Plasma brings fusion closer

A project by University of Wisconsin-Madison researchers claims to have overcome a barrier to plasma research, bringing the possibility of fusion energy a step closer.



The research team, headed by electrical and computer engineering professor David Anderson and research assistant John Canik, investigated the Helically Symmetric eXperiment (HSX). The HSX is a magnetic plasma chamber called a stellarator, which can overcome a major barrier in plasma research, by which stellarators lose too much energy to reach the high temperatures needed for fusion.



The new results show that the unique design of the HSX loses less energy, meaning that fusion in this type of stellarator could be possible.



Current plasma research builds on two types of magnetic plasma confinement devices, tokamaks and stellarators. The HSX aims to merge the best properties of both by giving a more stable stellarator the confinement of a more energetically efficient tokamak. ‘The slower energy comes out, the less power you have to put in, and the more economical the reactor is,’ says Canik.



Tokamaks, the current leader in the fusion race, are powered by plasma currents, which provide part of the magnetic field that confines the plasma. However, they are prone to disruptions.



‘The problem is you need very large plasma currents and it’s not clear whether we’ll be able to drive that large of a current in a reactor-sized machine, or control it. It may blow itself apart,’ says Canik.



Stellarators do not have currents, and therefore no disruptions, but they tend to lose energy at a high rate, known as transport. The external magnetic coils used to generate the plasma-confining field are partially responsible for the high transport rates in conventional stellarators. The coils add some ripple to the magnetic field, and the plasma can get trapped in the ripple and lost.



The HSX is the first stellarator to use a quasi-symmetric magnetic field. The reactor itself has twisted magnetic coils wrapped around the warped doughnut-shaped chamber, with instruments and sensors protruding at odd angles. But the semi-helical coils that give the HSX its unique shape also direct the strength of the magnetic field, confining the plasma in a way that helps it retain energy.



The team designed and built the HSX with the prediction that quasisymmetry would reduce transport, which the latest research supports. ‘This is the first demonstration that quasisymmetry works, and you can actually measure the reduction in transport that you get,’ said Canik.



The next step for the project is to establish how much symmetry in the coils is necessary to achieve low transport rates. They hope to make the coils easier to engineer, with the idea that the principles used in the HSX could someday be incorporated into fusion generators, the reason that Anderson and his team began designing the HSX 17 years ago.



‘It’s an exciting field. It’s something where one can contribute positively to mankind with an energy source that’s completely sustainable, doesn’t involve nuclear proliferation or radioactive waste, with a limitless fuel supply,’ said Anderson.