Alfven eigenmodes in shear reversed plasmas

Plasma configurations with shear reversal are known to be propitious for creating internal transport barriers in tokamaks. Experiments on JT-60 and JET have shown that these configurations are also prone to excitation of unusual Alfven eigenmodes by energetic particles. Surprisingly, these modes emerge significantly below the TAE frequency gap, where one would expect them to be strongly damped. The modes often appear in bunches and they exhibit a quasi-periodic pattern of predominantly upward frequency sweeping (Alfven Cascades) as the safety factor q changes in time. This work presents a theory that explains the key features of the observed unusual modes including their connection to TAE's as well as the modifications of TAE's themselves near the shear reversal point. The developed theory has been incorporated into a reduced numerical model and verified with full geometry codes. JET experimental data on Alfven spectroscopy have been simulated to infer the mode numbers and the evolution of qmin in the discharge. Work is currently underway on extending the numerical model to evaluate the mode growth rates for realistic energetic particle distributions. (author)