The world’s first quasi‐helically symmetric stellarator, built at the University of Wisconsin–Madison to prove that neoclassical transport in stellarators could rival tokamak performance.
The Helically Symmetric eXperiment (HSX) holds a singular place in fusion history: it was the first stellarator ever constructed with quasi-helical symmetry, a magnetic-field geometry predicted by theory but never tested in hardware until HSX produced its first plasma in 1999. Located at the University of Wisconsin–Madison, the device was conceived by a team led by physicist David Anderson and engineer John Canik to answer a question that had dogged the stellarator concept since Lyman Spitzer’s original Model A—could a stellarator be designed whose particle orbits behave as well as those inside a tokamak?
Conventional stellarators suffer from poor confinement of trapped particles. Because their magnetic-field strength varies in complicated ways along a field line, a significant fraction of ions follow so-called “superbanana” orbits that carry them rapidly out of the plasma. In the 1980s, Allen Boozer and Juergen Nuehrenberg independently showed that if the field magnitude—not the coil geometry—possessed a hidden symmetry, trapped-particle losses could be dramatically reduced. Quasi-helical symmetry is one such solution: the field strength repeats with a helical pattern even though the physical coils look nothing like a simple helix.1
HSX is a four-field-period device with a major radius of 1.2 metres and an average minor radius of roughly 0.15 metres. Its magnetic field, typically around 0.5 T on axis, is generated by 48 non-planar modular coils—each wound to a unique, computer-optimised shape. Building those coils in the mid-1990s pushed the limits of what university-scale fabrication could achieve; every coil had to be wound within a tolerance of about one millimetre to preserve the delicate symmetry.2
The vacuum vessel, a thin-walled stainless-steel structure, threads through the coil set in a helically shaped torus. Plasma is heated by a 100 kW electron-cyclotron resonance heating (ECRH) system at 28 GHz, later upgraded with additional microwave power to extend the operating range.
The defining experiment came in the early 2000s when the HSX team compared discharges in the quasi-symmetric configuration with discharges in a deliberately “spoiled” configuration, achieved by energising a set of auxiliary trim coils. The spoiled configuration broke the quasi-symmetry and raised the effective helical ripple by an order of magnitude. Electron thermal conductivity in the spoiled case rose to roughly three times the quasi-symmetric value, exactly as neoclassical theory predicted.3
Later campaigns explored plasma flows, demonstrating that quasi-symmetry produces intrinsic rotation in the plasma—a feature normally associated only with tokamaks. This finding has implications for turbulence suppression and for the design of future optimised stellarators.4
HSX provided the first experimental proof that quasi-symmetry works. Its results fed directly into the optimisation codes used for Wendelstein 7-X and influenced conceptual designs for compact stellarator power plants. Though modest in size and temperature, HSX changed the intellectual landscape of the stellarator programme by showing that clever magnetic-field design could overcome the stellarator’s historical disadvantage in particle confinement.