Plasma engine is powered by electricity
Chinese scientists have tested an engine that uses air and electricity to produce plasma. The concept might one day replace jet engines, which emit 2.5% of the world’s greenhouse gases.
zone where water flows on the surface, are rare exceptions in a universe dominated by plasma.
From Earth, we see the plasma of the Sun’s more-than-one-million-degree-hot outer atmosphere, the corona. From this flows a constant stream of plasma in the form of protons and electrons, striking Earth’s upper atmosphere, producing the auroras that can be viewed above the poles. The Sun also keeps plasma ‘bombs’ in its arsenal, with major solar eruptions known as coronal mass ejections, in which a huge bubble with billions of tonnes of hot plasma is shot from the corona, sometimes heading directly for Earth. At their worst, such a bubble could cut right through Earth’s magnetic shield, releasing high quantities of electrically-charged particles deep into Earth’s atmosphere, generating a geomagnetic storm that might disable technology, affecting electricity and telephone communication across entire continents.
So plasma involves extreme forces. And physicists aim to tame these forces to produce the fuel of the future.
In 1903 the Wright brothers paved the way for air travel when they achieved take-off with an engine-powered plane for the first time, flying (for all of 12 seconds) from a mound in North Carolina. Some 115 years later, aerospace engineer Steven Barrett and his colleagues from the Massachusetts Institute of Technology achieved a flight that might similarly go down in history. Their battery-powered model aircraft, with a wing span of 5 metres and a weight of 2.45kg, flew 55 metres. As with the Wright Flyer, this might not seem very impressive. But this plane was ‘powered’ by nothing but air.
Under the plane’s wings were four arrays of thin wires, strung like horizontal fencing along and beneath the front end of the plane’s wing. The wires act as positively charged electrodes, while similarly arranged thicker wires, running along the back end of the plane’s wing, serve as negative electrodes. The front electrodes have a positive voltage of 20,000 volts, and their powerful electric field converts air molecules into plasma. The positive nitrogen and oxygen ions from the air are then attracted by the rear electrodes, which are at negative 20,000 volts. On the path between electrodes, each positive ion strikes millions of neutral air molecules, kicking them backwards, making the plane move forwards. According to Steven Barrett, the technology could be used to make silent drones, and airliners in which air-powered ion engines can supplement ordinary jet engines. Scientists are working on lower voltage propulsion, and on using more of the plane’s surface.
The model aircraft from MIT travels forwards with a force of six newtons per kilowatt of electricity. (One newton corresponds to about the pressure you feel on your hand when you are holding an apple.)
Scientists at China’s Wuhan University recently took another step forward, developing an engine that generates 28 newtons per kilowatt. The engine works using microwaves that convert the air into plasma hotter than 1000 degrees. Compressed air is directed through the plasma, which then expands and forces itself explosively out of a pipe. In an experiment using a small prototype, the plasma engine was able to lift a 1kg metal ball placed on top of the pipe. If they manage to upscale the technology, the Chinese scientists estimate that the plasma jet engine, which ‘burns’ only air, could become sufficiently powerful to achieve the forces generated by modern jet engines.
If plasma engines are to go mainstream in jet planes throughout the world, they will require batteries that can provide as much energy for their weight as delivered by the fossil fuels currently used. Today’s lithiumion batteries can manage approximately 250 watt hours per kilogram, which is still some 30 times less than jet fuel. But if equity could be achieved, and the batteries charged by electricity from solar panels or wind turbines, then plasma engines converting air during flight could make aviation entirely climate neutral in the future.
Plasma could power space trucks
It is not only down on Earth that plasma holds potential as a fuel. In space, the extremely hot state is even more efficient, as engines don’t need to overcome air resistance and gravity as they do on Earth. And spacecraft have already flown by means of plasma produced in ion engines. When NASA’s Dawn satellite entered orbit around the large asteroid of Vesta and the dwarf planet of Ceres, its engine was using a powerful electric field to convert gas into plasma, after which the positive ions in the plasma were sent backwards through a nozzle, pushing the satellite forwards.
On long space missions, this type of engine has one important disadvantage: the plasma makes the electrodes corrode, limiting the engine’s life. To minimise the corrosion, only unreactive noble gases, such as xenon, can be used. We might overcome this challenge by developing plasma engines in which gas is converted into plasma using radio waves.
Another method which might pave the way for aerospace plasma engines involves trapping the plasma in a magnetic cage, so it does not touch the wall of the engine chamber. In that way, plasma engines can use a common gas such as hydrogen, which can be extracted in many places relatively close to Earth, such as the Moon or Mars. So plasma-powered engines could play a key role in humankind’s colonisation of the
Solar System, with spacecraft refuelled for a return or onward journey with hydrogen from bases built on different worlds.
The front-running plasma engine is the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), which has been under development in the US for decades, first by NASA and now with the Ad Astra company. In the engine chamber, gas is first heated to thousands of degrees, by which it is converted into plasma. Subsequently a magnetic field directs the electrically-charged plasma into another chamber in which radio waves heat the plasma to one million degrees, causing it to expand tremendously. Finally, a magnetic field will direct the plasma into space through a nozzle – at speeds of 50,000+ m/s.
At 200kW, the current plasma engine can generate a force of just 5 newtons, which is not sufficient to lift a rocket up through the atmosphere. But it is enough to send a craft deeper into space from an orbit around Earth. Ad Astra is now carrying out crucial tests of the plasma engine on the ground, where it must complete a continuous 100 hours of operation before it can be tested in space. The technology may be particularly suitable for ‘space trucks’ – craft designed to haul cargo to and from the Moon. It might also suit long space missions. In the inner Solar System, solar panels might generate sufficient energy, so that a small nuclear reactor would be required only for missions to the outer planets.
Ocean water as fuel source
The most ambitious plan for the use of plasma as fuel is fusion power generation in a power plant that imitates the Sun. Inside the Sun, hydrogen exists as plasma in which atomic nuclei and electrons have split, and the atomic nuclei continuously fuse into helium, a process in which immense quantities of energy are discharged. Physicists and engineers in several countries are constructing reactors in which temperatures of 100-200 million °C will produce hydrogen plasma, so the atomic nuclei can undergo fusion just as they do inside the Sun. The most important fuel for fusion power plants is heavy hydrogen, which can be extracted from ocean water, plus superheavy hydrogen, which is produced in the reactor by radiating lithium with neutrons emitted when hydrogen becomes helium. The known reserves of lithium are sufficient for 1000 years (Australia is the world’s leading producer), while ocean water is a practically infinite source of heavy hydrogen. Heavy hydrogen from 40 litres of ocean water and superheavy hydrogen from 5g of lithium (the content of a mobile phone battery) can supply as much energy as 40 tonnes of coal.
Fusion power plants seem to be safer than fission nuclear power because hydrogen fusion halts quickly if the supply of fuel to the reactor chamber is shut off, like a car running out of petrol. Fusion energy also leaves behind none of the highly radioactive fuel waste produced by fission energy in modern nuclear power plants, because the only waste product is helium.
If fusion can be successfully delivered, energy systems of the future might combine fusion energy with solar and wind farms to deliver the green energy revolution that so many scientists are working to create. And plasma, hitherto the forgotten state, could be the solution that takes us from coal and oil to a new world of inexhaustible and climate-friendly energy.