Description
The Helically Symmetric eXperiment (HSX) is a mid‑sized stellarator designed to investigate quasi‑symmetry as an approach to stellarator optimization. With its major radius of 1.2 m and magnetic fields up to 1.25 T, HSX routinely achieves electron temperatures above 1 keV, making it a valuable platform for addressing key scientific and technological challenges on the path toward a fusion power plant.
HSX began operation in 2001 and was designed using the same tools and methodologies employed in the development of the W7‑X stellarator. Early research centered on demonstrating its neoclassical optimization, which was confirmed through the observation of strong plasma flows, excellent fast‑particle confinement, and low core heat fluxes. Current studies focus on validating theories and numerical models of neoclassical, turbulent, and MHD‑driven transport, exploring novel divertor configurations, advancing millimeter‑wave heating schemes, and developing next‑generation fusion diagnostics.
The device’s operation flexibility is one of its key strengths. HSX allows rapid installation and testing of new components, and its magnetic geometry can be tailored using a set of 48 planar auxiliary coils. This flexibility has enabled the experimental investigation of quasi-symmetric turbulence‑optimized configurations with reduced transport, identified previously using gyrokinetic simulations. HSX can also be configured to operate in deliberately MHD‑unstable states, resulting in reduced confinement and strong mode activity measurable with magnetic pickup coils and density and temperature fluctuation diagnostics.
Looking ahead to the needs of the public sector and the broader push toward fusion power plant technologies, the HSX program is preparing for island divertor operation through the installation of new first wall components. Moreover, the device is being upgraded with a new 70‑GHz gyrotron to enable increased heating power and higher density operation.
By combining its experimental flexibility with a strong programmatic focus on plasma confinement and technology development, HSX has become a collaborative platform that actively pursues and welcomes partnerships across the public and private sectors to help close key gaps on the path toward a stellarator‑based fusion power plant.