Non-axisymmetric response of OH coils to Lorentz forces

Abstract

The central solenoid of the Compact Ignition Tokamak (CIT) is constructed from stacks of thick-turn, resistive pancakes. As the solenoid is operated at fields as high as 20 Tesla, it is desirable to determine accurately its non-axisymmetric response. This paper presents finite-element based methods for obtaining these results. The basic unit of analysis is the double-pancake, a model consisting of two pancakes, the insulation between turns in each pancake and the insulation between pancakes. An isolated double-pancake is shown to exhibit a significant non-axisymmetric response, with each pancake having a dilated or elliptical deflection profile. Peak stresses in an isolated double pancake are shown to be sensitive to the transition angle between circular turns. The response of the solenoid is then found by superimposing the number of double pancakes required to form the basic axial unit of the stack. In practice this is accomplished by using the double-pancake model and imposing additional boundary conditions to reflect the interaction with adjacent pancakes. The dilation observed in an isolated double pancake may be preserved or destroyed, depending on the axial symmetry of the stack. Peak stresses are found to be much less sensitive to transition angle, but are still 20–30% above the values calculated from an axisymmetric analysis.