Infoscience

Conference paper

A saddle coil system for TCV and RMP spectrum optimisation

A multi-purpose saddle coil system is proposed as part of a future upgrade of TCV (Tokamak à Configuration Variable). This system consists of 3 rows of 8 coils, each coil having independent power supplies, and provides simultaneously error field correction (EFC), resonant magnetic perturbation (RMP) and fast vertical control (VC). Other applications, like resistive wall mode (RWM) control and controlled plasma rotation, are also considered. Edge localized modes (ELM), related to the high confinement regime (H-mode), lead to a degradation of the plasma confinement and a release of energetic particles towards the vessel walls. Scaling the current experimental data to ITER predicts that the power flux related to ELMs will cause an intolerable erosion and heat load on the plasma facing components. Experiments on DIII-D and JET have demonstrated that the application of RMP is able to mitigate or suppress ELMs while keeping sufficient confinement properties. The description of the mechanism responsible for this phenomenon is still incomplete. The limits of the process, in terms of operation domain, are not yet accurately known. Experiments in different Tokamaks reveal opposite results for similar conditions. With that respect, TCV unique plasma shaping and positioning capability could extend the range of accessible magnetic perturbation modes for a given coil system geometry. Feeding the coil system with independent power supplies allows an optimisation of the coil current distribution, both toroidally and poloidally. A Lagrange method has been developed and appears to be an efficient way to minimize parasitic modes and current requirements while imposing the amplitude of a number of target modes. For example, the relative amplitude of edge modes can be increased at the cost of lower absolute amplitudes, demonstrating a degree of controllability on the localisation of the magnetic perturbation. In order to apply the Lagrange method, the characterization of the system in terms of spectral degeneracy is crucial to determine the set of simultaneously achievable target modes.

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