Using a combination of laser cooling techniques and strong magnetic fields, researchers at Colorado State University have for the first time created an ultracold neutral plasma with electrons cooled to temperatures measured to be within one degree Kelvin.
The work, highlighted in Physics of Plasmas , outlines an approach to better validate theory and refine models with this common state of matter under these and other extreme conditions. Among many potential applications, the findings will be helpful for the future development of fusion energy systems and the study of astrophysical systems, such as white dwarf stars. The project was supported by the Air Force Office of Scientific Research.
Plasma is the fourth state of matter and makes up 99% of the visible universe. It forms when a gas is supercharged with energy so that its atoms become ionized, meaning electrons are stripped from their atoms. The result is a mixture of free electrons, positively charged ions and, in many cases, some neutral atoms that move freely and interact with one another like ingredients in a bowl of hot soup. Researchers can create plasma by adding energy to electricity, lasers or heat. Plasma is also naturally occurring in stars where extreme heat and pressure can strip electrons away.
In contrast, the plasmas used in the CSU study were created by first cooling atoms to temperatures just above absolute zero before converting them into plasma. That approach slows the motion of the charged particles down to a speed that makes measuring their response easier than in the hotter versions. This allows researchers to observe interactions in a more controlled setting and make better comparisons with existing theoretical predictions and modeling about what should happen in the plasma.
Ryan Baker is a graduate student at CSU and first author on the paper. He said the most difficult part of the work was how complicated the plasma system gets at low temperatures.
"At the coldest temperatures, we were making 'plasmas' that were a mess of deeply bound atoms, loosely bound atoms, and free electrons that all interacted in unexpected ways. We had to figure out a completely new, simulation driven approach to pulling out the useful information from our experimental data," he said.
Fusion reactors attempt to re-create the plasma conditions found inside stars, which are incredibly hot, complex and volatile. Keeping that kind of plasma stable long enough to generate useful energy remains one of the biggest scientific and engineering challenges on the path to clean and limitless energy. Research from the paper will be useful for the development of better computer models that can predict behavior, improving the design of future fusion systems.
At low temperatures, the electrical forces at play between particles also become more important relative to their motion. That makes the CSU team's plasma especially useful for studying how charged particles respond to the introduction of magnetic fields, which can be used to restrict electron movement – limiting potential coupling reactions.
Professor Jacob Roberts led the work at CSU in the Department of Physics. He said it is not easy to develop the conditions needed to test complex plasma theory, but validating experiments and creating better models are worth pursuing for many reasons.
"Knowing the limitations from how hot or cold plasma is at given points can help us understand what types of experiments are even possible and what we need to investigate further as we try to develop the foundational science needed for things like clean fusion energy," he said. "These insights also help us better understand the dense plasmas believed to exist in white dwarf stars and other extreme environments throughout the universe that we can't easily visit to collect data from."
Roberts said the paper demonstrates how magnetic fields can be used to make plasma colder, depending on the need. It also shows that previously predicted limitations – such as how cold you could conceivably make them – are consistent with their new findings.
"There have been theories for decades about how cold you can conceivably make these plasmas, and our goal is to understand and test the basic rules that govern how they behave," he said. "It is satisfying to have created the coldest measured electrons in a plasma formed in a lab right here at CSU. But more important is what our work shows about how to create a range of extreme environments to continue building our understanding of this key state of matter."
