiling point [49] making it more favourable. Ion thrusters have the highest efficiency in comparison to other propulsion methods and very high specific impulses.…”

“…This allows for a continuous ion beam which removes the need for a neutraliser thus reducing the size and weight further [55]. As mentioned earlier, xenon is the typical propellant used but using iodine as a propellant under a low flow regime creates more thrust than xenon at similar operating powers [49,56]. The iodine radiofrequency ion thruster (IRIT4) from reference [56], produced 2.3 mN of thrust, a 2361 s specific impulse with nominal power of 95.8 W and grid voltages of 1800 V. Although iodine is a corrosive propellant [49], it is more abundant and cheaper than xenon which currently costs around USD 850/kg.…”

“…While the performances did not compare to that of xenon or iodine, water is a safer propellant and an abundant fuel source [57]. There is a fast and cheap screening method for assessing new propellants that exists in reference [49], though any new propellant needs to also be experimentally validated including the adaptions to grid geometries and whole diameter distributions. Investigating the properties and applicability of new propellants will become quite important as the growth of smallsat increases.…”