Conservation of Magnetic Helicity during Plasma Relaxation

The change in magnetic energy and magnetic helicity has been measured during the sawtooth relaxation in the Madison Symmetric Torus reversed-field pinch. The larger decay of the energy (4.0%--10.5%), relative to helicity decay (1.3%--5.1%), modestly supports the helicity conservation hypothesis in Taylor's relaxation theory. However, the observed helicity change is larger than the simple magnetohydrodynamics prediction. Enhanced fluctuation-induced helicity transport during the relaxation is observed.