Fusion experiment back on track
After nearly two years of planning and upgrades, the Pegasus Plasma Experiment is back online and better equipped to address the challenges facing low-aspect-ratio toroidal fusion experiments.
A fire in a power diode sidelined Pegasus in late-spring 2002, damaging the lab's superstructure and rendering the experiment's capacitor banks unusable. Cleanup and reconstruction began in summer 2002. "We took a very long look at it and with the help of the college and the state insurance company, we came up with a solution that is far, far better," says Professor and Pegasus Director Ray Fonck.
What once was a crowded laboratory now is airy and organized. A major part of the solution involves the experiment's power supplies. Gutting the lab, the group moved the old capacitors out-for good-and replaced inactive high-voltage transformers in an adjacent exterior underground vault with new nonflammable power supplies.
In the resulting lab space, an enclosed room now safely shields all of the cabling and control equipment from electrical noise and interference from the high power of the experiment itself.
With help from University of Washington fusion scientists (some of whom are UW-Madison alumni), the group installed advanced switching technologies on the lab's mezzanine. Those technologies will enable Pegasus researchers to break new ground in university fusion research. "We're making very flexible, high-energy, high-power energy supplies that give us what we call waveform programmability," says Fonck. "That allows us to control in real time what the currents and the magnets are doing."
Because of the infrastructure needed, it's a capability typically found in the national laboratories and Fonck says he hopes Pegasus will be a prototype for a new generation of university experiments. "The big difference between the small experiments and the national labs has always been how much control do you have over the magnetic structure and how much variability do you have," he says. "And these start to give us the tools to do that."
In addition to the new switches and power supplies, his group added a new interlocking magnet to the high-magnetic-field magnet in the experiment's center. The change is key to conquering plasma instabilities that previously held the researchers back. It also gives Pegasus a unique capability: "We can, in a few milliseconds, change our magnetic-field strength dramatically," says Fonck. "It allows us to sit at a very high magnetic field and make a high-quality plasma and then slam that field down. That gets us into the special high-pressure regime."
After the fire, Fonck had the unenviable task of breaking the news to several top-notch graduate students who had just joined the program. To his delight, they chose to attend UW-Madison anyway and with the lab's technical staff, have shouldered much of the reconstruction effort. "The really positive side is that the students are experiencing everything from the ground up," he says. "It's something that most people in large physical science laboratories don't get."
During the reconstruction, the group's $2.7 million Department of Energy grant remained intact. Now, in its place as the third-largest experiment of its kind in the world, Pegasus still will adhere to its mission, which is to test basic theoretical ideas about magnetic geometry and the ability to confine plasmas at very high pressure, says Fonck.
But the door of possibility is wide open. "From our point of view, because we've changed the magnetic structure and the power supplies are completely changed and we have a new generation of graduate students, this is a new program for us," he says.
The group will "catch up" within the context of the world fusion community and then embark on a new research direction by the end of 2004. "We have to stabilize these instabilities that were holding us back, we have to go beyond them and we have to get to very high-pressure plasmas in a regime where no one else has been," says Fonck. "We think we can do it with these tools."