Strongly Nonlinear Energetic Particle Dynamics in ASDEX-Upgrade Scenarios with Core Impurity Accumulation

Full text: In 2017 a new scenario on ASDEX-Upgrade for the dedicated investigation of energetic particle (EP) physics has been developed. This scenario is unique in two aspects. firstly, the neutral beam (NB) induced fast-ion β is comparable to the background plasma , and secondly, the ratio of the fast ion energy to the thermal background is of the order 100. At ASDEX-Upgrade we reach this previously unexplored regime by NB off-axis heating only and by letting impurities accumulate in the core. Due to strong radiation losses the background temperatures and pressures of both ions and electrons stay low, despite 2.5–5 MW NB heating. In the stable flat-top phase an unprecedented number of various EP driven instabilities (despite v_EP/v_Alfvén ≈ 0.4 ≪ 1) is simultaneously observed. EP-driven geodesic acoustic modes (EGAMs), -induced Alfvén eigenmodes (BAEs), reversed shear Alfvén eigenmodes (RSAEs) and toroidal Alfvén eigenmodes (TAEs), that are modulated by transient q = 2 sawtooth-like crashes, NTMs and ELMs. The physics reasons for these strong mode activity are discussed. During the stable flat-top phase meaningful EP distribution function measurements (FIDA) and analysis (TRANSP/FIDASIM) can be performed. First results indicate that the EP profiles differ significantly from neoclassical predictions. Bicoherence analysis using an advanced tool set for nonstationary processes reveals that nonlinear coupling processes between different frequency bands exist. In addition, TAE bursts are observed to trigger the onset of EGAMs which indicates coupling of these modes via the velocity space (EP avalanches). Linear and nonlinear tools (HAGIS/LIGKA, ORB5, XHMGC) are used for modelling mode onset and nonlinear phases. These experiments facilitate the experimental study of the interaction of AEs, zonal modes and turbulence and thus serve as an ideal validation opportunity for various nonlinear analytical and numerical models. In addition, the observed onset of EP avalanches can be quantified. The investigation and understanding of these-so far not accessible-physics elements is a prerequisite for a reliable prediction of the self-organization of a burning plasma. (author)