Boffins trap ultra-cold plasma-in-a-bottle, a move that may unlock secrets of exotic stars

Matter held not for long... but for long enough


Video Physicists have trapped a sliver of the world’s coldest plasma in a magnetic "bottle" for the first time, taking the first tentative steps towards building increasingly realistic simulations of the interiors of stars.

Plasma, otherwise known as the fourth state of matter, is normally formed in extremely hot environments, where the heat rips away the outer electrons from atoms leaving behind a turbulent sea of charged particles.

Strong magnetic fields are used to trap super hot plasma, though it requires a lot of energy to confine the matter for long enough to actually do something interesting with it, such as probing its properties or generate some electricity from fusion. Instead of studying super hot plasma, researchers at Rice University have turned to much colder temperatures to generate plasma that is easier to manage.

Hot plasma requires very strong magnetic fields to overcome the particles' large kinetic energies, Grant Gorman, a graduate student at America's Rice University working on the project, explained to The Register.

“One of the major problems is keeping the magnetic field stable for long enough to contain the reaction," he said. "Cold plasmas, on the other hand, move more sluggishly, reducing the flow of mass and energy. Confining these plasmas requires modest magnetic fields that are straightforward to produce in the laboratory by flowing current through wire.”

First, a stream of strontium atoms are chilled down to -272 degrees Celsius – just above absolute zero – and then ionised using lasers. Next, they’re trapped within a magnetic field generated by current flowing in two sets of coils. The amount of plasma created is tiny, and it dissipates within a millisecond, as explained in the video below.

Youtube Video

That may not sound like very long, but without the magnetic confinement the particles in the plasma tend to drift apart from each other and only last tens of microseconds.

“It’s really helpful to have the plasma so cold and to have these very clean laboratory systems,” said Tom Killian, co-author of the study published in the journal Physical Review Letters (here's a free preprint) and a professor of physics and astronomy at Rice University. “Starting off with a simple, small, well-controlled, well-understood system allows you to strip away some of the clutter and really isolate the phenomenon you want to see.”

The strength of the magnetic field in the coils was about 100 Gauss, approximately ten times stronger than a typical fridge magnet, Gorman added.

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The team believes that generating these magnetized clouds of cold plasma will help scientists better understand physical processes that are difficult to model computationally. For example, the cold plasma experiments can simulate environments of white dwarf stars or neutron stars.

Gorman said that cold plasma can simulate the internal workings of stars even though they’re much colder and smaller than astrophysical objects if it shares some of their properties. “One property is that the plasmas are strongly coupled," he said. "If you take the interaction energy and the forces between neighbouring particles, if they’re larger compared to their average kinetic energy then the plasma is said to be strongly coupled. This leads to very interesting phenomena, and the systems will share similar transport properties.”

The team mapped the motion of their cold strontium-made plasma by taking pictures of the individual ions. The particles are zapped with a laser beam and glow under the light, the emitted frequencies allow the team to figure out where they are and how fast they’re going. However, the magnetic field generated by the coils isn't uniform, and that changes the frequencies at which the particles will fluoresce.

“That’s one of the biggest challenges in doing this experiment,” Gorman explained. “The most important thing is that they’re very well controlled and have precise diagnostics. Things can be altered in ways that are subtle and complicates what you can see.”

In order for the plasma experiment to be applicable to useful areas in physics, researchers have to be able to control the substance for longer and be able to accurately measure its properties. “To understand how the solar wind interacts with the Earth, or to generate clean energy from nuclear fusion, one has to understand how plasma — a soup of electrons and ions — behaves in a magnetic field,” Killian added. ®


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