Plasma is the fourth state of matter, occupying a spot beyond solids, liquids and gases — and Saskia Mordijck says plasma needs more attention.
“Simply put, plasmas are everywhere,” Mordijck said. “Ninety-nine percent of all matter in the universe is a plasma, but we don’t study it as we should.”
Mordijck is an assistant professor in the Department of Physics at William & Mary. The National Science Foundation recently awarded her a CAREER award to continue her investigations into the mysteries of the fourth state of matter.
The nature of plasma can be explained in simple terms. Mordijck related a recent conversation with a sixth-grade teacher who was teaching her class about the states of matter. If you heat a solid enough, it becomes a liquid. Mordijck said. More heat, and the liquid becomes a gas. But, she said, the widely held misconception holds that “everything magically stops” once a gaseous state is reached.
“If you continue to heat a gas, you end up with a plasma,” Mordijck explained. “And plasmas are very different from gases. Funnily enough, they actually behave very much like fluids, but at much higher temperatures and for completely different reasons.”
Plasmas are often described to lay audiences as “ionized gases,” a definition that skirts an important distinction. A principal difference between a gas and a plasma, Mordijck said, is that a plasma is affected by electromagnetism, whereas a true gas is utterly indifferent to a magnetic field.
“I study plasmas because they’re fascinating states of matter that have been undervalued and underrecognized within the broader physics community as something interesting or useful to study,” she said.
Plasmas are interesting and useful to both science and society. Mordijck’s plasma studies in the past have been focused on their applications to the development of fusion energy. The CAREER award will allow her to expand her scope.
“This is really much more fundamental,” she said. “I’m going to look at fully understanding plasma physics and plasmas. There are aspects of what I’m studying that are relevant for fusion reactors, but there’s also aspects that are directly relevant for space plasma.”
Revelations about plasma have been unsettling “settled science” for the last century. Mordijck said that until 100 years ago, people thought the sun was, in essence, “a coal mine gone wrong” — a burning solid. Scientists actually did calculations of how long the sun would last, based on the burning-solid notion, she added.
The concept held not a few theoretical problems, not least of which was “space is fairly empty of oxygen, so burning anything in space is slightly tricky to do,” Mordijck noted. She said the true nature of the solar furnace began to emerge in 1925 when Cecilia Payne, working on her Ph.D. thesis, began a scientific examination of the colors of sunlight.
“And she discovered that the sun was mostly made out of hydrogen and helium — there was very little carbon,” Mordijck said. “Naturally, being a student and being a woman at that time, it took a little while for her esteemed colleagues to catch on, but after a few esteemed, established male colleagues confirmed her observations, this became the working theory.”
Payne’s observations worked well with the developing atomic model, and people realized that two hydrogen atoms were combining, releasing heat in what we now know is a fusion reaction.
“But you have to realize that at that time, they didn’t understand that the product of that fusion reaction was a plasma,” Mordijck said, the temperature of the reaction so extreme as to make the electrons pull away from the hydrogen nucleus.
Mordijck’s research on plasma will be conducted using the LAPD. It’s not the Los Angeles Police Department. “The people naming the instrument have a sense of humor,” she said. “And it is in Los Angeles — at UCLA.”
LAPD is the initials for the LArge Plasma Device. Mordijck describes the LAPD by comparing it to a fluorescent light tube.
“The LAPD is that kind of thing on steroids,” she said, “with magnetic fields around it. So it’s like a really long tube of plasma, nearly 20 meters and a fairly large radius.”
And inside the tube is a plasma. Mordijck said that the magnets encircling the LAPD allow researchers to “squeeze” the plasma.
“The magnets help to guide it,” she said. “The plasma becomes less of a random cloud; it has more directionality.”
Mordijck will use the partially tamed, more directional plasma in the LAPD to investigate a phenomenon in plasma known as drift wave turbulence. It’s very much like what happens to water in a pot on the stove.
“You have something with a hot temperature down near the burner and cold temperature up near the air,” she explained. “What happens in that pot of water is you can typically see a kind of circulation going. Because it’s not at an even temperature, you kind of get movements going around.”
Mordijck said the circulating swirls within a pot of warming water illustrate one of the ways in which plasmas behave much like fluids. Like the water in a heating pot, plasmas are rarely uniform.
“You can have a slightly higher density of plasma — or a slightly higher temperature of plasma —in one location than in another,” she said. “That unevenness allows for the existence of waves and turbulence.”
Plasma waves and turbulence are more complicated than a pan of hot water, though. And Mordijck said drift wave turbulence in plasma is not a new phenomenon. Indeed, drift wave turbulence is well understood as a fundamental theory.
“However, this work is done with what we call very pure plasmas and very specific plasma conditions,” she noted.
Mordijck has already done some studies at the LAPD and will build on those studies of impure plasmas — the kind actually found in nature, making up 99 percent of matter in the universe.
“We will be changing certain parameters that are not included in the current drift wave turbulence theory,” she explained. “We always start from the idea that you’re not dealing with a pure helium plasma or a pure hydrogen plasma.”
But in the real universe, plasmas are virtually never pure. A plasma might include some gas, much as a chunk of ice could contain areas of liquid water.
“You might have a plasma that’s mostly hydrogen, but has a little bit of helium in it as well,” Mordijck said. “That’s what’s in the solar wind; the solar wind contains a varying, but non-negligible, amount of helium — it’s not pure hydrogen.”
Her initial work with the LAPD turned up some results that seemed to be at odds with the current understanding of drift wave turbulence. She is scheduled to return to the LAPD in early 2022 with a set of new ideas to test by adjusting parameters on the device.
Mordijck notes she has done much of the preliminary work with undergraduates, notably Conor Perks, a North Carolina State graduate now at MIT. Thanks to the CAREER funding, she will also be bringing Leo Murphy ’24, a 1693 Scholar and physics/math major to the LAPD. The William & Mary LAPD team will also include a graduate student, to be named.
Joseph McClain, Research Writer