The physics of m=0 modes and the RFP sawtooth crash

In the RFP, poloidal mode number m=0 fluctuations are stable but important for nonlinear coupling of m=1 modes. We study the energy flow in/out of different modes and the role of resistivity and viscosity in the rise and fall of m=0 amplitude at a sawtooth crash. Nonlinear, visco-resistive MHD simulations using DEBS show that resistivity, viscosity, and their radial profiles all play a role. For example, increased resistivity near the m=0 resonance in the edge reduces crash duration and the m=0 rise and fall times. Energy flow into m=0 from other modes is initially positive but reverses sign early in the crash. The mean current profile becomes the energy source for the explosive increase even though the modes are linearly stable. The drop in m=0 amplitude at the end of the crash occurs because energy input from the mean current profile fades while energy transfer to other modes persists. Analysis of MST experiments yields m=0 rise times consistent with the code results but the decay is generally faster in experiment and more weakly dependent on Lundquist number. Temporal variation of the dissipation profiles in the experiment may partially explain the differences in m=0 fall time. This work was supported by the U.S.D.O.E.