Theory and measurement of structure formation and chaotic transport in stochastic magnetic fields
Final Report Abstract
Within the project new tools have been developed to study the effects of resonant magnetic perturbations (RMPs) on physical processes in the edge of a fusion plasma. The tools allowed to study new models for radial electric field generation, plasma rotation, and initial phases of edge localized modes (ELMs). The basic tools used in the project have been systematically improved. Symplectic mapping methods, coupled with integration codes, allowed precise descriptions of footprint structures and separatrix splitting. Compared to the situation at the starting of the project, the reliability of the tools was advanced and checked now at predictions from different experiments leading to a high degree of reliance and remarkable agreements. With the incorporation of ambipolar electric fields the tools gained a new quality. Now, ambipolar particle motion could be covered by the mapping, leading to new interpretations of radial electric field generation and plasma rotation in the presence of RMPs. Based on the still conventional picture of particle transport mainly along the perturbed magnetic field lines, the quite complex behavior of radial electric field structures could be explained on the basis of incomplete chaos with different openings of cantori for electrons and ions, depending on the control parameter (current in the perturbation coils). Finally, a thorough analysis of the found structures could be utilized to test a new conceptual model for the ELMs in the presence of non-axisymmetric magnetic perturbations. The nonlinear dynamics of the ELM instability, prescribed by the evolving separatrix topology, was discussed. The model invokes a feedback amplification mechanism that causes the stable and unstable invariant manifolds of the separatrix, comprising a "homoclinic tangle", to grow explosively as the topology of the separatrix manifolds unfolds. The amplification process is driven by the rapid growth of helical, field-aligned, thermoelectric currents that flow through relatively short edge plasma flux tubes connecting high heat flux wall structures, known as divertor target plates, on both sides of the plasma. These thermoelectric currents produce magnetic fields that couple to the separatrix and modify its 3D (topological) structure. As the lobes of the separatrix tangle grow, their area of intersection with the divertor target plates increases along with the size of the flux tubes connecting target plates on both sides of the plasma. This increases the thermoelectric current flow and completes the feedback loop. Numerical simulations have shown that the model is consistent with measurements of the currents flowing between the target plates and with camera images of the heat flux patterns on the divertor target plates.
Publications
- Ambipolar stochastic particle diffusion and plasma rotation. Phys. Plasmas 15, 052305 (2008)
A. Wingen and K.H. Spatschek
- Footprint structures due to resonant magnetic perturbations in DIII-D. Phys. Plasmas 16, 042504 (2009)
A. Wingen, T.E. Evans, and K.H. Spatschek
- Sheared plasma rotation in partially stochastic magnetic fields. Phys. Rev. Lett. 102, 185002 (2009)
A. Wingen and K.H. Spatschek
- Influence of differrent DED base mode configurations on the radial electric field at the plasma edge of TEXTOR. Nucl. Fusion 50, 034009 (2010)
A. Wingen and K.H. Spatschek
- Numerical modeling of ELM filaments on divertor plates based on thermoelectric currents. Phys. Rev. Lett. 104, 175001 (2010)
A. Wingen, T.E. Evans, C.J. Lasnier, and K.H. Spatschek
- Effect of thermoelectric current splitting on the magnetic topology in DIII-D. Phys. Plasmas, 18, 042501 (2011)
A. Wingen, T.E. Evans and K.H. Spatschek
