Implementation of a X-mode multichannel edge density profile reflectometer for the new ICRH antenna on ASDEX Upgrade

Abstract

Ion cyclotron resonance heating (ICRH) is one of the main heating mechanisms for nuclear fusion plas- mas. However, studying the effects of ICRH operation, such as power coupling efficiency and convective transport, requires the measurement of the local edge plasma density profiles. Two new three-strap an- tennas were designed to reduce tungsten impurity release during operation, and installed on ASDEX Upgrade. One of these ICRH antennas embedded ten pairs of small microwave pyramidal horn anten- nas. In this thesis, a new multichannel X-mode microwave reflectometry diagnostic was developed to use these embedded antennas to simultaneously measure the edge electron density profiles in front of the bottom, middle and top regions of the radiating surface of the ICRH antenna. Microwave reflectome- try is a radar technique that measures the round trip delay of probing waves that are reflected at specific cutoff layers, depending on the probing wave frequency, plasma density and local magnetic field. This diagnostic uses a coherent heterodyne quadrature detection architecture and probes the plasma in the range 40-68 GHz to measure plasma edge electron densities up to 2×1019 m-3, with magnetic fields between 1.85 T and 2.7 T, and a repetition interval as low as 25 μs. This work details the implementa- tion and commissioning of the diagnostic, including the calibration of the microwave hardware and the analysis of the raw reflectometry measurements. We study the automatic initialization of the X-mode upper cutoff measurement, which is the main source of error in X-mode density profile reconstruction. Two first fringe estimation algorithms were developed: one based on amplitude and spectral information and another using a neural network model to recognize the first fringe location from spectrogram data. Kalman filters are used to improve radial measurement uncertainty to less than 1 cm. To validate the diagnostic, we compared the density profile measurements with other electron density diagnostics on ASDEX Upgrade, and observed typical plasma phenomena like the L-H transition and ELM activity. The experimental density profile results were used to corroborate ICRH power coupling simulations under different gas puffing conditions and to observe poloidal convective transport during ICRH operation