The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7×10 10 at 2.0 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 3×10 10 . The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nΩ/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 a cryomodule, which provide critical information for the LCLS-II project.

Abstract. Recent coordination of thermometry with optical images has shown that obvious defects at specific locations produce heat or even quench superconducting radio frequency (SRF) cavities, imposing a significant limit on the overall accelerating gradient produced by the cavity. Characterization of the topography at such locations provides clues about how the defects originated, from which schemes for their prevention might be devised. Topographic analyses also provide understanding of the electromagnetic mechanism by which defects limit cavity performance, from which viability of repair techniques might be assessed. In this article we discuss how a variety of two-component siliconebased room-temperature vulcanizing agents can be routinely used to make replicas of the cavity surface and extract topographic details of cavity defects. Previously, this level of detail could only be obtained by cutting suspect regions from the cavity, thus destroying the cavity. We show 3-D profiles extracted from several different 1.3 GHz cavities. The defect locations, which were all near cavity welds, compelled us to develop extraction techniques for both equator and iris welds as well as from deep inside long 9-cell cavities. Profilometry scans of the replicas yield micrometer-scale information, and we describe various curious features, such as small peaks at the bottom of pits, which were not apparent in previous optical inspections. We also discuss contour information in terms of electromagnetic mechanisms proposed by others for local cavity heating. We show that production of the replica followed by high-pressure rinsing dose not adversely affect the cavity RF performance.

Superconducting accelerator cavities are now routinely operated at levels approaching the theoretical limit of niobium. To achieve these operating levels more information than is available from the RF excitation signal is required to characterize and determine fixes for the sources of performance limitations. This information is obtained using diagnostic techniques which complement the analysis of the RF signal. In this paper we describe the operation and select results from three of these diagnostic techniques: the use of large scale thermometer arrays, second sound wave defect location and high precision cavity imaging with the Kyoto camera.

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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 i n f l u e n c e d b y t h e w a y t h e c a v i t y i s c o o l e d d o w n . Moreover, different data sets exists, some of which indicate that a slow cool-down through the critical temperature is favourable while other data states the exact opposite. Even though there were speculations and some models about the role of thermo-currents and fluxpinning, the difference in behaviour remained a mystery.I n t h i s p a p e r w e w i l l f o r t h e f i r s t t i m e p r e s e n t a consistent theoretical model which we confirmed by data that describes the role of thermo-currents, driven by temperature gradients and material transitions. We will c le arly s ho w ho w the y imp ac t t he qu ali ty f ac to r o f 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 socalled nitrogen doped cavities in horizontal test.