Magnetic field induced by thermo electric current in lcls-ii cryomodules*

The Seebeck effect in metals plays an important role in cryomodule design and performance. As the cryomodule is cooled from room temperature to nominal cavity operating temperature of 2 K, components in the cryomodule experience varying temperatures. Some components such as power couplers have gradients from 300 K to 2 K. Such gradients and metallic paths form thermo-electric current loops that circulate through and around the niobium cavities. These currents will generate an additional magnetic field that could be trapped in the cavity wall during superconducting transition, as well as during cavity quench. This trapped field can degrade the cavity’s quality factor and increase heat load. A simple circuit model is proposed and compared to a calculated trapped field during LCLS-II cryomodule tests.