Eurofusion October 19th 2016
The current environmental concerns lead us to look for cleaner, safer and more efficient ways in which to produce power. We are in constant growth and our technological lifestyles are increasingly demanding for more energy. In addition to sustainable power from renewable sources, scientists want to find a solution which employs nuclear fusion. This has a huge benefit, it is a source of energy that is virtually unlimited!
Just like Prometheus
Achieving nuclear fusion can be compared to the Greek titan Prometheus feat: creating a little Sun on Earth! This is like stealing fire from the Gods. Fusion is going to become reality and European scientists are working hard towards it. The experts‘ goal is to exploit fusion to make power plants work. In order for this to be feasible, the fusion reaction must be able to produce more energy than it consumes. It has been a great challenge for years, nevertheless EUROfusion, the European Consortium for the Development of Fusion Energy, aims to realise fusion energy by 2050.
“Magnetic bottles” are the way
Building a fusion reactor is a bit more than just tricky. To achieve the densities and temperatures required for a successful thermonuclear reactor, a plasma must be contained by magnetic forces for a sufficiently long time to produce net thermonuclear power. Such a container is also called a‚ ‘magnetic bottle‘. One of the most important problems discovered in the attempts to achieve this confinement is stability; since a plasma confined by a magnetic field is not in thermodynamic equilibrium. It can collapse due to a large variety of instabilities.
Despite the stability problems, significant progress has been made in building larger and “smarter” machines. One of the first and most important testing grounds is JET: the world‘s largest operational magnetic confinement plasma physics experiment, located at EUROfusion’s Research Unit Culham Centre for Fusion Energy (CCFE) in the UK. The experiments began in 1983, when the very first JET plasma was burned. In 1991, a Preliminary Tritium Experiment achieved the world’s first controlled release of fusion power. Six years later, in 1997, another world record was set: JET produced 16 megawatts of fusion power. After more than 25 years of successful operation, JET is still at the forefront of fusion research and is closely involved in plasma physics research, systems and materials testing for ITER.
Towards the future: ITER
The international project ITER represents the next stage in the development of fusion energy. The tokamak ITER will be the biggest magnetic confinement experiment in the world capable of generating fusion energy surplus. That energetic surplus essentially comes from the difference between the sum of the masses of the two hydrogen isotopes tritium and deuterium and that of the reaction product: helium. Hydrogen is one of the most common elements in the universe. Deuterium can easily be found in sea water. Tritium is produced in a fusion reaction by splitting Lithium ions by a neutron (from the fusion reaction) into tritium and helium. Helium is “lighter” than the sum of the masses of the elements that generate it, so that “lost” mass is what is transformed into heat. Until now, the most difficult part has been initiating the entire process. It needs very high temperatures and, above all, an effective way to confine the very hot reaction. At those conditions matter is in a physical state of plasma, a mixture of free electrons and ions characterised, amongst others, by a temperature reaching millions of degrees.
Doughnuts for fusion
For these reasons, the singular “doughnut-shaped” tokamak is ideal. It is a machine with a specific ring-shaped structure which generates a strong magnetic field (5 to 10 Tesla). The tokamak is able to isolate a thermonuclear plasma from the walls of its container; the walls also directly absorb the heat. In a tokamak charged particles move with circular trajectories when immersed in a magnetic field. That is why such intense magnetic fields require superconducting magnets whereby the word “super” is well deserved, since they hold plasma at millions of degrees 1 meter away from the superconductor cable which has a temperature of only 4 to 4.5K.
In the meantime, in Greifswald in Germany the most progressive stellarator Wendelstein 7-X produced its first plasma last December. But, what in the world is a stellarator? Its ‘devilish design’ looks like something out of Star Trek or Star Wars when, in truth it is only a case of strong assonance. Yet, looking at it, a stellarator is very much akin to science fiction. The stellarator is the tokamak’s “cousin”. But unlike tokamaks, in which the magnetic field changes only in two dimensions, in stellarators it is three-dimensional: the twisting field is produced entirely by external non-axisymmetric coils. It is essentially, a fusion reactor with a twist, so hard to build that in fact it equired the help of supercomputers to model the design in advance.
In February, the German team working on Wendelstein 7-X was able to heat hydrogen gas to 80 million degrees for a quarter of a second. This was the proof of concept: the team wanted to increase microwave plasma heating power to 20 megawatts, scaling things up to heat the hydrogen gas to the 100-million-degree benchmark. Hydrogen releases a whole lot more energy, it is also a lot harder to heat. This was a huge milestone in the decades-long pursuit of controlled nuclear fusion.
Tokamak vs. stellarator
Both stellarators and tokamaks work and both of their concepts have innate advantages and disadvantages with regard to the technical and physical aspects of a fusion device working towards burning plasmas. In fact, tokamaks have their drawbacks too. The current in the plasma may falter unexpectedly, resulting in “disruptions”: sudden losses of plasma confinement that can damage the reactor. Stellarators, however, are immune to this problem: their fields come entirely from external coils, which do not need to be pulsed, and there is no plasma current to suffer disruptions. A tokamak has the advantage of being technically much simpler and more straightforward, while stellarators, as mentioned before, are complex.
Stellarators are steady state as there is no transformer action. They are almost currentless which implies that many of the instabilities occurring in tokamaks cannot occur in stellarators. What troubles fusion researchers are the fast ions in a stellarator’s plasma. The unconfined particles orbit and get lost along the twisted path. As a result, most fusion research since the 1950s has focused on tokamaks. Less effort has been concentrated on stellarator devices, essentially due to the technical complexity in comparison with tokamaks. Now, stellarators have shown significant achievements and researchers want to prove that a stellarator is as good as a tokamak when it comes to realising fusion energy.
In the meantime
Whether a commercial fusion power plant is a stellarator or a tokamak is still up in the air. But, while we wait for this promethean quest to be accomplished, research provides us with many short-term benefits. Fusion exploration is a complex and multidisciplinary field, but it has pushed advances in medical technology, the environment, theoretical physics, astrophysics, material sciences and telecommunication.
It is precisely its complexity that requires everyone to join in on collaborating their efforts.