Density and magnetic fluctuations at JET: experimental observation and numerical characterization

Density and magnetic fluctuations have been experimentally observed on JET in the inter ELM phases in low beta discharges.They have been characterized in terms of typical frequency range (60-80 kHz), wavenumber (0.01 $\le $ky $\rho $i$\le $0.1), radial localization (pedestal top) and correlation with the relevant kinetic quantities [1].A linear simulation with gyrokinetic code GENE [2], matching the experimental edge condition has been performed to gain insight on their possible physical interpretation. ITG modes turn out to be the most unstable modes for 0$\le $ky $\rho $i$\le $1, while microtearing modes (MTMs) are the dominant instabilities for ky $\rho $i $\le $ 0.1.A typical oscillation frequency of about 50-100 kHz is associated to both unstable modes, with opposite propagation direction.Different considerations suggest an interpretation in terms of MTMs for the observed magnetic fluctuations, while density fluctuations appear to be dominated by ITG instabilities.[1]G. De Masi et al., 57th DPP APS Conference, 2015, Savannah (GA) [2]http://www.genecode.org