What you need to know
The IFM Nano Thruster uses a porous tungsten crown emitter and Liquid Indium as propellant. The propellant is solid at room temperature and needs to be heated in order for the thruster to start. This results in the following unique features of the IFM Nano thruster, that have to be taken into account when implementing the technology:
- The whole thruster is inert during launch. You are basically launching a piece of metal with no stored energy (no gases or liquids, no pressurized components).
- The feeding of the propellant to the emitter works completely passively with capillary forces, therefore no propellant handling system is required.
- The high density of the propellant, the absence of a feeding system and the high mass utilization efficiency of the technology allow for a total impulse density which is about ten times higher than the one of Xe-based systems and exceeds also recently presented results from Iodine based systems.
- The electrode configuration is not designed for a specific operation point. Therefore, you can utilize the whole range of the performance map throughout the mission. This can be used to combine a high thrust orbit raising with high Isp station keeping.
- The thruster plume consists of highly energetic singly charged Ions and droplets of neutral Indium. It is therefore necessary to consider in an early stage that a conductive layer of Indium can be deposited on all surfaces directly in the field of view of the thruster. AMR can assist you with an assessment of this issue and suggest appropriate counter-measures.
The Indium FEEP Technology
Field emission is an effect which is closely tied to the presence of strong electric fields. In practice, this means that the fundamental structure on which field emission takes place is shaped like a needle, due to the field-enhancing effect at the tip. An important application of this effect is the so-called ‘Liquid Metal Ion Source’ (LMIS), because it uses the process of field emission to ionize a thin film of liquid metal covering a needle which has been biased to a few kV with respect to a counter electrode. The thusly created ions are then accelerated by the strong electric fields and can be used for ion implantation in semiconductor industry or micromachining in a focused ion beam (FIB).
The Department of Aerospace Engineering at FOTEC (the former department ‘Space Propulsion and Advanced Concepts’ at the Austrian Institute of Technology, AIT) has a long and successful heritage in manufacturing Liquid Metal Ion Sources for Space. They have been used for mass spectrometer (for example on ROSETTA) or for spacecraft potential control (for example on the NASA mission MMS). Up to date, FOTEC is the only source world-wide for Liquid Metal Ion Sources with flight heritage.
This principle of generating positive ions and accelerating them by the very same field can also be used to generate thrust. When a liquid metal ion source is used in this fashion, it is termed ‘field emission electric propulsion’ (FEEP).
Due to the accuracy with which it is possible to regulate the voltage between the needle and the extraction electrode, the ensuing thrust can be controlled with unmatched accuracy. The main advantage of using FEEP thrusters lies in their capability to produce thrust from the sub-µN level to several tens of µN per needle.
For more than 15 years, research has been carried out to use this technology for providing ultra-precise thrust in the µN-range to a spacecraft for applications related to formation flight of spacecraft.
In the course of these developments, several concepts proved to be less feasible than others, and so far attempts to develop a propulsion system for specific missions have not been successful. Difficulties that have been encountered during those projects have been related to the lifetime of the thruster and the large amount of emitter that had to be produced in order to get a few with adequate performance.
The main reason for these difficulties can be found in the manufacturing of the emission sites. These emission sites can be capillaries, needles or porous needles, all having in common, that liquid indium is transported to a very sharp tip via capillary forces. Applying an electric field to this sharp tip causes the formation of the so-called Taylor cone. At the tip of the Taylor cone, neutral atoms of the liquid metal are removed from the surface by the strong electric field and, as one or more electrons tunnel back to the surface, the formerly neutral atom is converted into a positively charged ion.
The sharper the emission site, smaller is the base of the Taylor cone, leading to a higher efficiency of the thruster at any given Ion current. The figure below shows three different ways of manufacturing small emission sites. On the left, a solid needle is covered with a film of indium. This technology is particularly attractive in terms of performance, but is also very prone to contamination or any effects that could compromise the indium film. The capillary emitter shown in the centre of the figure has the advantage, that it is very resistant to contamination and the manufacturing is very reliable, but the exit of the capillary cannot be manufactured as small as a needle tip, leading to a slightly lower Isp if used as a thruster.
As a consequence of the failed attempts to implement FEEPs in previous missions, it has been concluded together with ESA that for this technology too much basic development has been performed in the frame of flight missions with tight schedules. The FEEP technology has therefore been put forward in ESA’s ‘Basic Technology Research Program’ and in the frame of early preparatory studies for the Next Generation Gravity Mission (NGGM).
In this environment, the so-called porous tungsten crown emitter has been developed, which employs 28 needles for field emission. Apart from the multiple emission sites, the most important new feature is the porous tungsten matrix which enables internal flow of the liquid metal (similar to a capillary emitter) to a very sharp tip (similar to a solid needle). It therefore combines the advantages of both the capillary emitter and the solid needle.
Another key feature of the porous tungsten crown emitters is the rapid manufacturing method from using micro powder injection moulding. In this process, a mixture of tungsten powder and binding polymer is injected in a mould and in subsequent steps, sintered to final hardness. In the course of this procedure, the structure becomes porous and facilitates flow of material through its volume.
Depending on the voltage applied to the crown emitter, a certain number of needles is actually emitting ions. As indicated in the picture above at high thrust levels, almost all needles are contributing to the total current whereas at low thrust levels only a couple of needles (whose electrical impedance is lowest) are emitting ions. This behaviour allows operation at a thrust level ranging from few µN up to several mN.
In the following picture the typical performance and the excellent controllability of a 28-needle porous crown emitter are shown (left). The voltage / current characteristic is given for different extractor voltage levels (right). One recognizes the ability to shift the extractor voltage which allows a trade-off between high specific impulse and low power-to-thrust ratio operation.
In the frame of the development efforts for the ESA NGGM mission, extensive testing of this technology has been performed. This included voltage / current characteristic measurements, direct thrust measurements and the characterization of the start-up behaviour.
An ongoing lifetime test has demonstrated more than 13.000 h of operation without performance degradation.