1.0 Abstract
The notion of ion propulsion has existed for over a century. However, it is the recent evolution in this matter that has allowed the technology to be used for interplanetary missions. The report focuses on the general concept of the ion thruster with references to current examples which are being used on missions and future prospects for the technology. In order to fully understand the ion thruster function, one must be introduced to a detailed theoretical analysis – which this report partly aims at doing.
2.0 Introduction
Every action has an equal and opposite reaction. This is Newton’s third law. A generic rocket in space is a suitable example which demonstrates the law in action where the rocket exhausts hot gasses and the rocket itself goes in the opposite direction. However, since the chemically powered rockets are greatly inefficient (reaching 35% efficiency), other methods of propulsion in space had to be considered and the ion propulsion thrusters entered the game, with efficiency reaching 90%. The concept of electrically powered propulsion systems was first implied in Konstantin Tsiolkovsky’s article in 1911. However, Robert Goddard considered this concept in his personal notes in 1906 and was the first to patent an electrostatic ion thruster in 1920. Thrust is a force that is the product of a mass and acceleration. The largest force produced by the latest ion thruster X3 in laboratory tests is 5.4N. Since in space there is no friction and, when far from planet, little effect from gravity, this small force allows a spacecraft to reach speeds of up to 90,000m/s over a long time. The NSTAR ion thruster aboard the Deep Space 1 aircraft proved that an ion thruster is capable running for several years in just 82kg of Xenon propellant. The X3 ion thruster is still undergoing tests and is prospected to power the future missions to mars. In this report, one is firstly introduced to the theoretical principles behind the operation of an ion thruster in order for one to be able to apply these principles when one reaches the examples section.
3.0 Theory of an Ion Thruster
The general idea behind ion thrusters is that they consist of several main components: hollow cathode, propellant, magnets, acceleration grids, exit cathode.
There are three stages which occur in order to achieve propulsion forces. Figure 1 demonstrates a simplistic view of the ion thruster. In stage 1 of figure 1, the hollow cathode injects electrons into the discharge chamber. At the same time, a propellant gas, usually Xenon, is also injected into the discharge chamber.
Stage 1 Stage 2 Stage 3
Figure 1: Simplistic view of the ion thruster showing three main stages of the process [8]
Mercury, Bismuth, Iodine, Caesium were taken into consideration when testing propellants for ion thrusters. However, Xenon has been determined to be most suitable (and will be used in this section of the report for the purpose of explaining the theory of ion thrusters) as the propellant due to its superior properties over the other gases:
- Xenon is inert – it has five complete shells of electrons. Its outer shell, containing valence electrons, has 8 electrons making it stable due to the even number of electrons. It is odourless, colourless, and tasteless – less corrosion occurs to the chamber walls. Also, safer to use in comparison with potentially explosive chemical combustion rockets.
- Xenon is easily ionized – it takes only 12.1298 kJ/mol [1] of energy in order to knock one electron from the outer shell of the Xenon atom. When the cathode injects electrons into the discharge chamber, these electrons collide with the inert (neutral) atom of xenon and knock an electron from the outer shell of the Xenon atom (more on this in ‘Stage 2’ explanation of ion thruster principle).
- Xenon has a relatively high atomic mass – atomic mass of Xenon is 131.29amu (atomic mass units) which makes it a relatively dense gas that allows it to be contained at a lower pressure. For comparison, atomic mass of Oxygen is 16amu.
The main drawback of Xenon gas as a propellant is that it is considered to be quite rare. It can be found in trace gases in the earth’s atmosphere – 1 part in 20 million [2]. It can be found on Mars and on Earth near water springs. Industrial production equates to roughly 1 tonne of Xenon gas per year.
Stage 2 of figure 1 reveals what occurs inside the discharge chamber of the ion thruster. As mentioned above, Xenon is easily ionized. When consulting the periodic table, figure 2, Xenon is in fifth place on the very right of the periodic table meaning it has five complete electron shells making it inert. Due to there being five electron shells, the outer most shell electrons are easiest to knock out of place since they are furthest from the proton and the attractive force is much weaker than that on the closest shell.
