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Physicists Trap the World’s Coldest Plasma in a Magnetic Bottle

This 'genie in the bottle' could answer many questions about fusion power.

Plasma is one of four fundamental states of matter, but unlike the others, it isn't a part of our daily lives since it's generally found in very hot places such as the sun, or a lightning bolt. There are many questions that need to be answered about it which could further our understanding of clean energy, space weather, and astrophysics, and Rice University physicists have made a discovery that could just do that. 

The physicists discovered a way to capture the world's coldest plasma in a magnetic bottle,  according to a press release shared by the university.

The study, published in Physical Review Letters, details how the researchers were able to make a plasma about approximately -272 degrees Celsius (1 degree above absolute zero) by using laser-cooled strontium. This enabled them to trap the plasma briefly with forces from surrounding magnets, marking the first time an ultracold plasma has been magnetically confined and making studying plasmas in different settings a viable possibility. 

Trapping the world's coldest plasma

The researchers used a quadrupole magnetic setup that reportedly resembles the designs that were developed by fusion energy researchers in the 1960s. This was extremely challenging due to two problems: according to the researchers, the plasma for fusion needs to be about 2.7e+8°F (150 million degrees Celcius), and containing it magnetically can be a bit of a pickle since the magnetic fields change drastically throughout the plasma.  

"One of the major problems is keeping the magnetic field stable enough for long enough to actually contain the reaction," said study co-author Stephen Bradshaw, a Rice astrophysicist who specializes in plasma phenomena on the sun. "As soon as there’s a small sort of perturbation in the magnetic field, it grows and 'pfft,' the nuclear reaction is ruined. For it to work well, you have to keep things really, really stable. And there again, looking at things in a really nice, pristine laboratory plasma could help us better understand how particles interact with the field." 

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Just like hot plasma, the researchers' laser-cooled plasma is a soup of electrons and icons, but it's sensitive to relatively weak magnetic forces. When the team applied such forces with a non-uniform magnetic field, much like setting up a trap, the plasma expanded rapidly after being created at the center of the field and slowed after moving into the other, stronger region.

While the researchers weren't able to observe the plasma's escape from the magnetic confinement, they managed to contain it for at least half a millisecond which wouldn't be possible if not for this technique. 

"This provides a clean and controllable testbed for studying neutral plasmas in far more complex locations, like the sun’s atmosphere or white dwarf stars," said Rice Dean of Natural Sciences Tom Killian who is the corresponding author of the study. "It’s really helpful to have the plasma so cold and to have these very clean laboratory systems. 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."

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The researchers state that the next step is to combine magnetic fields with lasers to create even better magnetic traps, opening the way for many discoveries. 

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