Measurement of bursting behavior at sub-ion cyclotron frequencies in DIII-D using Radial Interferometer Polarimeter (RIP) diagnostic*

The study of Global/Compressional Alfv´en Eigenmodes (GAEs/CAEs) is relevant for future reactors which would have a large concentration of highly energetic particles. In present day tokamaks, these modes are driven by the energetic particles from Neutral Beam Injection (NBI). The magnetic field of these modes displays sharp periodic change in its amplitude (or bursts), the mechanism of which remains unclear. Recent upgrade in the Radial Interferometer Polarimeter (RIP) diagnostic at DIII-D enables the simultaneous measurement of density and radial magnetic field at frequencies close to the ion cyclotron frequency (f_ci). Fluctuations with frequencies 0.25 < f/f_ci < 0.65, typically associated with GAEs/CAEs, are observed to display bursts in time signals, resulting in their Probability Density Functions (PDFs) deviating from a Gaussian distribution to one with exponential tails. The characteristic density  amplitude of these bursts (n_e^c) normalized by the equilibrium plasma density (n_e) is nce/ne ∼ O(10⁻⁵ − 10⁻⁴). We find the timescales of growth (τ_g ), decay (τ_d) and waiting time (τ_w) between bursts respectively to be τ_g ∼ τ_d ∼ O(0.1) ms and τ_w ∼ O(1) ms. These observations are found to hold true for both positive and negative triangularity configurations of plasma.