Xenon
atom
Figure 2: The periodic table of elements [9]
Ionization is the gain or loss of an electron in an atom which then becomes an ‘ion’. Figure 1 shows the injection of excessive electrons in stage 1 into the discharge chamber in stage 2. In the discharge chamber, the excess electrons collide with the neutral atoms of Xenon resulting in a loss of an electron in the Xenon atom which is then released into the discharge chamber. The theory is that when an atom loses an electron, a positive ion is formed, and when an atom gains an electron, a negative ion is formed. Hence, positive ions are formed in the discharge chamber, one electron lost from Xenon atom allowed to float in chamber, and another electron (which bombarded the Xenon atom) is also left floating in chamber – refer to equation 1:
e– + Xe0 => Xe+ + 2e–
e– = Electron
Xe0 = Neutral Xenon atom
Xe+ =Positive Xenon ion formed
2e– =Two electrons (one from cathode, the other from Xenon atom)
Equation 1: Chemical equation showing the ionization of Xenon
The presence of magnets organizes the injected excess electrons to adopt a helical path towards the positive grid in stage 3 (since opposite charges attract). This greatly increases the chances of collision of electrons with neutral atoms of the Xenon gas rather than a straight line path.
Magnets also protect the discharge chamber from corrosion by preventing the cooling of the newly formed plasma (an electrically neutral gas consisting of positive ions and electrons) and, thus, by preventing the fusion of the plasma with the discharge chamber. The newly formed plasma is confined by the magnetic field at a safe separation distance from the discharge chamber walls. The plasma confinement effect can be observed in figure 3.
Figure 3: Simplistic view of ion thruster [10]
Figure 3 demonstrates the presence of anodes inside the discharge chamber. This is to attract the electrons and repel and form the positive ions into a beam to be ready to go through the accelerating grids in stage 3. This further preserves the discharge chamber from corrosive damage.
Finally, stage 3 is where the positive ion acceleration occurs. The positive ions go through the holes in the positive grid (at 1000V for example) and then through the negative grid (at 250V for example). The positive grid repels the positive ions in the direction of the negative grid. The negative grid attracts the positive ions. Since The voltage at the positive grid is higher than at the negative grid, the repulsive force is enough to repel positive ions in the direction of the negative grid, but at the same time the negative grid attracts the positive ions at a force much less than repulsive force of positive grid. Hence, the positive ions shoot through the holes of the negative grid. At this stage, the ions have gained so much momentum that they escape the acceleration grid at up to 90km/s!
There are two main factors which affect ion velocity as they are exhausted out of the engine: accelerator voltage, and charge to mass ratio of the ions [6].
As can be seen at stage 3 in figure 1, an outer cathode is present after the accelerator grid which ensures that the positive ions do not return to the negative grid of the accelerator by supplying the same amount of electrons as the number of positive ions being exhausted, effectively neutralising the cloud of atoms.
Hence, Newton’s third law is in action as positive ions leave the ion thruster. Unfortunately, the force exerted by the thrust of ions is the same as the force of three coins would exert when resting on a palm (roughly 0.5 Newtons). On the other hand, since there is vacuum in space, this force has the potential to significantly speed up the spacecraft over time (months), potentially up to 200,000 miles per hour when referring to the most modern ion thrusters.
4.0 Variation of Ion thruster – Hall thruster:
Despite similar working principle, the hall thruster has a different setup in order to increase efficiency. Figure 4 demonstrates a cross sectional view of the Hall thruster. The propellant is fed through the openings in the anode (positive electrode). As the atoms of Xenon go through the holes of the anode, they are still neutral. As the atoms progress through the chamber, they are ionized – the cathode outside of the thruster chamber feeds electrons (refer to yellow spheres in Figure 4) which also enter the chamber (anode attracts electrons) and collide with the neutral atoms of Xenon, forming a positive ion.
As the electrons enter the chamber, they adopt a helical path just like in the normal ion thruster. This speeds up the process of collision of electrons with neutral atoms.
