Magnetic helicity is conserved at a tokamak sawtooth crash

Sawtooth instabilities cause sudden changes in the magnetic topology during combined neutral beam and fast wave heating in the DIII-D tokamak. Measurements with a motional Stark effect diagnostic provide accurate determination of the equilibria before and after the sawtooth reconnection events. The global magnetic helicity ∫ A·B dV changes 0.2±0.9% at a sawtooth crash. The local change in the helical flux, χ, is roughly consistent with the Kadomtsev model within large errors. The volume in which the helical flux changes is 85±15% of the volume predicted by Kadomtsev, while the central value of χ is within 1% of the predicted value. (author). Letter-to-the-editor