Magnetic Mirror Cell
A long bottle whose only corks are made of magnetic field, holding a hundred-million-degree plasma between two invisible plugs.
Confines a fusion-relevant plasma in a straight solenoid by creating high-field zones at both ends. Ions and electrons spiral along field lines and reflect off the field gradient at each mirror via the conserved magnetic moment. Avoiding tokamak-style toroidal complexity opens the door to modular, compact reactors with simpler access for blankets and direct energy conversion.
Plasma is created in a central solenoid at moderate field B₀. At each end, mirror coils crank the field up to B_m, giving a mirror ratio R = B_m/B₀ that sets the loss-cone half-angle θ_c = arcsin(1/√R). Particles with pitch angles greater than θ_c bounce back; those inside the loss cone escape axially. Classical mirrors were plagued by this loss cone and by the MHD interchange (flute) instability. Modern revivals fix both: the Gas Dynamic Trap (GDT) at Budker Institute runs at high enough density that gas-dynamic flow partly fills the loss cone; WHAM at UW–Madison uses 17 T high-temperature-superconductor (REBCO) mirror coils together with edge-biased sheared flow to suppress interchange modes; and neutral-beam injection creates fast-ion populations that further stabilize the plasma.
In plain terms
Picture a long tube with a very tight squeeze at each end. If you roll a marble fast enough toward one end, the narrowing pushes it back to the middle — the marble never escapes, just bounces. A magnetic mirror does the same thing to charged particles: a stronger field at each end pushes them back toward the center. Most particles bounce forever; only those aimed nearly straight at the squeeze can sneak through, a known leak engineers work hard to plug.
Also called: open magnetic confinement, magnetic bottle, tandem mirror.
Why it matters
- Linear geometry allows modular reactor designs with direct end-cell access for tritium breeding, particle exhaust, and direct energy conversion.
- GDT-type machines are leading candidates for compact 14 MeV neutron sources for fusion-materials testing.
- WHAM's 17 T REBCO mirror coils set a record for the magnetic field around a confined plasma and inform Commonwealth Fusion Systems' tokamak magnets.
- The concept enables a class of reactor that does not require the giant superconducting toroidal magnet of a tokamak.
Where it lives
- GDT, Budker Institute of Nuclear Physics, Novosibirsk — operating mirror; B_center ~0.3 T, B_mirror ~10 T
- WHAM, Physics Sciences Lab, Stoughton WI — 17 T HTS mirror; first plasma July 2024 (UW-Madison / MIT / CFS / Realta Fusion)
- 2XIIB, LLNL (1971–1980s) — historical milestone, 100 MK ion temperature in 1975
- MFTF-B, LLNL (1986, never operated) — largest mirror machine ever built
Key numbers
- GDT mirror ratio R: ~30 (B_mirror 10 T / B_center 0.3 T)
- GDT electron temperature: ~900 eV–1 keV (Achieved 2013–2016)
- 2XIIB ion temperature (1975): ~100 million K (8.6 keV) (First fusion-relevant ion temperature)
- WHAM mirror coil peak field: 17 T (REBCO HTS, world record at a confined plasma (2024))
- WHAM ARPA-E grant: $10M (2020) (UW-Madison / MIT / CFS partners)
Surprising facts
- In 1975 the 2XIIB mirror at LLNL produced the world's first plasma above 100 million kelvin — hotter than the core of the Sun — using just 12 fridge-sized neutral-beam injectors.
- The Mirror Fusion Test Facility (MFTF-B), the largest mirror machine ever built at $372 million in 1986 dollars, was mothballed on the day it was dedicated and never operated.
- WHAM's 17 T REBCO mirror coils generate a field roughly 340,000 times stronger than Earth's magnetic field, in a magnet small enough to fit on a university lab floor.
- The Gas Dynamic Trap has demonstrated plasma beta values around 0.6 — about ten times higher than a tokamak — by letting fast NBI ions stabilize what was once thought to be a fatal instability.
Open questions
- High-beta loss-cone stability: GDT data show that fast-ion populations from neutral-beam injection suppress the classical drift-cyclotron loss-cone instability far better than early theory predicted, enabling plasma beta near 0.6. The detailed nonlinear saturation mechanism is still under active theoretical debate.
A short history
Richard Post at LLNL invented the magnetic-mirror concept in the 1950s. A series of LLNL machines (2X, 2XII, 2XIIB) reached fusion-relevant temperatures by 1975. The $372M MFTF-B was completed in 1986 and cancelled on dedication day. Russian physicists Dimov and Budker at Novosibirsk developed the Gas Dynamic Trap concept in the 1980s; GDT began operation in 1986 and is still active. The 2020s saw a renaissance: ARPA-E funded WHAM in 2020, its 17 T HTS coils were installed in July 2024, and Realta Fusion is now commercializing the concept.
Sources
- WHAM project page (UW-Madison) (https://wham.physics.wisc.edu)
- Wisconsin Plasma Physics Lab — WHAM HTS axisymmetric mirror (https://wippl.wisc.edu/wisconsin-hts-axisymmetric-mirror/)
- OSTI — WHAM physics basis paper (https://www.osti.gov/biblio/2001162)
- Science First Hand — GDT description (Budker Institute) (https://scfh.ru/en/papers/trap-f-o-r-fusion-/)
- LLNL archive — Mirror Fusion Test Facility history (https://www.llnl.gov/archives/1980s/mirror-fusion-test-facility)