Radio-frequency losses induced by trapped magnetic flux in superconducting cavities containing defects and grain boundaries

Abstract Superconducting cavities inevitably contain defects during the manufacturing process, such as point defects and grain boundaries. Magnetic flux vortices are easily pinned by these defects, and the trapped flux undergoes high-frequency oscillations under radio-frequency (RF) magnetic fields, forming hotspots that increase energy dissipation in the cavity and degrade its performance. This study employs time-dependent Ginzburg–Landau theory to numerically simulate RF losses caused by trapped magnetic flux in superconducting cavities, investigating the effects of defects on heating power and quality factor under RF fields. The results demonstrate that when no magnetic flux vortices are trapped, the quality factor of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mi>Sn</mml:mi> </mml:mrow> </mml:mrow> </mml:math> multilayer superconducting cavities increases with the thickness of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mi>Sn</mml:mi> </mml:mrow> </mml:mrow> </mml:math> layer, perfectly aligning with theoretical predictions. The energy dissipation caused by the trapped flux is dependent on not only the amplitude and frequency of the RF field but also the number, spacing, and position of defects. Greater numbers of defects, closer spacing between defects, and proximity of defects to the inner boundary of the cavity all lead to higher RF losses. Furthermore, RF losses in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>Nb</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mi>Sn</mml:mi> </mml:mrow> </mml:mrow> </mml:math> multilayer cavities initially increase and then decrease with longer grain boundary lengths, while losses increase with the strength of the grain boundaries. These findings provide critical insights for evaluating the impact of defects on the performance of superconducting cavities.