A real-time tracking for optimal wave injection in overdense plasma heating experiments at 140 GHz in FTU

Experiments on overdense plasma heating through the mode-coupling scheme known as “O-X-B Double Mode Conversion”, obtained launching a narrow beam of millimeter-waves at 140 GHz frequency and 400 kW power, are scheduled for the next experimental campaigns of the FTU tokamak. Such a scheme, not yet demonstrated at electron density higher than the critical one (2.4·10²⁰m^-3) for the 140 GHz ordinary mode, and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. In the specific case of FTU the overall efficiency of this scheme is mainly determined by the coupling efficiency between the O- and the X-wave, which can occur only for ordinary polarized radiation propagating in a very narrow angular range at the cutoff region. The simulations performed with a single ray tracing, show that the required precision in the injection of the wave into the plasma is very high and an angular deviations of ±1° with respect to the optimal injection, in either vertical or horizontal direction, implies a 50% drop in the power transmitted to X-mode. Moreover, the application of models able to take into account the real shape of the incident beam, show that the maximum reachable efficiency, under optimal wave injection, is expected not to exceed 45% of the EC power injected. The new ECH&CD launcher, now being installed in FTU, will be able to provide the angular precision required for the steering. The basic idea of a control algorithm, aimed to track in real-time the optimal angular window for the wave injection in experiments on O-X-B mode conversion, is presented in the paper. The control will use the stray gyrotron radiation as observable, which is detected by a set of sniffer probes located at different toroidal positions in the FTU vessel.