Solenoid-Free Plasma Initiation in the Pegasus-III Experiment
Abstract: Reducing the need for induction from a central solenoid during plasma initiation, ramp-up and sustainment of a tokamak plasma is a critical challenge for tokamaks. Solenoid-free plasma initiation techniques such as helicity injection (HI) and microwave injection offer the potential to simplify the cost and complexity of fusion energy systems by reducing the requirements of, or need for, a central solenoid. The Pegasus-III Experiment (A ~ 1.2, Ip < 0.3 MA, BT < 0.6 T) is a new solenoid-free, extremely low aspect ratio spherical tokamak facility focused on studying innovative non-solenoid tokamak plasma initiation techniques. Pegasus-III is equipped with a new, higher-power local helicity injection (LHI) system. Future capabilities will include a coaxial helicity injection (CHI) system and a 28 GHz gyrotron-based system for initial electron Bernstein wave and electron cyclotron heating. The first physics campaign replicated prior LHI scenarios at 0.15 T and extended them to the 0.3 T, Ip > 0.2 MA regime, consistent with expectations from global Taylor relaxation and helicity balance limits. Studies of Te scaling, impurities, and electron Bernstein wave emission during LHI have begun, supported by new and upgraded diagnostics. Experiments at 0.6 T are in progress. Near-term plans include tests of a new noncircular injector that may optimize the Taylor and helicity limits and installation of a dual-electrode CHI system for T-CHI scenarios. Pegasus-III will provide key enabling fusion power plant relevant technology to directly test proposed plasma initiation and ramp-up scenarios envisioned for larger scale devices, investigating methods to synergistically improve the target plasma for consequent bootstrap and NBI current sustainment.
Bio: Dr. Stephanie (Steffi) Diem is an Assistant Professor in the Nuclear Engineering and Engineering Physics Department at the University of Wisconsin-Madison. Diem is the PI of the Pegasus-III Experiment, a large university-scale magnetic confinement fusion energy experiment focused on studying unique and innovative plasma initiation techniques to reduce cost and complexity of future fusion energy systems and has recently started an interdisciplinary team investigating the societal-environmental-economic-technological impacts of fusion. She was selected as the first U.S. Science Envoy for Fusion Energy for the U.S. Department of State and is a member of the all-women 2024 U.S. Science Envoy cohort. Diem is a member of Cohorts 2021 and 2024 of the New Voices in Science, Engineering and Medicine of the National Academies and a recipient of the 2023 Excellence in Fusion Engineering Award from the Fusion Power Associates. Prof. Diem received her Ph.D. in Plasma Physics from Princeton University where she studied microwave emission on the National Spherical Tokamak Experiment at the Princeton Plasma Physics Laboratory. Prior to joining UW-Madison, Prof. Diem was a Research and Development Scientist in the Fusion Energy Division at Oak Ridge National Laboratory and spent two years on long-term assignment at the DIII-D National Fusion Facility at General Atomics in San Diego, CA.
