Stochastic sawtooth reconnection in ASDEX Upgrade

Abstract

In this paper we investigate non-complete Sawtooth reconnection in ASDEX Upgrade tokamak.Such reconnection phenomena are associated with internal m/n=1/1 kink mode which does not vanish after the crash phase (as would be the case for complete reconnection). It is shown that this sawtooth can not be fully described by pure m/n=1/1 mode and that higher harmonics play an important role during the Sawtooth crash phase. We employ the Hamiltonian formalism and reconstructed perturbations to model incomplete Sawtooth reconnection. It is demonstrated that stochastization appears due to excitation of low-order resonances which are present in the corresponding q-profiles inside the 1 q = surface which reflects the key role of the 0 q value.Depending on this value two completely different situations are possible for one and the same mode perturbations: (i) the resonant surfaces are present in q-profile leading to stochasticity and sawtooth crash ( 0 0.7 0.1 q ≈ ± ); (ii) the resonant surfaces are not present which means no stochasticity in the system and no crash event ( 0 0.9 0.05 q ≈ ± ). Accordingly central safety factor value is always less than unity in case of non-complete sawtooth reconnection. Our investigations show that stochastic model agrees well with experimental observations and can be proposed as a promising candidate for explanation of the sawtooth reconnection.

Diffusion in a stochastic magnetic field in ASDEX Upgrade

“…In the former case stochastization is caused by a simple overlapping of the (3,2) and (4,3) modes, while in the later case stochastization strongly depends on a subtle interplay of the low-order rational surfaces [4] resulting in tooth-like structures in Poincare plots (figures 3 and 4) and corresponding spikes in diffusion coefficient plots (figures 5 and 6).…”

“…The (2,2) and (3,3) harmonics are also present [4] and we scale them with the same law as the primary (1,1) Now using equations (12), (13), (14) and (15), we obtain a time-dependent parameterization for the averaged electron thermal diffusivity. The resulting curves are shown in figure 11.…”

Structure and dynamics of sawteeth crashes in ASDEX Upgrade

“…In the following, we investigate the local behavior of the safety factor for the same case as in Ref. 7 ͑see Fig. 9 in the Appendix͒.…”

“…7. The applied method uses experimental mode perturbations and different safety factor profiles to trace the field lines of the magnetic field.…”

“…For instance, it can explain neither the measured safety factors [3][4][5] nor the existence of the ͑1,1͒ mode after the crash. 6,7 It will be shown later that further doubts about the validity of this model are found in ASDEX Upgrade experiments. As a result, a number of different theories was proposed to explain the crash dynamics and the short crash duration.…”