High-Field Slope in Superconducting Cavities Due to Magnetic Field Enhancement at Grain Boundaries

Quality ( ) degradation of superconducting, fieldemission-free cavities at high accelerating gradient ( acc MV/m) has been observed at a number of laboratories. This slope presents the main impediment to achieving gradients in excess of 25 MV/m needed for future superconducting high-energy accelerators. Presented here are measurements and numerical simulations with SUPERLANS and ANSYS that provide insight into the mechanism responsible for the degradation. A model, based on magnetic field enhancement at the grain boundaries, is developed and used to simulate the measured results. It can explain differences in cavity behavior following 150 C and 850 C heat treatment. The model also explains a number of other observations, such as the improved cavity performance following electropolishing, and the predominance of cavity breakdown at the equator weld in chemically etched cavities.