Stress and fatigue in the magnets in DEMO in pulsed and steady-state operation using geometric scaling factors

One of the most important lifetime-limiting effects for a pulsed tokamak reactor, the effect due to pulsing the magnets, is addressed here. Pulsed operation imposes stresses with significant alternating components on all the coils. This paper analyses the effects of fatigue on the coils. (Fatigue of the superconductor itself is not discussed.) Initial work has concentrated on the IF coils. The form of the finite element model is simple to permit parametrisation so that the results can be used in systems studies using codes such as PROCESS. The parameters used so far are geometric scaling factors describing the TF and CS coils only. Once sufficient parameters have been introduced, the resulting equations for stress can give realistic results in a systems code, which allows for the interdependence of all the variables. The magnetic field in the TF coil winding pack due to all the coils is calculated using the Biot-Savart law. This gives the Lorentz force in the winding pack, which is then used for the stress strain calculation. The alternating stress is calculated by subtracting the stress tensors between two different load cases. In order to meet the lifetime fatigue goal the coil support structure needs to be made stronger than the structure intended for the small number of cycles of a steady-state reactor design, which will add to its radial build. We have quantified these requirements, allowing systems codes to estimate the additional costs. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.