The success of Rice University scientists in trapping an ultracold form of plasma by magnetic confinement can significantly advance research into nuclear fusion energy as well as in astrophysics.

The researchers applied a laser cooling technique to strontium atoms to develop a low-temperature, low-density plasma — the world’s coldest at -272° C, briefly trapped with forces from surrounding magnets. A quadrupole magnet setup, similar to that used to confine plasma in experimental fusion energy systems, effectively confined the ultracold plasma in place for several hundredths of a second.

The plasma for fusion needs to be about 150 million° C and magnetically containing it is a challenge, as the manner in which the plasma and magnetic fields interact and influence one another is not well understood.

“One of the major problems is keeping the magnetic field stable enough for long enough to actually contain the reaction,” said study co-author and astrophysicist Stephen Bradshaw. “As soon as there’s a small sort of perturbation in the magnetic field, it grows and ‘pfft,’ the nuclear reaction is ruined.”

With continuing improvements such as increased magnetic field gradient and field of view for laser-induced fluorescence imaging, the researchers hope to characterize scaling of trapping behavior with magnetic fields and study plasma flow in loss gaps, which will support development of a quantitative model of plasma dynamics.

A paper on this development appears in Physical Review Letters.

Images produced by laser-induced fluorescence show how a rapidly expanding cloud of ultracold plasma (yellow and gold) behaves when confined by a quadrupole magnet. Ultracold plasmas are created in the center of the chamber (left) and expand rapidly, typically dissipating in a few thousandths of a second. Source: T. Killian/Rice University