Utilization of collinear ECE detection/ECRH heating for active stabilization of plasma instabilities

Experiments on TEXTOR have successfully demonstrated the proof of principle of using Electron Cyclotron Emission (ECE) measured along the Electron Cyclotron Resonance Heating (ECRH) line-of-sight for the control of magnetohydrodynamic (MHD) modes. This work, which was done with a quasi-optical system, motivates the further development and implementation of a similar, but in-waveguide system for detection and control of Neoclassical Tearing Mode (NTM) in larger fusion machines. Progress on the implementation of such a system on ASDEX-Upgrade, based on waveguides equipped with a Fast Directional Switch (FADIS), is presented. Since FADIS, will be installed at ASDEX, to switch the gyrotron power between different launchers, FADIS can also be employed as a frequency filter serving the purpose of separating a low-power ECE signal from the high power ECRH radiation as required in the co-aligned ECE-ECRH setup. Because additional absorptive filtering is required after the FADIS ECE output in and a non resonant two-beam Mach-Zehnder-type interferometer was chosen. This system has been built and partially tested.