Since the ions are positive, they are accelerated by the electric field that is produced between the anode and the electrons that are injected from the cathode. Also, due to the ions being positive, the electrons are attracted to the positive ions, neutralising them. Hence, the same number of electrons and ions leave the thruster in the form of an electrically neutral beam.
The direction of thrust is opposite to direction of travel. This applies to both ion thrusters.
Figure 4: Simplistic view of the Hall Thruster [11]
5.0 Brief History
The first mentions of the idea of ion propulsion dates back to the start of the 20th century. Actually, the idea was greatly emphasized by two main figures: Konstantin Tsiolkovsky and Robert Goddard. It was Tsiolkovsky who first publicly introduced the concept of rocket propulsion in 1903 in an article – refer to figure 5.
The Tsiolkovsky equation is used to find out how much fuel is needed in order to change the velocity of the rocket by the amount Vbo:
Figure 5: The Tsiolkovsky equation to calculate velocity [12]
It was in 1911 when Tsiolkovsky first publicly mentions in an article the concept of electricity in order to achieve rocket propulsion: “It is possible that in time we may use electricity to produce a large velocity for the particles ejected from a rocket device” [3]. In the same article, Tsiolkovsky conceptualizes the fact that electrostatically accelerated atoms (through the use of cathode tubes) are superior, in terms of maximum velocity, to thermo accelerated atoms.
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Independent of Tsiolkovsky and his work, Robert Goddard was an Amercian Physicist who was greatly focused on accelerating electrons near to the speed of light and how this would affect their mass. It was he who suggested to exploit the ionisation process and the need to neutralize ions once they are spewed out of the exhaust. One of Goddard’s patents, approved in 1920, was an electrostatic ion accelerator – the first ion thruster.
Prior to the electrostatic ion generator, Goddard received two patents: a liquid fuelled rocket, and a solid fuelled two/three stage rocket. In 1926, this allowed him to launch the first liquid fuelled rocket which accelerated for 2 seconds at 60 mph.
6.0 Examples of use of ion thrusters
6.1 Space Electric Rocket Test 1 (SERT – 1)
This is the first space mission where ion thruster engines were used in conjunction with batteries – two ion thruster engines were used in order to test their effectiveness.
The first ion thruster was Caesium propellant based. Caesium is a very soft metal that has a melting point of 28.44°C. The cathode electrodes which supplied the electrons were made of tungsten. The presence of a boiler was necessary in order to provide a constant flow of the Caesium propellant. The theoretical concept of the ion thruster is adopted – Caesium atoms are ionized by utilizing the electrodes in order to remove the single electron present in the outermost shell of the Caesium atom to form a positive ion. The electric field then exhausts the Caesium ions – creating thrust.
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The second ion thruster utilized Mercury as the propellant. The mercury was heated (by an onboard boiler) in order to produce vapour gasses. This was then injected into the discharge chamber. One electron, out of two, was knocked out of the outermost shell of the mercury atom in order to produce a positive ion. The general principle of the ion thruster is then followed.
In both cases, the exhaust, consisting of a cloud of ions, is neutralised by a cathode providing the same amount of electrons as ions.
Mercury and Caesium have very similar properties and atomic structure. Both have six shells of electrons which make them rather easy to ionise by reducing the number of electrons in the outermost shell by one electron. Mercury has an atomic mass of 200.59amu and has an ionization energy of 10.43kJ/mol (refer to figure 6) and Caesium has an atomic weight of 132.91amu and an ionization energy of 3.89/mol (refer to figure 7). When comparing the ionization energy and atomic weight to Xenon, it makes sense to consider Mercury and Caesium (especially) to be used as the propellant.
Figure 6: Simplistic view of the Mercury atom [13] Figure 7: Simplistic view of the Caesium atom [14]
One thing to consider when using liquid metals as the propellants is the fact that when the rocket is leaving earths’ orbit, it is subject to a lot of physical stress (vibrations etc.). This needs to be taken into account when launching such test spacecraft since the propellant atoms can come in contact with the container material and knock electrons out. These excess electrons can then affect the ion beam in such way that efficiency is reduced.
