The increasing demand for high-performance superconducting radiofrequency (SRF) cavities in particle accelerators,especially in free electron lasers, has led to intensified research on optimizing cavity treatments. One promisingrecent development is the application of medium temperature heat treatments, which are typically performed at250 ◦C - 350 ◦C in a vacuum furnace for several hours. It has been shown that these mid-T heat treatmentsenhance cavity performance by increased quality factors. However, the improved performance comes with thetrade-off of an increased sensitivity to trapped magnetic flux of the cavity, which leads to lower quality factors. Forthe first time at DESY, the sensitivity to magnetic flux trapping is deduced from parasitic cavity measurements.The data retrieval and analysis in the scope of this project was only accessible due to a new systematic approachto both experimental test conditions and data acquisition systems. The sensitivity to trapped magnetic flux issystematically investigated on four cavities with very different heat treatments. Magnetic field measurements wereconducted using three magnetic sensors mounted on the cavity equator. Additionally, the temperature gradientsduring the cooldown procedures are analyzed and showed consistencies in regard to the according cavity qualityfactors. Finally, results are compared to data of previous studies. These findings display the complex interplaybetween chemical and heat treatments, material properties and thermal dynamics on SRF niobium cavities. It canbe confirmed that mid-T heat treated cavities show significantly increased sensitivity values 3-5 times higher thanfor cavities without this heat treatment. Moreover, differences in sensor signals after applying an external magneticfield highlight the influence of cavity characteristics, such as grain structure and defects. These findings provide astrong foundation for future optimization of cavity treatments for a second generation of EuXFEL cavities, whichare required for the planned upgrade of the European XFEL accelerator.
Jennifer Ademoye (Wed,) studied this question.