Runaway electron diagnostics in FTU

Runaway Electrons (REs) are beams of electrons within a tokamak plasma, accelerated to velocities close to the speed of light. They represent a major concern in tokamaks because they are lost to the first wall, resulting in significant local energy deposition, local melting and damage of plasma-facing components. Key to the successful operation of tokamaks, and in particular of ITER and future energy-generation reactors such a DEMO, is the ability to avoid and control REs to mitigate their impact on the machine operation. A comprehensive experimental program on the study of REs has been developed at FTU by the integration of diagnostic, hardware and software tools in real-time (RT) plasma control system, including the CO2 scanning interferometer for RE position control, a fission chamber to monitor RE interactions with plasma-facing components and the hard x-ray radial profile monitor for the measurement of in-plasma bremsstrahlung from REs. Further, two new diagnostics have been developed specifically to measure in-flight and lost REs, respectively, the RE Imaging Spectrometer system (REIS) and a Cherenkov probe. The REIS provides both image and (visible and infrared) spectra of RE synchrotron emission, providing information on the RE energy distribution function. The Cherenkov probe, installed within the plasma scrape-off layer, detects RE losses, enabling the study of RE dynamics in the presence of magnetic islands. This probe provided also first evidence of RE losses in the presence of beta induced Alfvén eigenmodes using a non-magnetic diagnostics. A three-channel Cherenkov probe is being installed, to permit energy discrimination and simultaneous data collection from different toroidal/poloidal positions. An analysis of the experimental data obtained in FTU