Influence of DEMO vacuum vessel shell thickness on its electromagnetic response

Abstract

In tokamaks during plasma transients, such as vertical displacement events (VDE), a certain amount of current is induced in the vacuum vessel and the in-vessel passive structures. As the current in the vacuum vessel (VV) crosses the magnetic field produced by tokamak magnets strong electromagnetic (EM) loads are generated that cause deflections of the VV. The horizontal displacement of the VV within the magnetic field generates additional currents and loads opposing the displacement that can be represented with a system of a spring and a damper coupled in series between the VV and the magnet structures. Stiffness and damping coefficients are strictly related to the current flowing in the VV and hence its electrical resistivity. Given the early stage of the DEMO VV design, a parametric law is useful to scale the values of the spring and damper for different choices of VV shells thicknesses. In this work, different stiffness and damping coefficients couples, related to specific material and shell thicknesses, are extrapolated using transient EM simulations. The forces in the VV are evaluated considering two different analyses in which the TF coils are in nominal and displaced positions. The stiffness of the system is directly proportional to the force peak occurring in the VV during time, while the damping is related to aforementioned stiffness and to the time decay constant of the force curve over time.