The Helically Symmetric Experiment (HSX) at the University of Wisconsin-Madison is a unique modular coil stellarator that is optimized for quasi-helical symmetry. The device is designed to investigate plasma transport, turbulence, and confinement in a quasi-helically symmetric magnetic field, with the aim of advancing fusion reactor technology. The HSX began operation in 1999 and has since made significant contributions to the physics of quasisymmetric stellarators.
The HSX uses a set of 48 twisted coils arranged in four field periods to generate a magnetic field for plasma containment. The vacuum vessel is made of stainless steel and is helically shaped to follow the magnetic geometry. Plasma formation and heating are achieved using 28 GHz, 100 kW electron cyclotron resonance heating (ECRH). The device also features a second 100 kW gyrotron for heat pulse modulation studies.
Experiments at HSX have shown that edge magnetic islands can affect particle fueling and exhaust. The presence of a magnetic island chain at the plasma edge can increase the plasma sourcing to exhaust ratio but reduces fueling efficiency by 25%. Moving the island radially inward decreases both the effective and global particle confinement times, which can effectively control plasma fueling and helium exhaust times. These findings suggest that the magnetic island chain in the plasma edge can be a crucial element in the design of a divertor system.
HSX has also shown significant improvements over conventional stellarator designs. The device has measured large ion flows in the direction of quasisymmetry, reduced flow damping, reduced passing particle deviation from a flux surface, reduced direct loss orbits, reduced neoclassical transport, and reduced equilibrium parallel currents due to the high effective transform.
Ongoing experiments at HSX include investigations into plasma flows, impurity transport, radio frequency heating, supersonic plasma fueling and the neutral population, heat pulse propagation experiments to study thermal transport, and more. These experiments are being conducted by students, staff, and faculties, often in collaboration with other universities and national laboratories in the USA and abroad.