A US-German collaboration has delivered a new set of upgrades to the Wendelstein 7-X (W7-X), German’s nuclear fusion reactor, improving the performance of the machine. These upgrades include a system of five large magnetic copper “trim” coils.
The W7-X is a stellarator, one of two kinds of nuclear fusion reactors currently being tested. The other is called a tokamak. Both of them are donut-shaped, but the stellarator is a twisty donut, which allows it to operate continuously with low-input power and without disrupting the fusing plasma inside. The drawback is that it is more difficult to design and build than a tokamak because of how its magnets are organized.
The new coils in the stellarator help divert the excess heat and particles that leave the plasma. The second run of testing, which started last December after the successful first run, showed that the upgrades did indeed deliver the heating and measurement capabilities that the reactor requires to become commercially viable. Specifically, the recent experiments demonstrated that the coils can be used to measure and correct extremely small variations in the system.
"The trim coils have proven extremely useful, not only by ensuring a balanced plasma exhaust onto the divertor plates, but also as a tool for the physicists to perform magnetic field measurements of unprecedented accuracy," Thomas Sunn Pederson, Max Planck director of stellarator edge and divertor physics, said in a statement.
To achieve this accuracy in modifying the magnetic field within the stellarator, the team conducted preliminary experiments on the coils, which confirmed the prediction that this exceptional control could be achieved with one-tenth of the full power of the coils.
"The fact that we only required 10 percent of the rated capacity of the trim coils is a testament to the precision with which W7-X was constructed," added Samuel Lazerson from the Princeton Plasma Physics Laboratory. "This also means that we have plenty of trim coil capacity to explore divertor overload scenarios in a controlled way.”
The stellarator is so complex that it had to be designed with 3D software. It took 19 years to complete and has 425 tonnes (470 tons) of superconducting magnets. The magnets need to be cooled down to almost absolute zero (to become superconducting) and there are 250 ports for diagnostics, supply and removal of fuel, and heating up the plasma with microwaves.