Dynamics of chaotic magnetic lines. Intermittency and noble ITBs in the tokamap
A purely magnetic approach is used to describe internal transport barriers (ITB's) observed in tokamaks, generally "around" rational values of the safety factor q, and even in ohmic and positive shear situations. A Hamiltonian twist map "tokamap" is used to describe magnetic line section points in a poloidal plane in presence of a magnetic perturbation of strength L, inducing the appearance of magnetic islands and chaotic zones ("incomplete chaos"). For a low value of L less than 1, before breaking of the boundary circle on the plasma edge, a regular zone with robust magnetic surfaces is found in the plasma core, surrounded by two chaotic regions, separated by a transport barrier across which a magnetic line performs an intermittent motion. By using number theory and convergent rational island chains along Fibonacci series, the ITB is identified as composed of two permeable Cantori (broken Kolmogorov-Arnold-Moser surfaces) with "most noble" q values. No paradox however exists with experimental observations identifying the barriers as being "near" rational q values: this is also the case here for the two Cantori, but more precisely, on neighboring most noble q values. Dispersion studies for an ensemble of magnetic lines in the large L regime where the boundary circle has been broken, shows an asymptotic radial sub-diffusion, with different transient regimes. Comparison is performed with the flux diffusion in the standard map.
Bibliographic Reference: An article published in: Physics of Plasma, Vol.8, No.5 (2001), pp.2132-2138
Record Number: 200113849 / Last updated on: 2001-10-05
Original language: en
Available languages: en