During the SERT-1 mission, a fault (short circuit) developed in the Caesium based engine in the high voltage part of the system. Unfortunately, all attempts to restart the engine were unsuccessful. On the other hand, the Mercury based engine yielded fruitful and pleasing results with the sensors (accelerometer, sun intensity, beam current and voltage) sending data proving that the ion thruster was producing thrust – 28mN highest recorded value during this mission. This, in turn, proved that the neutralization of ions at the exhaust was completed [4]. The Mercury based ion engine functioned for 31 minutes 16 seconds.
6.2 NSTAR ion thruster aboard the ‘Deep Space 1’ spacecraft
The Deep Space 1 (DS1) mission was launched on October 24 1998. This was the first interplanetary deep space mission which utilized an ion thruster. The ion thruster was Xenon propellant based – as explained in the theory section of the report. There was 82kg of Xenon propellant on the spacecraft. Although only 30cm in diameter, the ion thruster consumed a range of 0.5kW to 2.3kW of power which expelled from 19mN to 92mN of thrust [5].
Unlike SERT-1, DS1 was powered by solar arrays. As mentioned before, as long as there is enough propellant and input power, the ion thruster is capable of functioning for many years by providing mN of thrust which greatly accelerates the spacecraft with prolonged time.
After two years, the NSTAR thruster provided 6630 hours of thrusting.
Figure 8 is an extract from the NSTAR mission completion document. One can see that as input power is increased, the amount of thrust also increases – as expected.
Figure 8: Power processing unit input power vs produced thrust relationship
6.3 X3 Thruster prototype
The X3 ion thruster is the latest advancement in ion thrusters. Weighing 230kg and being 80cm in diameter it is the largest and most powerful thruster which is being tested by the University of Michigan, NASA, and the US air force.
In 2017, the X3 thruster yielded record breaking results – 5.4N of force produced, 250A operating current, and 102kW operating power. When compared with the two previous examples above, these results are truly astonishing and gives prospective for future advancement in this type of technology.
Figure 9 demonstrates a generic hall thruster. One can observe the presence of a single plasma ring which can be reflected to Figure 4 which shows a single path where the positive ions shoot out. With the X3 ion thruster, shown in figure 10, one can clearly see the presence of multiple plasma paths that are narrower but larger in circumference. This is the reason for the large overall dimensions, but is also the reason why the thruster is able to produce such record breaking results.
Figure 10: The X3 Hall thruster demonstrating multiple plasma rings [16]
Figure 9: Example of a generic hall thruster showing only one plasma ring [15]
7.0 Future for ion drive propulsion systems
Despite their proven uses for interplanetary missions, ion thrusters can also be used to clear the debris that is orbiting the Earth and blocking flight paths for new space rockets. Debris such as used fuel tanks and old satellites can be cleared using ion thrusters.
A joint concept has been presented by Engineers from Tohoku University (Japan) and Australian National University in order to tackle the problem of debris [7]. Their proposed idea, Figure 11, is of a spacecraft that is fitted with two ion thrusters which would be set up on opposite sides of the spacecraft. The idea is that one ion thruster would be working in order to keep the satellite in place so that it wouldn’t move once it exerts a force on debris. The other ion thruster would work in order to decelerate the debris.
Figure 11: The proposed idea of an ion thruster debris clearing spacecraft [7]
Once the debris reduced altitude and gets closer to earth, it falls and disintegrates in the process. However, due to the fact that there have to be two ion thrusters, this greatly complicates the overall design and increases weight.
8.0 Conclusion
Firstly introduced independently by Konstantin Tsiolkovsky and Robert Goddard in the early 20th century, the technology has advanced over the decade to allow for the use of ion thruster in interplanetary space missions. By researching into the missions that have utilized ion thrusters, it seems that Xenon as the propellant is most favourable due to its properties like being inert, easy to ionize, and having high atomic mass. Having affiliated theory to the real examples of ion thruster and the prospective future, it is clear that the technology is still evolving and is highly likely to be used in the upcoming missions.
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