July 13, 2020
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When magnetic walls are closing in, wily plasma slips out between magnetic field lines. A Michigan-led team pioneered a way to keep more plasma contained.
A new spin on the magnetic compression of plasmas could improve materials science, nuclear fusion research, X-ray generation and laboratory astrophysics, research led by the University of Michigan suggests. The study shows that a spring-shaped magnetic field reduces the amount of plasma that slips out between the magnetic field lines.
Known as the fourth state of matter, plasma is a gas so hot that electrons rip free of their atoms. Researchers use magnetic compression to study extreme plasma states in which the density is high enough for quantum mechanical effects to become important. Such states occur naturally inside stars and gas giant planets due to compression from gravity.
The research group led by Ryan McBride, an associate professor of nuclear engineering and radiological sciences at U-M, tests ways to achieve states like this by imploding plasma cylinders with magnetic fields. These cylinders have a tendency to break up in a “sausage link” fashion when the magnetic field finds tiny divots in the cylinder’s surface and cuts into them. (The technical term is “sausage instability.”)
“It’s like trying to squeeze a stick of soft butter with your hands,” said McBride. “The butter squishes out between your fingers.”
The butter in McBride’s analogy is plasma and the fingers are magnetic field lines. His group looked for a way to keep the magnetic field from digging into the imperfections in the cylinder, instead causing the field to press more uniformly on the cylinder’s outer surface. They did this by twisting the magnetic field into a helix, that spring-like shape, and varying the angle at which the helix pressed on the plasma cylinder. This made it harder for the magnetic field to slice in—the field moved across many divots rather than pressing into any one divot for too long.
The most twisted magnetic configurations tested in these experiments reduced the length of the escaping plasma tentacles by about 70 percent. The research was done in collaboration with Sandia National Laboratories and the Laboratory of Plasma Studies at Cornell University.
The team changed the shape of the magnetic field by changing the way that the electrical current—over 1 million amperes—ran through the compression device. The electrical current typically runs up through the central cylinder that is to be compressed and then back down through straight “return current” columns that surround the central cylinder. This produces a cylindrical magnetic field that surrounds the central cylinder. To transform the cylindrical field into a helix, the team twisted the return-current columns around the central cylinder. The central cylinder starts out as a metal foil, but the huge electrical current quickly transforms the metal into a plasma. They ran the experiments on the Cornell Beam Research Accelerator (COBRA).
“Designing the return current structures was an interesting balancing act,” said Paul Campbell, first author on the paper and a PhD student in nuclear engineering and radiological sciences at U-M. “We weren’t sure we could even get these structures machined, but fortunately, metal 3D printing has advanced far enough that we were able to get them printed instead.”
Campbell explained that when the structures are more twisted, less current runs through them, so the columns had to be placed closer to the imploding plasma to compensate. At the same time, they needed gaps in the structure so that they could see what was going on with the implosion.
Post-implosion images of the plasma cylinders. On the left, plasma tentacles stretch out from the sides of the conventional, straight-column design. With the 14-tesla and 20-tesla twisted structures in the middle and right, respectively, the plasma tentacles are much shorter. This reflects more uniform compression by the magnetic field. Credit: Paul Campbell; Plasma, Pulsed Power and Microwave Lab; University of Michigan.
A paper on this research is published by the journal Physical Review Letters. The research will also be featured in an invited talk at the annual conference of the American Physical Society’s Division of Plasma Physics in November 2020.
The study was funded by the National Science Foundation and the Department of Energy. The opinions, findings and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the U.S. Department of Energy.
See the full article here .
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The University of Michigan (U-M, UM, UMich, or U of M), frequently referred to simply as Michigan, is a public research university located in Ann Arbor, Michigan, United States. Originally, founded in 1817 in Detroit as the Catholepistemiad, or University of Michigania, 20 years before the Michigan Territory officially became a state, the University of Michigan is the state’s oldest university. The university moved to Ann Arbor in 1837 onto 40 acres (16 ha) of what is now known as Central Campus. Since its establishment in Ann Arbor, the university campus has expanded to include more than 584 major buildings with a combined area of more than 34 million gross square feet (781 acres or 3.16 km²), and has two satellite campuses located in Flint and Dearborn. The University was one of the founding members of the Association of American Universities.
Considered one of the foremost research universities in the United States,[7] the university has very high research activity and its comprehensive graduate program offers doctoral degrees in the humanities, social sciences, and STEM fields (Science, Technology, Engineering and Mathematics) as well as professional degrees in business, medicine, law, pharmacy, nursing, social work and dentistry. Michigan’s body of living alumni (as of 2012) comprises more than 500,000. Besides academic life, Michigan’s athletic teams compete in Division I of the NCAA and are collectively known as the Wolverines. They are members of the Big Ten Conference.
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