Simulation of magnetic flux expulsion on 5-cell and half-wave coaxial niobium superconducting radio frequency cavities*

Superconducting radio frequency (SRF) cavities are utilized in most modern particle accelerators for their ability to store large amounts of energy with little to no dissipation. Niobium metal is a common yet strong choice of material due to its superconducting properties, however some dissipation of RF power occurs due to the trapped magnetic flux during the cooldown of these cavities. Two approaches are generally used to mitigate residual magnetic flux trapping: implementing magnetic shielding around the cavity and/or optimizing the cooldown process with significant temperature gradient to push out the magnetic flux during the cooldown when the cavity transitions to the superconducting state. Several experimental studies have been done in single and multi-cell cavities to optimize the flux expulsion from the cavity surface. This work aims to simulate the dynamics of magnetic flux when the cavities transition from the normal to superconducting state on multi-cell and complex coaxial cavities using COMSOL Multiphysics software, and these simulation results are compared with experimental data. The simulation results are qualitatively in agreement with the experimental findings, confirming what is demonstrated experimentally in more detail through simulation. The data also provide a means of observing, in more precise detail, how these cooling processes influence the field on the cavity surface.