A NEW KIND OF FUSION
The combatants confront each other: The simple, well-known, however impractical tokamak model versus the sophisticated, untested, but efficient stellarator model. One of the two reactors is the key to successful fusion energy in the future.
Tokamak reactors are the safe tech when it comes to fusion, but now the technically complex stellerator could spark a revolution.
TOKAMAK Simple. Symmetrical. Hopelessly Impractical?
PROS: The tokamak reactorer is the most simple design. It is easier to build a large fusion power plant with this type of reactor.
CONS: As the magnetic coils are located closer to each other on the inside than on the outside, the ring is weaker on the outside. So far, hydrogen can only be kept contained for a few seconds.
STELLERATOR Sophisticated. Stable. Maybe Impossible?
PROS: In a stellarator, twisted magnets make sure that the fuel is affected by the exact same field strength from all sides. Theoretically, it can be kept stable for months.
CONS: Stellarators are extremely complex plants that make up a huge engineering challenge. Consequently, they pose a considerable technical challenge.
On 3 February, German Chancellor Angela Merkel hit the red button, injecting hydrogen into Germany’s new fusion reactor for the first time. The extremely hot and electrically charged hydrogen gas was captured in a magnetic cage for a quarter of a second. That doesn't sound like a lot, but Merkel’s touch may have been the first small step towards taming the forces that provide the Sun with energy and so towards solving Earth’s energy problem once and for all. The day was a huge fusion technology breakthrough.
The fuel of fusion power plants must keep up a temperature of no less than 100 million degrees, and so, the hydrogen cannot be cooled by touching the reactor wall. The magnetic cage is a must, when hydrogen atoms are to fuse into helium, releasing large quantities of energy. The new reactor, Wendelstein 7-X, aims to capture the hydrogen in the magnetic cage for half an hour at a time. If successful, that could pave the way for power plants, in which fusion fuel can be captured in the cage for months. And this very perspective makes the new German reactor a worthy challenger of the huge international ITER reactor, which is under construction in France.
ITER is a huge tokamak. The reactor project will pave the way for many fusion power plants with pulsed power supply systems, in which the fuel is introduced into the reactor ring and captured in the magnetic cage for a few minutes or, at best, for an hour at a time. The cage cannot be kept up any longer, and subsequently, the reactor ring is completely emptied, and new fuel is introduced over and over.
TWISTED SHAPE IS INGENIOUS DESIGN
The Wendelstein 7-X is a stellarator, in which the magnets surrounding the reactor ring are twisted into odd shapes. That allows the creation of a stable magnetic cage, which can theore ti c a l l y
keep "floating" fuel trapped for ever. This very characteristic will be a great practical advantage in a fusion power plant that is to be operated 24/7, 365.25 days a year. The disadvantage is that the design is extremely complex, will no doubt throw up many technical problems engineers haven't yet considered. So, the simpler tokamaks have been the darlings of fusion scientists’ since the 1960s. But if the new German stellarator lives up to expectations, the scores will be even immediately before the next long leap ahead – a demonstration power plant, which is to show that fusion reactors are able to generate competitively priced power.
Physicists and engineers from all over the world have been working on making the dream of generating fusion energy come true for the past seven decades, as that would mean that we can forget about energy shortage for the next 1,000 years. The fuel is heavy hydrogen, which can be extracted from seawater, and superheavy hydrogen, which is made by radiating lithium with neutrons in the reactor. Fusion generates so much energy that 40,000kg of coal can be replaced by 40 litres of seawater and 5 grams of lithium – about as much as in a smartphone battery.
Fusion energy will not produce radioactive fuel waste, which must be deposited safely for 100,000 years, and a power plant would neither emit unhealthy air pollution nor greenhouse gases. In addition, fusion power plants are very safe, as there is no risk of out of control chain reactions like in a traditional nuclear power plant, where the reactor tank contains uranium for many months.
Hydrogen fusion can be halted fast and efficiently by cutting the continuous supply of fuel to the reactor ring. So, the fusion reactor will come to a halt in the same way as a car engine, which runs out of petrol.
NO MATERIAL CAN HANDLE THE HEAT
The major fusion energy challenge is to make hydrogen fuse into helium. To obtain so much fusion that the process results in a surplus of energy as compared to the quantity of
energy used to heat the fuel, temperatures of 100-200 million degrees are required. No materials can tolerate such temperatures, so the fuel must be captured in the magnetic cage. To create an efficient cage, we need huge, superconducting magnets all the way around the reactor ring. The magnets must be cooled to a temperature of minus 269 degrees by means of liquid helium, and the frozen magnets must function right outside the reactor ring with the extremely hot fuel.
EXPERIMENTS BEGIN IN 2035
So far, tokamaks have been the best solutions, but the challenge remains a huge one. At the giant ITER test reactor, in which the largest superconductive magnets measure 25 m from "head to foot" and weigh 400 tonnes, the first experiments were meant to begin in 2020, but the date has now been postponed to 2025, at which point the reactor is to use ordinary hydrogen. Experiments with energy generation and real fusion fuel consisting of heavy and superheavy hydrogen will begin in
2035. Physicists wait as long as possible, because superheavy hydrogen is radioactive. Once they have introduced it into the reactor ring, they can no longer send engineers in to make repairs or technical improvements. In the first experiments with energy generation, ITER must keep the fuel captured for a few minutes at a time and produce 10 times more energy than the reactor consumes.
STELLARATOR WAS ALMOST DITCHED
The aim of the Wendelstein 7-X-stellarator is not to generate power, but rather to keep the fusion fuel magnetically caged for half an hour at a time. If the experiments are successful, the capture time could theoretically be extended to almost forever in future fusion power plants, which could operate 24/7 by adding new hydrogen to the reactor ring and removing surplus helium.
The first experiments look promising, but the construction of the reactor challenged engineers. The ring with the twisted magnets had to be designed on a supercomputer to get a stable magnetic cage, and the shape of the magnets was so technically demanding that one third of the industrially made magnets were scrapped in 2003, after the Germans had been working on them for three years. By then, the project was close to being ditched, but Germany continued the work, only to discover that the construction of the tricky helium system around the twisted magnets was a true nightmare. The components had to be changed over and over again. A million working hours went into building Wendelstein 7-X. Thankfully, it works.
If the new German fusion reactor is succeeded by an energy-generating stellarator, this type of reactor could become a serious competitor to tokamaks like ITER, when fusion energy takes its next leap forward. The next step will be construction of a demonstration plant, which is to supply power to the grid. The choice of reactor will be made when the design phase begins in the 2030s. The aim is to build the first commercial fusion power plants in the 2050s. At that point, fusion energy will have been underway for 100 years, but then, the huge effort will earn the world an almost inexhaustible energy source.