Grant Gorman, a graduate student at Rice University, works in Rice’s Ultracold Atoms and Plasmas Lab. Photo by Jeff Fitlow / Rice University
Laser cooled plasma-in-a-bottle can answer questions about the sun, fusion power.
Rice University physicists have discovered a way to capture the world’s coldest plasma in a magnetic bottle, a technological achievement that could advance research into clean energy, space weather and astrophysics.
“To understand how the solar wind interacts with the earth, or to generate clean energy from nuclear fusion, one must understand how plasma – a soup of electrons and ions – acts in a magnetic field,” said Tom Killian, Rice Dean of Natural Sciences. , the corresponding author of a published study on the work in Physical overview letters.
Using laser-cooled strontium, Killian and graduate students Grant Gorman and MacKenzie Warrens made a plasma about 1 degree higher absolute zero, or about -272 degrees Celsius, and captured it briefly with forces from surrounding magnets. This is the first time an ultra-cold plasma has been magnetically restricted, and Killian, who has been studying ultra-cold plasmas for more than two decades, said it opens the door to studying plasmas in many environments.
“It provides a clean and controllable test bed for studying neutral plasmas in much more complex places, such as the atmosphere of the sun or white dwarf stars, ”said Killian, a professor of physics and astronomy. ‘It’s very useful to have the plasma so cold and to have these very clean laboratory systems. If you start with a simple, small, well-controlled, well-understood system, you can remove the clutter and really isolate the phenomenon you want to see. ‘
MacKenzie Warrens, a graduate student at Rice University, is conducting a laser cooling experiment in Rice’s Ultracold Atoms and Plasmas Lab. Credit: Photo by Jeff Fitlow / Rice University
This is important for study co-author Stephen Bradshaw, a Rice astrophysicist who specializes in studying plasma phenomena on the sun.
‘Throughout the sun’s atomic atmosphere, the (strong) magnetic field has the effect of changing everything compared to what you would expect without a magnetic field, but in very subtle and intricate ways that can make you really disappear if you don’t really.’ a good understanding of it, ”said Bradshaw, associate professor of physics and astronomy.
Solar physicists rarely get a clear observation of specific properties in the atmosphere of the sun, because part of the atmosphere lies between the camera and the features, obscuring unrelated phenomena in the intermediate atmosphere that they want to observe.
“Unfortunately, because of this vision problem, observational measurements of plasma properties hold a great deal of uncertainty,” Bradshaw said. “But as we improve our understanding of the phenomena, and most importantly, use the laboratory results to test and calibrate our numerical models, we can hopefully reduce the uncertainty in these measurements.”
Images produced by laser-induced fluorescence show how a rapidly expanding cloud of ultra-cold plasma (yellow and gold) behaves when limited by a quadruple magnet. Ultra-cold plasmas are created in the center of the chamber (left) and expand rapidly, usually disappearing within a few thousandths of a second. Using strong magnetic fields (pink), Rice University physicists captured and held ultra-cold plasmas for a few hundredths of a second. By examining how plasmas interact in such experiments with strong magnetic fields, researchers hope to answer research questions related to clean fusion energy, solar physics, space weather and more. Credit: Image courtesy of T. Killian / Rice University
Plasma is one of the four fundamental states of matter, but unlike solids, liquids and gases, plasmas are not usually part of everyday life because they tend to be in very hot places like the sun, a lightning bolt or to prevent candle flame. Like the hot plasmas, Killian’s plasmas are electrons and ions, but they are made cold by laser cooling, a technique developed a quarter of a century ago to capture matter with light.
According to Killian, the magnetic setup of the quadrilateral used to capture the plasma is a standard component of the ultra-cold setup used by his laboratory and others to make ultra-cold plasmas. But finding out how plasma can be captured with the magnets was a tricky problem because the magnetic field has a great deal of destruction with the optical system that physicists use to look at ultra-cold plasmas.
“Our diagnosis is by laser-induced fluorescence, where we shine a laser beam onto the ions in our plasma, and if the frequency of the beam is just right, the ions will scatter photons very effectively,” he said. ‘You can take a picture of them and see where the ions are, and you can even measure their velocity by looking at the Doppler shift, just like with a radar rifle to see how fast a car is moving. But the magnetic fields actually shift around the resonant frequencies, and we need to disrupt the shifts in the spectrum that come from the magnetic field from the Doppler shifts we want to observe. ”
This complicates experiments considerably, and to make matters even more complicated, the magnetic fields change dramatically throughout the plasma.
Rice University physicists (from left) Grant Gorman, Tom Killian and MacKenzie Warrens have discovered how to capture the world’s coldest plasma in a magnetic bottle, a technological achievement that could research clean energy, space weather and solar physics. promote. Credit: Photo by Jeff Fitlow / Rice University
“So we have to deal not only with a magnetic field, but also a magnetic field that varies in space in a fairly complicated way to understand the data and figure out what’s going on in the plasma,” Killian said. said. “We’ve been trying for a year just to find out what we see once we get the data.”
The plasma behavior in the experiments is also made more complicated by the magnetic field. That is why the capture technique can be so useful.
“There’s a lot of complexity as our plasma expands over these field lines and the forces start to feel and get trapped,” Killian said. “It’s a very common phenomenon, but it’s very complicated and something we really need to understand.”
One example from nature is the solar wind, streams of high-energy plasma from the sun causing the aurora borealis, or northern lights. When plasma from the solar wind hits the earth, it varies with the magnetic field of our planet, and the details of these interactions are still unclear. Another example is fusion energy research, where physicists and engineers want to recreate the conditions in the sun to create a large amount of clean energy.
Rice University plasma physicist Stephen Bradshaw studies solar flares, heating in the sun’s atmosphere, solar wind and other phenomena in solar physics. Credit: Jeff Fitlow / Rice University
According to Killian, the magnetic setup of the quadrilateral he, Gorman and Warrens used to bottle their ultra-cold plasmas is similar to designs developed by fusion energy researchers in the 1960s. The plasma for fusion must be about 150 million degrees Celsius, and containing it magnetically is a challenge, Bradshaw said, in part because of unanswered questions about how the plasma and magnetic fields interact and affect each other.
“One of the biggest problems is keeping the magnetic field stable long enough to contain the reaction,” Bradshaw said. ‘Once there is a small kind of disturbance in the magnetic field, it grows and’ pfft ‘, the nuclear reaction is destroyed.
“To work well, you have to keep things really, really stable,” he said. “And there again, when we look at things in a very beautiful, pristine laboratory plasma, it can help us better understand how particles interact with the field.”
Reference: “Magnetic Confinement of an Ultracold Neutral Plasma” by GM Gorman, MK Warrens, SJ Bradshaw and TC Killian, 25 February 2021, Physical overview letters.
DOI: 10.1103 / PhysRevLett.126.085002
The research was supported by the Air Force Office of Scientific Research and the National Science Foundation Graduate Research Fellowship Program.