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In tokamak operation, the control of dangerous MHD instabilities, possibly in real-time scenarios, must rely on prompt robust diagnostics of the state and stability of the system. The set of magnetic signals measured on the outside of the plasma boundary, based on the Zakharov–Shafranov, Shkarowsky, Wootton (ZSSW) current moments, has been always used for reliable monitoring of key characteristics of the instantaneous equilibrium condition, such as the quantities $\Delta _{\mathrm{ Sh}}$ , the Shafranov centroid shift, $\beta _{p} $ related to the thermal energy content, and $l_{i}$ related to the current profile peakedness. In addition, the fast pickup coils monitor the external magnetic field fluctuations due to internal MHD activity, however, without the possibility of radial localization of the source. Here, we explore the potential usefulness of more complete use of ZSSW moments in association with the information from fast B perturbation signals to detect tearing stability conditions. For clarity, we set up an analysis of the measurable response to tearing perturbations based on an exact equilibrium model, which is an extension of the Solov’ev case with the addition of an equilibrium, nonuniform plasma rotation $\Omega (\psi)$. The relation of the selected (externally measurable) ZSSW moments to the calculated stability index is mapped for different rotation values. The footprint of the stability condition $\Delta '< 0$ on some current moments on the outer surface can then identify stability boundaries for different rotation conditions. This first discussion on an idealized exact model is proposed for testing the concept for application to realistic equilibria since it relies on a few externally monitorable quantities and very basic assumptions on the tearing modes physics. [ABSTRACT FROM AUTHOR]

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 \cdot 10^20\ \m^-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 \pm 1^\circ 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 shows that the maximum reachable efficiency, under optimal wave injection, is expected not to exceed 45% of the EC power injected, while the diameter of the angular window corresponding to 50% of power conversion keeps \leq 3^\circ in both polidal and toroidal directions. The new E CH&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. [ABSTRACT FROM PUBLISHER]