SUDDEN FLOW CHANGES NOT RELATED TO FIELD ERRORS
A.K. Hansen, J.T. Chapman, D.J. Den Hartog, C.C. Hegna, S.C. Prager, J.S.
Sarff--University of Wisconsin--Madison, E. Martines--Istituto Gas Ionizzati,
University of Padua
It has heretofore been assumed1 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. We have observed that the mode deceleration
occurs earlier for sawteeth in which permanent locking occurs than those
where there is reacceleration; also, the core mode amplitudes increase
earlier in the sawtooth cycle which immediately precedes locking.
1Den Hartog et. al., Phys. Plasmas 2 (6) 2281.
This work was supported by the U.S. Department of Energy.