JLab, Newport News, Virginia, USA
SRF accelerating cavities are often operated in superfluid helium of temperature near 2 K to enhance the cavity quality factor Q0 and manage cryogenic heat loads, which are particularly important at large SRF accelerator facilities. This temperature paradigm, however, need not put SRF technology out of the reach of small institutions or even limit SRF operation at large facilities to provide 10-100 MeV beam energy. At the Jefferson Lab CEBAF accelerator there are regularly scheduled maintenance periods during which the liquid helium temperature is raised to 4 K, reducing cryogenic plant power consumption by ~50% and saving megawatts of electrical power. During such a recent period, we accelerated a continuous-wave electron beam at the CEBAF photo-injector to 6.3 MeV/c with current ~80μA using two niobium cavities at helium temperature of 4 K. This contribution describes the SRF and cryogenic performance and uses measured beam quality and energy stability as key metrics. These measurements indicate that 4 K operation of niobium SRF cavities in CEBAF and at small institutions may be a sensible and cost effective mode of operation, provided the cryogenic load associated with lower Q0 is manageable for the number of SRF cavities needed. For Jefferson Lab, this enhances our scientific reach allowing additional low-energy ~10 MeV experiments each year.
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Virginia Polytechnic Institute and State University, Blacksburg, USA
The potential for higher operating temperature and higher gradient have motivated SRF cavity researchers to pursue Nb3Sn as an alternative to Nb for nearly fifty years. Far and away the most common embodiment has been a few micron-thick Nb3Sn layer on the cavity interior surface obtained by vapor diffusion coating, with one or another set of parameters. While many cavities have been made and RF tested, reports of dissecting a cavity in detail to examine the coating and relate it to RF measurements are rare. We coated a BCP-treated single cell cavity in a typical process of tin/tin chloride activation at 500 C followed by tin vapor deposition at 1200 C. After RF-testing, we cut and examined sections from several locations to learn composition, thickness topography of the interior surface. The effect of process variables, such as surface preparation, process temperature and duration, and vapor chemistry needs to be explored.
JLab, Newport News, Virginia, USA
Operations at 4 K instead of 2 K have the potential to reduce the operational cost of an SRF linac by a factor of 3, if the cavity quality factor can be maintained. Cavities coated with Nb3Sn have been shown to achieve the accelerating gradients above 10 MV/m with a quality factor around 1010 at 4 K. Because such performance is already pertinent for CEBAF injector cryomodule, we are working to extend these results to CEBAF accelerator cavities envisioning coating of two CEBAF 5-cell cavities with Nb3Sn. They will be installed in an injector cryomodule and tested with beam. The progress on this path is reported in this contribution.
The College of William and Mary, Williamsburg, Virginia, USA
JLab, Newport News, Virginia, USA
Nb3Sn has the potential to achieve superior performance both in terms of operating temperature (4.2 K vs 2 K) and accelerating gradient resulting in significant reduction in both initial and operating costs of SRF linacs. Cavity interior surface coatings are obtained by two-step vapor diffusion: nucleation followed by deposition. To gain more understanding of nucleation and its effect on the subsequent coating, we investigated the effect of varying parameters in a typical tin/tin chloride process. We report findings obtained by SEM/EDS, AFM, SAM and other materials characterization approaches.
