This workshop will review the current understanding on flux trapping and magnetic field expulsion for pure or infused/doped bulk niobium, coated, and A15 superconducting cavities. The goal is to identify areas, which are not yet understood and to define an experimental program to improve our understanding over the coming years.
The workshop will be divided into the following sessions:
I) Magnetic Shielding (conveners: S.K. Chandrasekaran, FNAL; R. Laxdal, TRIUMF; M. Masuzawa, KEK)
Since a long time, it has been known that SRF cavities may trap the ambient magnetic field present when the cavity is undergoing its transition to the superconducting state. This trapped flux is responsible for additional dissipation at RF frequencies, and SRF engineers have strived to suppress the extra losses by suitable magnetic shielding. In this session, we would like to review the techniques which are available to achieve conditions as close as needed to zero field cooling in cryomodules. Magnetic hygiene, passive and active shielding, computer simulations, and techniques of measurement of the residual field will be discussed; the aim is to highlight the engineering efforts and to estimate the resources required to achieve different levels of ambient field reduction, as appropriated to the SRF material under consideration and to the application at hand.
II) Flux Expulsion Efficiency (conveners: J. Koeszegi, HZB; D. Longuevergne, IPNO; K. Umemori, KEK)
In real superconductors, the Meissner effect is almost invariably incomplete. The ability to expel the magnetic field upon transition to the superconducting state varies amongst different materials and cavity geometries. In the extreme case, the expulsion does not happen at all, and all the ambient magnetic field is trapped in the form of vortices. This is most often the case of Nb/Cu thin films. Recently the efficiency of flux expulsion in SRF cavities has been the object of several studies, which highlighted dependencies from material parameters and from the cool down dynamics.
A related topic is the possible presence of thermoelectric currents, stirred by temperature gradients across the cavity during transition.
Advancing towards a full understanding of all factors controlling flux expulsion in SRF cavities would contribute to i) optimise the performance of SC cavities, and to ii) relax the need of a highly performant magnetic shielding. In this session, we plan to review and discuss the existing theoretical frameworks and data sets, and their possible evolution and integration.
III) Sensitivity to Trapped Flux (conveners: M. Checchin, FNAL; A. Miyazaki, CERN)
Once flux is trapped in a SRF cavity, the associated dissipation will contribute to the total surface resistance. The loss mechanism is thought to be related to the forced oscillations that vortices may undergo under the effect of the Lorentz force when RF fields penetrate into the superconductor. For a same amount of trapped flux, however, different materials display different levels of RF dissipation, which is expressed by the so-called sensitivity. The surface resistance induced by a given amount of ambient magnetic field is therefore dependant on i) flux expulsion in-efficiency and ii) sensitivity to the trapped flux, which are both functions of material parameters. Sensitivity is as well a function of the applied RF frequency and amplitude. Several models have been proposed, to explain all the observations, but we still lack a predictive theory. Besides being a fascinating subject in itself, understanding and controlling the sensitivity to trapped flux in SRF cavities would have a direct impact on the cavity performances and on cryomodule costs. As for the case of flux expulsion, the session will be devoted to review and discuss the existing theoretical models and experimental work, and to their possible developments.
Each session will start with an invited review talk, followed by invited and contributed talks on proposed theories, observed phenomena, and experimental results. Each session will close with a discussion that shall result in a proposal for future experiments.
The chairs are then asked to summarise their respective session in a write-up, which will be published after the workshop.
The workshop will be held as a TTC topical workshop and it will receive funding from the European Union's Horizon 2020 Research and Innovation programme under Grant Agreement No 740871.