Stability and Statistical Analysis of Transient Waves in Fusion Plasmas

Licentiatavhandling, 2007

Thermonuclear energy generation in magnetically confined plasmas offers an appealing energy source for coming generations. Some of the still unsolved problems in fusion plasma physics involve plasma transport and stability of plasma waves. Two such problems are addressed in the present thesis. Transient magnetohydrodynamical (MHD) modes were typically detected on the Wendelstein 7-AS stellarator in correlation with transient transport events, called ELM-like modes. The temporal and spatial structure of these transient modes are analysed in the thesis using custom developed methods based on Mirnov-coil and lithium beam emission spectroscopy measurements. The statistical connection between the power modulation of transient MHD modes and the temporal changes of electron transport related quantities is investigated and possible theoretical explanations are proposed. Runaway electrons already pose serious problems in connection with disruptions in present-day tokamak-type fusion devices, and the problems are expected to become even more severe in next generation machines, like ITER. There are still discrepancies between the theoretical understanding and experimental results concerning the generation of runaway electrons, some of which might be explained by an interaction between the runaway electron beam and plasma waves. Linear and quasi-linear evolution of magnetosonic-whistler waves excited by runaway electrons is considered in the thesis, and it is shown that these waves can indeed be destabilized and can scatter resonant electrons from the beam.