Plasma Physics Seminar (Phys 769)

Dr. Olgerts Dumbrajs, Euratom/U. of Latvia The role of stochastization in fast MHD phenomena in ASDEX upgrade Studies of fast MHD events in ASDEX Upgrade suggest that stochastization plays an important role in these processes. In spite of the short time duration and the small region, stochastization can lead to significant changes in plasma confinement. The main reason for this is a strong mixing of the magnetic field lines which destroys magnetic surfaces and strongly increases the radial transport. In the present talk two such phenomena will be discussed: the frequently interrupted regime of neoclassical tearing modes [1,2] and the sawtooth crash [3]. The role of stochastization of magnetic field lines is analyzed by applying the mapping technique to trace the field lines of toroidally confined plasma [4]. The proposed stochastic model for the sawtooth crash is able to explain fast losses of heat from the plasma core with simultaneously small changes of the safety factor profile [5]. Here the sawtooth crash does not change the position of the q=1 surface. This observation disagrees with ordinary sawtooth models, but is in agreement with the stochastic picture of the crash. The sawtooth crash dynamics is also studied by means of the spectral analysis and reconstruction of phase trajectories using the delay coordinates to identify the road to chaos in different physical systems [6]. References [1] O. Dumbrajs, V. Igochine, D. Constantinescu, H. Zohm, and ASDEX Upgrade team, Phys. Plasmas 12 (2005) 110704. [2] V. Igochine, O. Dumbrajs, D. Constantinescu, H. Zohm, and G. Zvejnieks, Nuclear Fusion 46 (2006) 741. [3] V. Igochine, O. Dumbrajs, H. Zohm, and A. Flaws, Nuclear Fusion 47 (2007) 23. [4] D. Constantinescu, O. Dumbrajs, V. Igochine, and B. Weyssow, Nucl. Fusion 48 (2008) 024017. [5] O. Dumbrajs, V. Igochine, and H. Zohm, Nucl. Fusion 48 (2008) 024011. [6] V. Igochine, O. Dumbrajs, and H. Zohm, Nucl. Fusion 48 (2008) 062001. Return to main page