An overview of the in-vessel ICRF-diagnostics in the ASDEX Upgrade tokamak

Abstract

Radio Frequency (RF) waves in the Ion Cyclotron Range of Frequency (ICRF) are successfully used to heat fusion plasmas. For a better understanding of the ICRF wave propagation, absorption and other processes in the plasma, the ICRF-related in-vessel diagnostics has been extended recently in the ASDEX Upgrade tokamak.A number of high frequency B-field probes have been installed which, arranged as an array, pick up the magnetic field components of the radiated wave, and can thus be used to characterize the radiation spectrum of the ICRF-antennas during plasma discharges. Dual channel logarithmic RF-detectors with phase detection capability have been designed and installed to track the fast variations of the probe signals during events like edge localized modes (ELMs). The same probes are also used to measure the ion-cyclotron emission (ICE) of plasmas. A fast, open source data acquisition system, which digitizes the RF-signal with 125 MSamples/s and 14 bit amplitude resolution, records the ICE signals. To allow ICE measurements during ICRF operation notch filters are used to suppress the ICRF signal by 50 dB. In order to characterize drive mechanisms and consequences of RF sheaths on the ICRF-antenna structures, 12 antenna limiter tiles are equipped with shunts to allow the measurements of RF- and DC-currents during operation, with some of the locations also covered by the spectroscopic lines of sight measuring the emission of neutral tungsten at 400.9 nm (WI). The antenna-embedded X-mode reflectometry with 3 channels operating simultaneously in the extended U-band helps to study ICRF wave coupling, whereas the RF current probes in the 3-strap ICRF antennas provide information on the phasing between central and outer straps.