PPPL physicist Sam Lazerson has teamed with German scientists to confirm the remarkable accuracy of the magnetic fields produced by the Wendelstein 7-X (W7-X) stellarator in Greifswald, Germany. Sellarators are fusion facilities that confine plasma in twisty, 3-D magnetic fields, compared with the symmetrical 2-D fields that more widely used tokamaks create. The W7-X at the Max Planck Institute for Plasma Physics is the most advanced stellarator in the world.

Twisty fields enable stellarators to control plasma without the current that tokamaks must induce in the gas to complete their magnetic fields. Stellarator plasmas thus run little risk of disrupting, as can happen in tokamaks, causing the plasma current to abruptly halt and fusion reactions to shut down.

The magnetic field measurements revealed an error field -- or deviation from the designed configuration -- of less than one part in 100,000 in the W7-X. Such results could become a key step toward verifying the feasibility of stellarators as models for future fusion reactors.

PPPL, the lead U.S. collaborator on W7-X, has designed and delivered five barn door-sized trim coils that fine-tuned the magnetic fields and made the measurement possible. PPPL also conducts research on the facility, which was completed in 2015.

“We’ve confirmed that the magnetic cage that we’ve built works as designed,” said Lazerson, who led roughly half the experiments that validated the configuration of the field. “This reflects U.S. contributions to W7-X and highlights PPPL’s ability to conduct international collaborations.”