Overview of recent plasma experiments in WHAM

The Wisconsin HTS Axisymmetric Mirror (WHAM) is a magnetic plasma confinement experiment developed by UW–Madison and Realta Fusion. One of the experiment’s primary objectives is to demonstrate the classical mirror ion confinement regime, which could be scaled up to a Q ~ 1 fusion power gain factor in a next-generation device. A significant focus of the experimental program will address plasma microstability—long considered a key challenge in realizing the full potential of simple mirror systems. As a cornerstone of Realta’s fusion development roadmap, WHAM is generating critical data to support advanced mirror concepts, such as the axisymmetric tandem mirror reactor.

The report presents the current configuration of the device and describes the progress in the development of key subsystems, including improvements to the vacuum systems as well as experience accumulated with various vacuum conditioning methods implemented for the first time in WHAM. In a series of experiments with high-throughput titanium gettering pumps (~ 40 m³/s), the machine was able to reach regimes with NBI-only plasma sustainment, where a dense (up to 10²⁰ m⁻³) plasma is confined without support from ECH heating. The report discusses various operational regimes and the factors currently limiting confinement, such as MHD activity, impurities, and vacuum conditions.

A brief update is given on the diagnostic development in WHAM, particularly the ion probe array embedded in the plasma-facing electrode structure in the expander vessel. The array is made of miniature (⌀2 cm) gridded ion energy analyzers, which accept the flux of ions accelerated by the ambipolar potential toward the endplate and can deflect ions with energies of up to 3 keV. The diagnostic provides time-resolved radial and energy distributions of the ion flux, which carries information about the plasma potential distribution in the confinement volume and ion temperature. The report presents the first data obtained from the diagnostic in various operational regimes.