Search

In a tokamak plasma, toroidicity induced Alfven eigenmodes (TAEs) are excited by and alter the orbits of resonant fast ions with the character of these modes defined by both thermal and supra-thermal fast ion populations. To explore the stability of TAEs in ASDEX Upgrade, the effects of magnetic shear and density on TAEs, the decay and growth rate of individual TAEs and fast ion drive rates have been studied. In particular, the mode structure of TAEs, the dependence of the ICRF power-density threshold on the toroidal mode number n, the effect of magnetic shear at the plasma edge on low-n TAEs (n [?] 2) and a comparison of measured decay rates of TAEs with predictions made by the LIGKA code are discussed. In addition, fast ion pressure profiles generated by the ICRF power deposition codes PION and TORIC and the equilibrium code CLISTE have been examined to see if they lead to a sufficient fast ion drive rate for the observed TAEs, and to benchmark a formula for volume averaged bfast using measurable quantities in ICRH-only plasmas. [ABSTRACT FROM AUTHOR]

The radial structure of toroidal Alfvén eigenmodes (TAEs) is of great importance for comparison with theoretical predictions. A dual-channel fast frequency hopping millimeter-wave reflectometer installed on the ASDEX Upgrade tokamak is capable of measuring density fluctuations from the plasma edge to core, allowing the radial eigenfunction of n = 4 TAE and edge MHD modes to be obtained using phase perturbation and coherence data analysis techniques. The two techniques reveal similar results, and in particular the radial structure of the n = 4 TAE is found to be in good agreement with numerical predictions from linear gyrokinetic simulations. The first results of the radial localization of Alfvén cascades are also presented. [ABSTRACT FROM AUTHOR]