As a soupy mix of electrons and ions that forms under certain conditions in the most extreme of environments, plasma is an inherently difficult thing to observe. Scientists have made a significant breakthrough in the way we study this phenomenon, managing to trap an ultracold form of it in a magnetic "bottle" for the first time, an achievement that could act as a springboard for research into nuclear fusion energy, and what we understand of the Sun and the stars.
Generally speaking, it takes extremely hot temperatures for plasma to form, like those found in the Sun or in a lightning strike. Scientists at Rice University have been exploring how a technique known as laser cooling, developed in the 1990s to slow atoms down almost to a halt, can be used to create low-temperature, low-density plasma and study how it behaves in the lab. In 2019, the team published a paper describing its method of creating laser-cooled plasma around 50 times colder than that found in space.
The plasma the scientists were working with in their latest experiments is described as the world's coldest, at around one degree above absolute zero, or -272 °C (457.6 °F). This ultracold plasma expands rapidly once it is created, dissipating entirely within just a few thousandths of a second. Using what's known as a quadrupole magnet setup, which is similar to the systems used to confine plasma in experimental fusion energy systems, the team was able to trap and hold its ultracold plasma in place for several hundredths of a second instead.
“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,” says Tom Killian, 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.”
An example of this clutter is the interactions that take place inside fusion reactors, where streams of plasma are heated to temperatures as high as 150 million °C, and stabilized with magnets to produce electricity. Holding the plasma in place for long enough for these reactions to occur, or understanding the reasons why it doesn't, is key to the pursuit of clean nuclear fusion energy.
“One of the major problems is keeping the magnetic field stable enough for long enough to actually contain the reaction,” says co-author 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. 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.”
The team's so-called bottled ultracold plasma could have ramifications in other fields of science, too. It could allow researchers to study the reactions that take place when the plasma in solar winds that emanate outward from the Sun collide with the Earth's magnetic field, or investigate special features in the sun's atmosphere that would otherwise be difficult to see with our cameras and scientific instruments.
“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,” says Killian.
The study was published in the journal Physical Review Letters, while Killian explains the research in the video below.
Source: Rice University