Detection of signals in the ion cyclotron frequency range at the edge during ICRF heating on ASDEX Upgrade

The ICRF system on ASDEX Upgrade is equipped with an arc detector based on the observation of frequencies between 5 and 25 MHz in the RF feeding line [RI). On some shots, the detector was triggered in an anomalous way: tenths of detections over several hundreds of milliseconds on detectors of all antennas at the same time [R2]. It was suggested that these frequencies were not excited by arcs but came from the plasma. We installed a RF probe inside the vacuum vessel and connected it to a fast acquisition system. We indeed observed frequencies during ICRF heating of several shots. We present here the features of these frequencies, their possible conditions of existence in terms of plasma and RF parameters (magnetic field, density, temperature, RF frequency, power injected) and we compare the source of these frequencies with known phenomena like ICE, CAE and surface waves.