Over the past years it became evident that the quality factor of a superconducting cavity is not only determined by its surface preparation procedure, but is also influenced by the way the cavity is cooled down. Moreover, different data sets exist, some of which indicate that a slow cooldown through the critical temperature is favorable while other data states the exact opposite. Even though there were speculations and some models about the role of thermocurrents and flux-pinning, the difference in behavior remained a mystery. In this paper we will present a consistent theoretical model which we confirmed by data that describes the role of thermocurrents, driven by temperature gradients and material transitions. We will clearly show how they impact the quality factor of a cavity, discuss our findings, relate it to findings at other labs and develop mitigation strategies which especially address the issue of achieving high quality factors of so-called nitrogen doped cavities in horizontal tests.

The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7x10^10 at 2 K and 16 MV/m accelerating gradient. Two niobium 9-cell cavities, prepared with nitrogen-doping at Fermilab, were assembled into the Cornell Horizontal Test Cryomodule (HTC) to test cavity performance in a cryomodule that is very similar to a full LCLS-II cryomodule. The cavities met LCLS-II specifications with an average quench field of 17 MV/m and an average Q0 of 3x10^10. The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nOhm/mG during fast cool down. In two cool downs, a heater attached to one of the cavity beam tubes was used to induce large horizontal temperature gradients. Here we report on the results of these first tests of nitrogen-doped cavities in cryomodule, which provide critical information for the LCLS-II project.

Energy-Recovery Linacs (ERLs) are proposed as drivers for hard x-ray sources because of their ability to produce electron bunches with small, flexible cross sections and short lengths at high repetition rates. Cornell University has pioneered the design and hardware for ERL lightsources. This preparatory research for ERL-lightsource construction will be discussed. Important milestones have been achieved in Cornell’s prototype ERL injector, including the production of a prototype SRF cavity that exceeds design specifications, the regular production of long-lived and low emittance cathodes, the acceleration of ultra-low emittance bunches, and the world-record of 65 mA current from a photoemission DC gun. We believe that demonstration of the practical feasibility of these technologies have progressed sufficiently to allow the construction of an ERLbased lightsource like that described in Ref [1].