JLab, Newport News, Virginia, USA
Jefferson Lab (JLab) is in collaboration with Fermi Na-tional Accelerator Laboratory (Fermilab) to build 18 cryomodules to install at the SLAC National Accelerator Laboratory's tunnel as part of the Linac Coherent Light Source upgrade project (LCLS-II). Each LCLS-II cry-omodule hosts 8 superconducting niobium cavities that adopt the nitrogen doping technique, which aims to en-hance the cavity quality factor Qo to reduce the consumption of liquid helium used to cool down the cavities. It is known that the Qo of niobium cavities is affected by cavity surface magnetic field. Traditionally, magnetic shields made of high magnetic permeability mu-metals are employed as a passive shielding of the ambient magnetic fluxes. During the LCLS-II cryomodule development, magnetic hygiene control that includes magnetic shielding and demagnetization of parts and the whole-machine is implemented. JLab and Fermilab worked closely on developing magnetic hygiene control procedures, identifying relevant tools, investigating causes of magnetization, magnetic field monitoring, etc. This paper focuses on JLab's experiences with LCLS-II cryomodule magnetic hygiene control during its fabrication.
Authored by Jefferson Science Associates, LLC. The U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for Government purposes.
JLab, Newport News, Virginia, USA
Cryomodule production for LCLS-II is well underway at Jefferson Lab. This paper explains the process flow for production cavities, from being received at the Test Lab to being assembled onto cavity strings. Taking our facility and infrastructure into consideration, process optimization and process control are implemented to ensure high quality products.
JLab, Newport News, Virginia, USA
The College of William and Mary, Williamsburg, Virginia, USA
Topography arising from the final etch step in preparing niobium superconducting radiofrequency (SRF) accelerator cavities is understood to significantly impact cavity performance at high field levels. This study investigated the effect of process temperature and time on the etch rate and topography arising from the widely-used buffered chemical polishing (BCP). This study aims to understand more thoroughly the genesis of topography in BCP of polycrystalline niobium, with the ultimate aim of finding a path to surface smoothness comparable to that obtained by electropolishing (EP). It was found that the etch process is controlled by the surface reaction; and that the etch rate varies with crystallographic orientation. The familiar micron-scale roughening necessarily results. Gas evolution has an impact, but is secondary. The major outcome is that surface smoothness comparable to EP appears to be inherently unachievable for polycrystalline niobium using BCP, setting an upper limit to the gradient for which it is useful.