It has heretofore been assumed 1 that in the Madison Symmetric Torus RFP the slowing down of core-resonant tearing modes during a sawtooth crash is caused by external field errors.
New evidence suggests other torques are responsible.
In plasmas which have been electrostatically biased to produce reversed toroidal rotation, the rotation speed increases at a crash, i.e. the usual trend is preserved. This is contrary to a torque exerted by a field error, which should always decrease the speed of the modes. Examples of torques possibly responsible for the flow changes during the crash are internal electromagnetic torques between the modes and a fluctuation-driven torque acting on the plasma flow. These torques may also provide an explanation for the observed bifurcation1 between reacceleration and permanent locking of the modes at an individual crash. 1 Den Hartog et.