Overview of plasma emissions observed in the ion cyclotron frequency range on ASDEX upgrade

In this work we present an overview of ion cyclotron emission (ICE) signals observed on ASDEX Upgrade (AUG) with a B-dot probe array [1]. The array was recently augmented with an 8 channel digitizer (up to 250 MHz sampling rate) that makes it possible to measure a phase from up to 8 (out of 12) probes. This diagnostic upgrade allows to measure the field-aligned (∼toroidal) propagation direction and the mode number for ICE. The high-frequency Alfvén eigenmode is a commonly observed instability in NBI-heated plasmas on AUG. The frequency of this mode is typically around half the on-axis cyclotron frequency of the driving fast ion species. The mode characteristics measured with the improved ICE diagnostic are consistent with the shear-polarized (or global) Alfvén eigenmode: the mode propagates toroidally in the counter-current, counter-injection direction with a high (>20) mode number. Core ICE is also commonly observed in NBI-heated discharges on AUG. These signal spectra match the on-axis cyclotron frequency (and higher harmonics) of the driving fast ion species and the excitation mechanism is consistent with the magnetoacoustic cyclotron instability (MCI). The recent toroidal phase measurements show that we are dealing with a nearly perpendicularly (to the background magnetic field) propagating wave, in line with the MCI picture.