Interaction of fast particles and Alfvén modes in burning plasmas

In this paper we study the interaction of fast particles with Alfvénic instabilities in Tokamak plasmas, with reference to present-day experiments that exploit strong energetic particle heating (namely, JT-60U [1]) and the consistency of proposed ITER burning plasma scenarios [2]. Concerning JT-60U, two different types of bursting modes have been observed by MHD spectrography in auxiliary heated (NNB) discharges. One of these modes has been dubbed fast frequency sweeping (fast FS) mode. It is characterized by a timescale of the order of few milliseconds and frequencies branching upwards and downwards. The other mode, called the abrupt large-amplitude event (ALE), has shorter timescale (order of hundred microseconds) and larger amplitude. On the occurrence of ALEs, a significant reduction of the neutron emission rate in the central plasma region is observed. Such a change has been attributed to a redistribution of the energetic ions, with a marked reduction of their on-axis density. We present an interpretation of these experimental observations, based on the results of nonlinear particle simulations performed by the Hybrid MHD-Gyrokinetic Code HMGC [3]. Concerning ITER, monotonic-q (scenario 2) and reversed-shear (scenario 4) equilibria are considered. Also an ITER hybrid scenario is examined and quantitatively compared with the previous ones. The transition from the low-amplitude Alfvénic instability saturation to the secondary excitation of a stronger mode is addressed, and its effect on the energetic particle transport analyzed.