Proof of principle of electron cyclotron emission as a complementary diagnostic for the plasma position

Abstract

Today's tokamaks use magnetic diagnostics to control the plasma position. In future fusion reactors (e.g. DEMO) the performance of magnetic diagnostics will be compromised by integrator drifts over long pulses and by radiation. Microwave diagnostics are good alternatives, as they are expected to be robust to the harsh radiation environment. Reflectometry has shown its potential to measure the density profile and hence to derive the separatrix position. This submission demonstrates how Electron Cyclotron Emission (ECE) can do the same via the electron temperature (Te) profile. It will be shown that Te at the separatrix can be calculated as a function of several well-diagnosed plasma parameters for a wide range of scenarios. From ECE measurements, the Te profile can be derived, taking into account problematic effects such as shinethrough and corrupt measurement channels. Trivially, finding the position where the profile matches the separatrix temperature gives the separatrix position. In a proof-of-principle experiment on ASDEX Upgrade, the radial plasma position was controlled using the separatrix position derived from ECE measurements. After initial success, problems with cutoff and latency led to a loss of control. An outline is given on how to remedy these issues and develop this proop-of-principle into a viable diagnostic for fusion reactors.