Understanding how a Pegasus plasma forms
Because the UW-Madison experiment Pegasus is among the world’s most compact tokamaks, the device plays a unique role in global efforts to develop fusion as a viable energy source.
In particular, Pegasus—a very-low-aspect-ratio tokamak that has a small center hole and nearly spherical shape—serves as a testbed for plasma startup techniques that ultimately could scale to larger tokamak devices.
For example, led by Engineering Physics Professor Ray Fonck, Pegasus researchers demonstrated a technique that enables them to start the experiment and create a stable plasma by injecting current through a small plasma torch as an alternative to traditional induction-based startup.
Now, Engineering Physics Professor Carl Sovinec and student John O’Bryan are solving theoretical models that help them understand how the plasma grows, from its start as a simple, localized current across electrodes through its evolution into a plasma that fills the entire tokamak.
Starting and sustaining a plasma in a tokamak isn’t as simple as flipping the “on” switch to power up the device. Rather, the process entails physical effects that span spatial and temporal scales and involve a phenomenon called magnetohydrodynamic activity—essentially, how electrical and magnetic fields interact to move, or “drive,” the plasma.
Sovinec is among the developers of the NIMROD code for solving magnetohydrodynamic equations. With support from the U.S. Department of Energy, he and O’Bryan used the code to
conduct numerical simulations of the Pegasus startup technique on computers both at UW-Madison and at Lawrence Berkeley National Laboratory.
Their simulations reproduced the initial behavior in which the current in Pegasus follows the vacuum magnetic field set by the experiment’s external magnetic coils. It snakes up a helical path—somewhat like a loosely coiled spring. The simulations also show how those passes interact with each other as the current increases, the plasma doubles back around, and fills itself in.
The research, says Sovinec, will help Pegasus scientists implement the technology on a larger scale—the National Spherical Torus Experiment tokamak at Princeton Plasma Physics Laboratory. “The more we learn through simulation, the more we can understand and optimize noninductive startup and make it work well at Princeton,” he says.