Avalanching and mesoscale transport events in DIII-D plasmas

We present experimental observations of non-diffusive transport events in DIII-D plasmas. Avalanche-like events were found in the core plasmas using electron cyclotron emission (ECE) diagnostic in L-mode and hybrid discharges at low line-averaged density (n_e<3e19 m^-3) and higher neutral beam heating power (P_NBI>2 MW). Avalanches were identified as a fast formation of ‘blob-void’ pairs with ‘blobs’ moving radially outward and ‘voids’ moving radially inward [1, 2, 3]. Larger and more frequent avalanches were observed during resonant magnetic perturbations (RMP). Dynamics of long-wavelength (k*ρ_i < 1) density fluctuations as well as the evolution of turbulent velocity field were measured with beam emission spectroscopy (BES) diagnostic during and between the avalanches and will be discussed.

Fusion reactors will most likely operate near marginal stability where profile gradients are limited by the nonlinear threshold boundary for turbulent transport [4]. In this regime larger mesoscale transport events (avalanches) cause higher transport than predicted by gradient-driven models [5]. Understanding turbulence dynamics associated with avalanches is important as it can reveal transport inhibiting mechanisms such as zonal flow layering [3] and help improve predictive models to make robust projections to ITER and FPP.