Active and fast particle driven Alfvén eigenmodes in Alcator C-Mod J A Snipes, a) N Basse, C Boswell, E Edlund, A Fasoli, b) N N Gorelenkov,

We consider propagation of coherent light through a nonlinear periodic optical structure consisting of two alternating layers with different linear and nonlinear refractive indices. A coupled-mode system is derived from the Maxwell equations and analyzed for the stationary-transmission regimes and linear time-dependent dynamics. We find the domain for existence of true all-optical limiting when the input-output transmission characteristic is monotonic and clamped below a limiting value for output intensity. True all-optical limiting can be managed by compensating the Kerr nonlinearities in the alternating layers, when the net-average nonlinearity is much smaller than the nonlinearity variance. The periodic optical structures can be used as uniform switches between lower-transmissive and higher-transmissive states if the structures are sufficiently long and out-of-phase, i.e., when the linear grating compensates the nonlinearity variations at each optical layer. We prove analytically that true all-optical limiting for zero net-average nonlinearity is asymptotically stable in time-dependent dynamics. We also show that weakly unbalanced out-of-phase gratings with small net-average nonlinearity exhibit local multistability, whereas strongly unbalanced gratings with large netaverage nonlinearity display global multistability.

The stability properties of Alfvén Eigenmodes (AEs) are investigated directly using external antenna excitation and detection of stable modes in a variety of plasma configurations in different devices. Dedicated methods to measure the AE damping rate separately from the fast ion drive have been pioneered at JET, using low toroidal mode number internal saddle coil antennas. Other experiments have since installed localised in-vessel antennas to drive and detect MHD modes in the Alfvén frequency range, first on C-Mod, then on MAST.Experiments on C-Mod proved for the first time that intermediate-n TAEs can be driven and detected, and point out significant differences with respect to the low-n regime on JET in the values and scaling of the damping rate with plasma parameters, e.g. the edge shape. On JET, a new antenna system, comprising two assemblies of four toroidally spaced coils each, was developed to replace the low-n saddle coil structure and excite AE modes in the toroidal mode number range that is expected to be most unstable in ITER, with n up to ~10. Experiments with the new JET antennas confirm that excitation is possible in a large volume plasma, together with real time tracking of core modes throughout the limiter and divertor phases of high performance discharges, with significant additional heating. The similarities and differences between the active MHD antenna systems, as well as a comparison of the results on C-Mod, JET and MAST are illustrated. Both C-Mod and JET results underline the fact that a precise reconstruction of the mode structure and its spectrum, important for a quantitative comparison with theoretical models, represents a significant challenge in the intermediate-n range and in the presence of several modes.