Suprathermal Electrons and Alfvén Eigenmodes in Sawtoothing Tokamak Plasmas

Licentiatavhandling, 2007

In the Joint European Torus (JET) tokamak, monster sawtooth stabilisation by ICRH-accelerated hot ions is lost if plasma density falls below a certain density threshold, or if energetic-ion driven Alfvén eigenmodes inside q = 1 occur. In the first case, a transition away from long period monster sawteeth to a regime with shorter period, chaotic sawteeth (grassy sawteeth) takes place, while in the second case the monster sawtooth crash happens, degrading plasma confinement. The transition away from monster sawteeth to grassy sawteeth is found to correlate with an increase of measured fast electron bremsstrahlung emission, indicating an increased population of suprathermal electrons. A Fokker-Planck theory for relativistic electrons is derived by applying an asymptotic expansion technique. The results of the Fokker-Planck theory for a suprathermal electron tail are used to calculate fast electron bremsstrahlung emission during the transition from ohmic to grassy sawteeth. The difference of measured fast electron bremsstrahlung emission between the two sawtooth regimes is verified, and the density fraction of suprathermal electrons in the grassy sawtooth regime is estimated. Observations of toroidal Alfvén eigenmodes inside q = 1 (tornado modes) preceding monster sawtooth crashes are used in magnetohydrodynamic spectroscopy to obtain information on the equilibrium in the core of the JET tokamak plasma. A suite of equilibrium and MHD codes facilitate modelling of the observed spectrum of Alfv´en eigenmodes. An investigation of bi-directional tornado modes, observed in JET plasmas, includes a study of the fast ion velocity anisotropy effect on tornado mode growth rate as well as numerical modelling in order to determine locations, widths and spectral evolution of the modes.