STOCHASTIC PROCESSES IN ASDEX UPGRADE TOKAMAK
O. Dumbrajs and G. Zvejnieks,
V. Igochine, H. Zohm and A. Flaws (MPI fur Plasmaphysik, Garching, Germany),
D. Constantinescu (University of Craiova, Romania).

One of major problems in fusion tokamak is plasma stability. In collaboration with Max Planck Institute, Garching, the diffusion coefficient was determined (by means of the mapping technique) was determined by means of the mapping technique for stochastic field lines arising in fast reconnection phenomena in magnetized fusion plasma during the frequently interrupted regime of neoclassical tearing mode (FIR-NTM), as well as in non-complete Sawtooth reconnection in ASDEX Upgrade tokamak in ASDEX Upgrade tokamak. It was found that the maximum values of the local field line diffusion coefficient are of the order 10^-5 m²/m in the case of FIR-NTM and of the order 10^-4 m²/m in the case of Sawtooth. The maximum values of the local field line diffusion coefficient are of the order 10^-5 to 10^-6 m²/m which correspond to the electron thermal diffusivity of the order of 10² to 10³ m²/s. Solution of the non-stationary diffusion equation with variable diffusion coefficient predicts that the temperature profile during the FIR-NTM event is shifted towards the plasma boundary within 30 ms and during the sawtooth reconnection at 10 ms.

In collaboration with MPI PP, Garching, we investigated 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 was shown that this sawtooth cannot be fully described by pure m/n=1/1 mode and that higher harmonics play an important role during the Sawtooth crash phase. We employed the Hamiltonian formalism and reconstructed perturbations to model incomplete Sawtooth reconnection. It was demonstrated that stochastization appears due to excitation of low- order resonances, which are present in the corresponding q-profiles inside the q=1 surface, which reflects the key role of the q₀ value. Depending on this value, two completely different situations are possible for the same mode perturbation: (i) the resonant surfaces are present in q-profile leading to stochasticity and sawtooth crash (q₀≈0.7±0.1); (ii) the resonant surfaces are not present which means no stochasticity in the system and no crash event (q₀≈0.9±0.05). Accordingly, the central safety factor value is always less than unity in case of non-complete sawtooth reconnection. Our investigations have shown that stochastic model agrees well with experimental observations and can be proposed as a promising candidate for explanation of the sawtooth reconnection.

The role of stochastization of magnetic field lines in fast reconnection phenomena occurring in magnetized fusion plasma is analyzed in collaboration with the MPI (Garching), Helsinki University of Technology (Finland), and University of Craiova, (Romania) . A mapping technique was applied to trace the field lines of toroidally confined plasma where the perturbation parameter is expressed in terms of experimental perturbation amplitudes determined from the ASDEX Upgrade tokamak. It was found that fast reconnection observed during amplitude drops of the neoclassical tearing mode instability in the frequently interrupted regime can be related to stochastization. It was also shown that stochastization can explain the fast loss of confinement during the minor disruption. This demonstrates that stochastization can be regarded as a possible cause for different MHD events in ASDEX Upgrade.

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