Design, Installation, and First Results from the Ion Cyclotron Emission Diagnostic on TCV

TCV is a medium sized tokamak equipped with a large suite of plasma diagnostics, a versatile array of poloidal shaping coils, an electron cyclotron heating system, and a two source neutral beam injection (NBI) system. The NBI system is capable of a simultaneous injection of energetic neutrals in the co-and counter-current directions. The resulting fast ion (FI) populations are used to study a multitude of FI-driven instabilities. While plasma instabilities in the below 1 MHz frequency range can be observed via standard diagnostics such as soft x-ray detectors, magnetics, fast ion loss detectors, and reflectometers, fluctuations in the range of >10 MHz require a specialized diagnostic. For this purpose the TCV tokamak has been equipped with a dedicated ion cyclotron emission (ICE) diagnostic, following a similar approach to AUG [1] and W7-X [2]. The diagnostic consists of a pair of magnetic coils (8 turns, 177 nH, 16 mm long, 7 mm in diameter), oriented orthogonally to each other to detect magnetic field fluctuations in the toroidal and poloidal directions. The coils are housed in a stainless steel electrostatic shield with a slit and are installed on the torus low field side at the midplane location, behind graphite protection tiles. The coil electric outputs are routed to the vacuum feedthroughs via a pair of coaxial cables, at which point one of the outputs is grounded at the feedthrough, from the airside. The second output is then routed to a rectifying radio frequency wave detector and a fast digitizer in the diagnostics rack. The rectifying detector is sensitive to signals in the 10-100 MHz frequency range and can measure the signal amplitude and the phase difference between the two probes, while the frequency information of the signal is lost. The benefit of this detection method is that the output signal can be digitized at a “slow” speed (200 kHz in the case of TCV) with a low cost digitizer and the data volume per plasma discharge is low. The fast digitizer, on the other hand, preserves the frequency information, albeit with a larger data volume. For the case of TCV, a 250 MHz sampling rate digitizer has been temporarily used, while a dedicated 1 GHz digitizer is currently being implemented. First results of high frequency (>10 MHz) instabilities detected in the presence of energetic ions will be presented.