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

Experiments on TEXTOR have successfully demonstrated the proof of principle [1] of using Electron Cyclotron Emission (ECE) measured along the Electron Cyclotron Resonance Heating (ECRH) line-of-sight [2] 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 [3], 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 [4], 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 [5]. Because additional absorptive filtering is required after the FADIS ECE output in [5] and [6] a non resonant two-beam Mach-Zehnder-type interferometer was chosen. This system has been built and partially tested