JLab, Newport News, Virginia, USA
The benefits of reduced RF losses from interstitial "doping" of niobium are well established. Many of the details involved in the process remain yet to be elucidated. The niobium surface reacted with low-pressure nitrogen at 800°C presents a surface with chemical reactivity different than standard niobium. While standard "recipes" are being used to produce cavities, we seek additional insight into the chemical processes that may be used to remove the "undesirable" as-formed surface layer. This may lead to new processing routes or quality assurance methods to build confidence that all surface "nitrides" have been removed. We report a series of alternate chemistry treatments and subsequent morphological examinations and interpret the results. We also introduce a new standardized Nb sample system in use for efficient characterization of varying doping protocols and cross-laboratory calibration.
TUFUA3
JLab, Newport News, Virginia, USA
Virginia Polytechnic Institute and State University, Blacksburg, USA
In early 2018, preliminary RF date from the LCLS-II HE program suggested two new high temperature doping recipes developed at Jefferson Laboratory (3N60) and Fermi Nation Laboratory (2N0) produced quench fields outside expectations.* Both recipes showed quench fields (while maintaining high Q0) outside the simplified model where the quench field scaled purely with the RF surface doping level. In late 2018 we developed a qualitative going on a quantitative model based on preliminary SIMS/SEM measurements of the new recipes that would explain the quench field distribution. Unfortunately, subsequent measurements invalidated the developing model. We will present our original qualitative model and new data where the model breaks down; showing the multi-variable dynamics which we now think we need to understand in order to fully model and maximize quench fields for high temperature doping.
* Palczewski, A.D. and Bafia, D., contributions TESLA Technology Collaboration University of British Columbia, Vancouver, Canada, February 5 - 8 2019
JLab, Newport News, Virginia, USA
Foreign particles residing on the field carrying surface of accelerator cavities are a known mechanism for field emission. Developing the methods and tools for collecting and characterizing particles found in an accelerator enables process development towards field emission free SRF cavities. Methods are presented for sampling assemblies, components, processes, and environmental conditions utilizing forensic techniques with specialized tooling. Sampling activities to date have produced an inventory of over 850 GSR spindles. Traditional SEM + EDS analysis of this volume of spindles is challenged by labor investment, spindle sampling methods, and the subsequent data pipeline which ultimately results in a statically inadequate dataset for any particulate distribution characterization. A complete systematic analysis of the spindles is enabled by third party software controlling SEM automation for EDS data acquisition. Details of spindle creation, collection equipment, component sampling, automating particle assessment, and data analysis used to characterize samples from beamline elements in CEBAF are presented.
JLab, Newport News, Virginia, USA
While the techniques used to provide "UHV clean" and "particle-free" beamline components, including SRF cavities, continue to evolve, "real-world" operating machines must deal with actual accumulated and latent contamination issues that produce non-trivial cryogenic heatload, radiation, activation, and degradation via field emission. We have developed a standardized and automated particulate contamination assay method for use in characterizing particulates found on beamline components and in cleanroom assembly environments. We present results from using this system to analyze samples taken from reworked cryomodules from CEBAF. Particle sizes are much larger than anticipated. Utility for feedback on sources to enable improved source reduction is explored.