Sawtooth heat pulse propagation in tokamaks

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Abstract
Standard analysis treats the sawtooth-induced heat pulse as a diffusive phenomenon by linearizing the diffusion equation around the unperturbed state of the plasma. This implies that the heat pulse is governed by the incremental thermal diffusivity. This approach has been criticized by Frederickson et al. (1990 Phys. Rev. Lett. 65, 2869), who claim that turbulence associated with the sawtooth collapse affects the propagation of the heat pulse. However, this criticism is based on an incorrect application of the standard diffusive models. Furthermore, it is shown that their modelling with a time-dependent diffusivity (the 'ballistic' heat pulse) does not describe the measurements. The authors demonstrate that the measurements impose a temporary change of the effective thermal diffusivity during the heat pulse which is similar to the implicit time behaviour imposed by the linearized treatment.