In the realm of fusion energy research, a team from the University of Wisconsin-Madison, led by Dr. Chris Schaefer and including researchers Andrew Sontag, Nick Ferraro, Jake Weberski, and Stephanie Diem, has been exploring innovative methods to initiate and sustain tokamak plasmas without the use of solenoids. Their work, published in the journal Physics of Plasmas, focuses on a technique known as local helicity injection (LHI), which could significantly reduce costs and simplify the design of fusion energy systems.
Tokamaks, which are devices used to confine hot plasma with magnetic fields for fusion reactions, typically require large, expensive solenoids to initiate the plasma current. However, solenoid-free startup techniques are crucial for spherical tokamaks and could offer substantial benefits for the energy industry. LHI is one such approach that uses compact edge current sources to drive open field line current, initiating and sustaining the plasma.
The researchers conducted their study on the recently commissioned Pegasus-III spherical tokamak, which provides a platform for advancing solenoid-free startup methods. They investigated the effect of LHI on magnetic topology in Pegasus-III plasmas. By representing the injected current with a helical filament model, they calculated the linear plasma response to its 3D field using the M3D-C1 code.
Their findings revealed substantial flux surface degradation in all modeled cases. The onset of overlapping magnetic structures and large-scale surface deformation began at a normalized poloidal flux of approximately 0.37, indicating a broad region of perturbed topology extending toward the edge. In rotating plasmas, both single-fluid and two-fluid models exhibited partial screening of the n=1 perturbation, with two-fluid calculations showing stronger suppression near the edge. Conversely, the absence of rotation led to strong resonant field amplification in the single-fluid case, while the two-fluid case with zero electron rotation mitigated this amplification and preserved edge screening.
Magnetic probe measurements indicated that modeling the current stream with spatial spreading—representing distributed current and/or oscillatory motion—better reproduced measured magnetic power profiles than a rigid filament model. The results underscore the role of rotation and two-fluid physics in screening stream perturbations and point to plasma flow measurements and refined stream models as key steps toward improving predictive fidelity.
For the energy industry, these findings could pave the way for more efficient and cost-effective fusion energy systems. By understanding and optimizing the LHI process, researchers can potentially reduce the reliance on expensive solenoids and simplify the design of tokamaks, making fusion energy more accessible and viable for large-scale energy production. The research was published in the journal Physics of Plasmas, providing a valuable contribution to the field of fusion energy research.
This article is based on research available at arXiv.