Impact of cooldown conditions on trapped flux in superconducting niobium

Superconducting radio frequency cavities are a key technology in modern accelerators, and, over the past years, their performance improved such that additional losses from trapped magnetic flux are a limiting factor in their performance. This is especially important for accelerators operating in continuous wave mode where high losses in the cavity make operation too energy consuming. For this reason there are many experiments investigating how trapped flux can be reduced. It is investigated how different materials and their treatments influence trapped flux, and how it is affected by cooldown parameters during the transition from the normal to superconducting state. These experiments are often done using cavities as samples. This makes changing material parameters expensive and time consuming.
Additionally, the tests themselves are very time consuming so that the number of obtainable data points are often limited. Within the scope of this thesis a new experimental setup is designed which uses flat, rectangular samples to investigate trapped flux. Using these samples has the advantage that different materials and treatments can be tested more easily. Additionally, geometric effects during transition are easier to model, and understand. Besides the easier sample preparation the new setup allows for more cooldowns in a shorter period of time so that around 300 thermal cycles can be performed in one day. This is roughly two orders of magnitude more than what is achieved with cavities. With the new setup cooldown parameters like the temperature gradient across the sample, the cooldown rate, and the external magnetic field can be independently controlled so systematic investigations how each parameter influences trapped flux can be performed.
Measurements conducted with different niobium samples confirm effects reported from other experiments. For example a decrease in trapped flux for increasing temperature gradient is observed as well as a linear increase of trapped flux with external magnetic field under certain conditions. But the ability to record more data points and a relative large parameter space also revealed unexpected results: For large grain niobium it is observed that when a sample is cooled down with a temperature gradient across the sample flux gets only trapped when the external field is larger than a certain threshold field which depends on the temperature gradient. Additionally, it is noticed that very fast cooldowns lead to high trapped flux magnitudes almost independent of the temperature gradient. Besides these newly discovered effects the measured dependence of trapped flux on temperature gradient during cooldown does not agree with an existing model. For this reason a new phenomenological model is developed in cooperation with Prof. T. Kubo.