Speaker: Dr. Benedikt Geiger, University of Wisconsin-Madison
Title: "Exploring Quasi-Helical Symmetry: Recent Results from the HSX stellarator"
Abstract:
The Helically Symmetric eXperiment (HSX) is a mid-sized stellarator designed to investigate quasi-symmetry as a path toward optimized three-dimensional magnetic confinement [1]. With a major radius of 1.2 m and magnetic fields up to 1.25 T, HSX routinely achieves electron temperatures exceeding 2 keV, providing a versatile platform for addressing key scientific and technological challenges on the path to fusion energy.
Since beginning operation in 2001, HSX has played an important role in advancing the understanding of quasi-helically symmetric configurations. Designed using methods similar to those employed for the Wendelstein7-X stellarator, early experiments successfully demonstrated neoclassical optimization through observations of strong symmetry-aligned plasma flows, excellent fast-particle confinement, and reduced core heat transport. Building on this, current research focuses on validating theories and simulations of neoclassical, turbulent, and MHD-driven transport, while also advancing divertor concepts, millimeter-wave heating techniques, and next-generation diagnostic capabilities.
A defining strength of HSX is its exceptional operational flexibility. The device enables rapid integration and testing of new components, while its magnetic configuration can be tailored using 48 planar auxiliary coils. In particular, HSX can access turbulence-optimized configurations that combine a high degree of quasi-symmetry with reduced trapped-electron-mode growth rates [2]. Experimental results investigating improved confinement in these regimes will be presented, together with studies of targeted density ramps that modify the local density gradient and its impact on transport. In addition, configurations operated near MHD stability limits exhibit periodic crash-like behavior while maintaining enhanced confinement, achieving higher stored energies than the standard HSX configuration.
Looking ahead, the HSX program is advancing toward studies of both non-resonant and island divertor operation [3], as well as long-pulse operation at reduced magnetic fields. The device is also being upgraded with a 70 GHz gyrotron to access higher-density, high-performance plasmas, alongside the implementation of neutral beam injection. Together, these developments position HSX to address key physics and technology gaps on the path toward a stellarator-based fusion power plant.
Bio: Benedikt Geiger is an Associate Professor at the University of Wisconsin–Madison, where he leads the Helically Symmetric eXperiment (HSX) and heads the plasma turbulence and spectroscopy research group. He earned his PhD from LMU Munich in 2013, specializing in fast-ion research at the ASDEX Upgrade tokamak, and was appointed a Helmholtz Young Investigator Group Leader in 2015 to study impurity transport on Wendelstein 7-X (W7-X). Since joining UW–Madison in 2019, his experimental research has spanned major magnetic fusion facilities including DIII-D, NSTX-U, and W7-X. Prof Geiger serves as co-chair of the National Stellarator Coordinating Committee (NSCC) and represents the U.S. on the Executive Committee for the Technology Collaboration Program on Stellarators and Heliotrons.
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