A compact mirror experiment at the University of Wisconsin–Madison that achieved a record 17 tesla magnetic field on a mirror-confined plasma, reviving interest in the mirror concept as a pathway to simpler, cheaper fusion systems.
The magnetic mirror was one of the earliest confinement concepts pursued in fusion research, but by the 1980s it had fallen out of favour after the Mirror Fusion Test Facility (MFTF-B) at Lawrence Livermore was cancelled and tokamaks came to dominate funding. Four decades later, advances in high-temperature superconducting (HTS) magnets have reopened the question. WHAM, built at the University of Wisconsin–Madison, is the first experiment to exploit HTS technology to push mirror-trap field strengths far beyond what was previously achievable.1
WHAM is a simple axisymmetric mirror—two high-field coils separated by a lower-field central cell. The superconducting mirror coils, wound from rare-earth barium copper oxide (REBCO) tape, generate peak on-axis fields of 17 T at the mirror throats, producing a mirror ratio high enough to dramatically reduce end losses that historically plagued mirror machines.2 The vacuum vessel is compact, roughly one metre in length between mirrors, and the plasma is heated by neutral-beam injection and electron-cyclotron resonance heating. An axisymmetric field geometry was chosen deliberately: unlike the tandem mirrors of the 1980s, WHAM avoids the engineering complexity of non-axisymmetric minimum-B anchors, betting instead that high mirror ratio and kinetic stabilisation can maintain macroscopic stability.
WHAM achieved first plasma in 2024 and rapidly demonstrated stable, high-beta plasma confinement at mirror-ratio conditions well beyond the reach of earlier copper-magnet experiments. The programme confirmed that axisymmetric mirrors stabilised by sloshing-ion distributions can suppress the interchange and loss-cone instabilities that were considered fatal in earlier designs.3 Measured ion temperatures exceeded 1 keV, and confinement times, while modest in absolute terms, scaled favourably with the high mirror ratio, consistent with theoretical predictions for HTS-class mirrors.
WHAM has re-energised a community of mirror-concept advocates and attracted attention from both public funders and private fusion ventures. Realta Fusion, a Madison-based start-up, is developing a commercial mirror reactor concept that draws directly on WHAM physics.4 The experiment also serves as a technology testbed for REBCO magnets under fusion-relevant conditions, contributing operational data that benefits the wider HTS magnet ecosystem shared by tokamak, stellarator, and mirror programmes alike.
By demonstrating that a simple, low-cost university experiment can achieve record fields and stable mirror plasmas, WHAM challenges the assumption that only large, complex toroidal devices can make meaningful progress toward fusion energy.5