For the first time, a fusion device at the University of Wisconsin in Madison has generated plasma, inching one step closer toward using nuclear fusion as a a new source of carbon-free energy.

The university’s physicists and engineers have been building and testing the device at a lab in Stoughton for the last four years, which is referred to as the Wisconsin HTS Axisymmetric Mirror or WHAM. The magnetic mirror device became operational on July 15.

Researchers worldwide have been working for decades to harness energy from nuclear fusion reactions that power the sun and the stars. That reaction relies on heated plasma, which is a gas of hot ions and free-moving electrons.

Cary Forest, a UW-Madison physics professor, said generating plasma is an exciting step.

Until now, nuclear power has come from fission reactors that split atoms. Fusion reactors would join atoms together. Unlike current nuclear plants, Forest said fusion would not produce radioactive waste, adding it’s also safer.