Physics design of a saddle coil system for TCV

Highlights: ► The design of a saddle coil system for TCV is presented. ► The system is designed for ELM control, error field correction and vertical control. ► The issue of optimal design determination is addressed. ► Electrical properties in the presence of a conducting wall are quantified. ► Coil heating due to Joule effect and impact of plasma disruptions are considered. - Abstract: The upgrade project for TCV (Tokamak à Configuration Variable) includes the installation of a set of saddle coils, namely the saddle coil system (SCS), located and powered such as to create a helical magnetic perturbation. Using independent power supplies, the toroidal periodicity of this perturbation is tunable, allowing simultaneously edge localized modes (ELM) control through resonant magnetic perturbation (RMP), error field correction and vertical control. Other experimental applications, like resistive wall mode and rotation control, are also in view. In this article, the adequacy of two SCS designs, an in-vessel one and an ex-vessel one, is assessed with respect to the desired experimental applications. The current requirements and the system performances are also characterized. The conducting vessel wall is accounted for in a model used to determine the coupled response functions of the SCS, the screening of the magnetic perturbation by the wall, the induced voltages and currents during a plasma disruption and the maximal magnetic forces exerted on the SCS. A scaling of the SCS parameters with the number of coil turns is presented and the issue of coil heating and cooling is discussed.