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A stellarator is a magnetic confinement fusion device which generate all required magnetic fields to confine high temperature plasma by external magnetic coils. Unlike in tokamaks and reversed field pinches, no toroidal plasma current is required in stellarators to confine the plasma. The lack of this large externally driven plasma currents makes stellarators suitable for steady-state fusion power plant. However, due to non-axisymmetric nature, conventional stellarators have a combination of toroidal and helical modulation of the magnetic field on a magnetic field line that leads to high transport of plasma out of the confinement volume at fusion relevant conditions. This large transport in conventional stellarators can limit their performance as a fusion reactor. This problem can be largely reduced by tailoring the magnetic field geometry. The dramatic improvements in computer modeling capability in the last two decades has helped to "optimize" the magnetic geometry to reduce this transport resulting to a new class of stellarators called "quasi-symmetric stellarators". Computer modeled odd looking electromagnets will directly produce the needed magnetic field configuration. These devices combine good confinement properties of tokamaks and steady-state nature of conventional stellarators. The Helically Symmetric Experiment (HSX) at the University of Wisconsin-Madison is a quasi-helically symmetric stellarator (helical axis of symmetry). HSX is the first and only operating quasi-symmetric stellarator in the world and will continue to have this distinction until Wendelstein 7-X stellarator, currently being built in Germany, is operating.

The magnetic field in HSX is generated by a set of 48 twisted coils arranged in four field periods. HSX typically operates at a magnetic field of 1 Tesla at the center of the plasma column. A set of auxiliary coils is used to deliberately break the symmetry to mimic conventional stellarator properties for comparison. HSX vacuum vessel is made os stainless steel, and is helically shaped to follow the magnetic geometry. Plasma formation and heating is done using a 28 GHz, 100 kW electron cyclotron resonance heating (ECRH). A second 100 kW gyrotron has recently been installed on HSX to do heat pulse modulation studies. Plasmas as high as 3 kilo electron Volt in temperature and densities about 8x10^12/cc are routinely formed for various experiments.

HSX has a large set of diagnostics to measure properties of plasma and magnetic fields. The following gives a list of major diagnostics and subsystems.

• Thomson scattering
• Diagnostic neutral beam
• Electron Cyclotron Resonance heating system
• Electron Cyclotron Emission radiometers
• Charge Exchange Recombination Spectroscopy
• Interferometer
• Motional Stark Effect
• Heavy ion beam probe (coming soon)
• Laser blow-off
• Hard and soft-X-ray detectors
• Mirnov coils
• Rogowski coils
• Passive Spectroscopy

HSX has made and continues to make fundamental contributions to the physics of quasisymmetric stellarators that show significant improvement over the conventional stellarator concept. These include:

• Measurement of large ion flows in the direction of quasisymmetry
• Reduction of flow damping in the direction of quasisymmetry
• Reduction of passing particle deviation from a flux surface
• Reduction of direct loss orbits
• Reduction of neoclassical transport
• Reduction of equilibrium parallel currents because of the high effective transform

A large number of experimental and computational research works are being done in HSX by students, staff and faculties. Some of them are in collaboration with other universities and national laboratories, both in the USA and abroad. Major research projects at present are listed below:

• Effect of quasi-symmetry on plasma flows
• Impurity transport
• Radio Frequency heating
• Supersonic plasma fueling and the neutral population
• Heat pulse propagation experiments to study thermal transport
• Interaction of turbulence and flows in HSX and the effects of quasi-symmetry on the determination of the radial electric field
• Equilibrium reconstruction of the plasma density, pressure and current profiles
• Effects of viscosity and symmetry on the determination of the flows and the radial electric field
• Divertor flows, particle edge fluxes
• Effect of radial electric field on the bootstrap current
• Effect of quasi-symmetry on fast ion confinement