DESY, Hamburg, Germany
Fermilab, Batavia, Illinois, USA
JLab, Newport News, Virginia, USA
IFJ-PAN, Kraków, Poland
Within a global effort to understand and standardize the nitrogen-infusion and the low T bake procedure, one large grain and two fine grain single-cell cavity were treated and tested at FNAL and then send to other labs including DESY and JLab for further studies.
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.
SLAC, Menlo Park, California, USA
Fermilab, Batavia, Illinois, USA
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
JLab, Newport News, Virginia, USA
The LCLS-II HE project is a high energy upgrade to the superconducting LCLS-II linac. It consists of adding twenty additional 1.3 GHz cryomodules to the linac, with cavities operating at a gradient of 20.8 MV/m with a Q0 of 2.7·1010. Performance of LCLS-II cryomodules has suggested that operations at this high of a gradient will not be achievable with the existing cavity recipe employed. Therefore a research program was developed between SLAC, Fermilab, Thomas Jefferson National Accelerator Facility, and Cornell University in order to improve the cavity processing method of the SRF cavities and reach the HE goals. This program explores the doping regime beyond what was done for LCLS-II and also has looked to further developed nitrogen-infusion. Here we will summarize the results from this R\&D program, showing significant improvement on both single-cell and 9-cell cavities compared with the original LCLS-II cavity recipe.
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
DESY, Hamburg, Germany
Ettore Zanon S.p.A., Nuclear Division, Schio, Italy
RI Research Instruments GmbH, Bergisch Gladbach, Germany
JLab, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
SLAC, Menlo Park, California, USA
Fermilab, Batavia, Illinois, USA
During the LCLS-II project a second batch of niobium was procured from Tokyo Denkai Co Ltd in order to make additional cavities. The original production material came from Two vendors Tokyo Denkai Co., Ltd. (TD) and Ningxia Orient Tantalum Industry Co., Ltd. (OTIC/NX)). It was found TD niobium required a lower annealing temperature (900°C) to obtain satisfactory flux expulsion characteristics compared to NX which required a slightly higher annealing temperature (950°-975°C). In order to ensure the new TD material performed equivalent to the niobium produced 4 year before after 900°C annealing; each heat lot of niobium had its flux expulsion characteristics parametrized and custom thermal treatments developed for each lot. Subsequent pure heat lot 9 cell cavities were made and tested. We will look at the flux expulsion characteristics of each lot, and RF loss of the 9-cell cavities produced using the individual heat lots.
IIT, Chicago, Illinois, USA
Fermilab, Batavia, Illinois, USA
SLAC, Menlo Park, California, USA
JLab, Newport News, Virginia, USA
JLab, Newport News, Virginia, USA
In recent years, efforts have been invested to leverage the different processes involved in energetic condensation to tailor Nb film growth in sequential steps. The resulting Nb/Cu films display high quality material properties and show promise of high RF performance. The lessons learned are now applied to 1.3 GHz Nb on Cu cavity deposition via high power impulse magnetron sputtering (HiPIMS). RF performance is measured at different temperatures. Particular attention is given to the effect of cooldown and sensitivity to external applied magnetic fields. The results are evaluated in light of the Nb film material and superconducting properties measured with various microscopy and magnetometry techniques in order to better understand the contributing factors to the residual and flux induced surface resistances. This contribution presents the insights gained in exploiting energetic condensation as a path towards RF Q-slope mitigation for Nb/Cu films, correlating film material characteristics with RF performance.
SLAC, Menlo Park, California, USA
JLab, Newport News, Virginia, USA
Fermilab, Batavia, Illinois, USA
Nitrogen doping has been proven now in several labs to enhance Q0 values of 1.3 GHz cavities in the gradient domain favored by CW operation. The choice of doping for the LCLS-II project has given the community a wealth of statistics and experience on the challenge of transferring the doping technology to industry. Overall, industry-produced nitrogen-doped cavities have shown excellent performance, however some technical issues have arisen. This talk focuses on lessons learned from the production of over 300 nitrogen-doped cavities for LCLS-II and how issues were mitigated to further improve performance. Finally, I will discuss pushing the boundaries of nitrogen-doping further by exploring different doping regimes in order to maintain excellent Q0 performance, while reaching higher quench fields.