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Thursday, May 26, 2022

UW works to understand cosmos

The University of Wisconsin-Madison is home to the Plasma dynamo experiment led by physics professor Cary Forest and his colleagues. This experiment has been designed to study plasma physics, which is the study of plasma state of matter. Plasma is a matter different from solids, liquids and normal gases in that it is a highly ionized gas composed of ions, electrons and neutral particles.

“The number of applications of plasmas initially was the most exciting thing," Forest said, adding, "The idea that we could solve the energy problem with plasma physics and nuclear fusion was a very compelling idea…Now, I am most interested in questions that are inspired by astronomy and astrophysics.”

The team involved in this experiment spent over four years constructing a vessel, an aluminum sphere with a three meter diameter that could imitate plasmas similar to those of a sun or star in space. This year, they began running experiments and taking measurements. Currently, there are two devices that conduct plasma—one generates plasma that reaches over 100,000 degrees Celsius and the other, specifically for the fusion experiment, generates plasma over 10,000,000 degrees Celsius.

Fusion is defined as a nuclear reaction where large nuclei are created from the combination of several nuclei while energy is released. In the fusion experiment, the idea of burning hydrogen into helium where the plasma state of matter is required is being studied.

"The last decade of space telescopes has brought… these incredible images of what’s happening out there far away and every image has a great plasma physics question embedded in it and we can look at those and look at our field as a field that describes what is going on. Even if we can’t really test it there, we can do things here on earth that maybe have some impact on understanding the cosmos.

What makes the study of plasma difficult is if plasma touches the walls of its container, it cools off and loses its properties. The most common question of study in plasma physics is how magnetic fields keep plasmas away from walls so the plasma can remain extremely hot. Large electromagnets can be used to keep plasma in a given space but that disrupts the actually property of plasmas where in nature they are not strongly magnetized. Another property of plasma is that it is not a rigid structure, but one that is turbulent and tends to flow like a river. Thus, while there are magnetic fields, their magnitude is smaller and weaker allowing for this constant flow. Incidentally, the vessel that reaches over 100,000 degrees Celcius works better at keeping the plasma off the wall.

“In the lab, the magnetic fields dominate and hold the plasma still and rigid. In nature, the flow dominates and the flow is the strong component that then carries the magnetic fields around and wraps them up and does things to them.”

The researchers, however, have developed techniques to make flow-dominated plasma rather than magnetically dominated plasmas to study space phenomena, like the sun, that have a weak magnetic field. They placed a strong magnetic field on the boundary while the inside stayed unmagnetized so that the vessel is similar to a magnetic bucket on the surface.

“We are trying to understand where magnetic field comes from… if you have these flowing turbulent plasmas, they can spontaneously make their own magnetic field and we call that the dynamo process. We are trying to do that in lab—make a fast flowing turbulent plasma, stirring it and then watch magnetic fields bubble out of it,” Forest said.

The researchers also created a technique to spin unmagnetized plasma where a cylinder is used to stir the plasma from outside so that is rotates like a rigid body. While doing these experiments, the researchers had made measurements of plasma viscosity, density, and velocity. It is needless to say we can expect more discoveries like this in the future.

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