Experts on superconducting radio frequency technologies (SRF) from all over the world came to the SLAC National Accelerator Laboratory in December for the Tesla Technology Collaboration (TTC) meeting.
Akira Yamamoto, Asian director of the International Linear Collider (ILC), said the TCC reminded him of a family reunion where everything from technical aspects to failures and mistakes were discussed without hesitation. He emphasized that the TTC community is an important technical partner around the globe for the ILC (International Linear Collider) even though TTC will of course encourage other large and small projects. He foresees fast developments and advances within the field of high and low beta cavities for a great variety of different accelerator projects. “I have been working in (the) superconducting magnet field… more than 30 years, and I am really feeling the SRF boom and golden age,” said Yamamoto. “(T)he TTC meeting (is) one of the most critically important meetings to prepare for the inevitably important technology for the ILC – including real industrialization experience.”
It was the very first TTC Meeting hosted by SLAC and the lab used the opportunity to not only invite experts to its lab but also to encourage its own employees to attend. “With LCLS-II using this technology we wanted SLAC staff to comfortably attend in spite of them being relative newcomers,” said Marc Ross, project manager for LCLS-II at SLAC. “I think it worked and we had an unusually exciting meeting for everyone involved in SRF.”
In his opening talk TTC Chair Hasan Padamsee called recent developments in the field the beginning of a Golden Age of SRF technology. “With the rapidly growing recognition from the accelerator community of the significant benefits that SRF has to offer to high energy physics, nuclear physics, materials science and accelerator-driven systems with high-power proton beams, the golden era of SRF enables a wide range of exciting new applications all over the world.”
Plenary presentations and working groups followed over the next several days, allowing scientists both the opportunity to present work and the time necessary to discuss experimental results, theoretical models and technical solutions.
Participants in the Cavity Working Group discussed high quality factors—and therefore nitrogen doping and magnetic flux expulsion—and field emission measurements. High quality factors (Q) are desirable for keeping the refrigeration cost as low as possible. Therefore maintaining high Qs throughout the running time of the machine is as important. Nitrogen doping of cavities has shown to produce high quality factors. “Nitrogen doping is a useful tool but there remains a need for more research to determine the actual physics behind it,” explained Ari Palczewski, staff scientist at Jefferson Laboratory (CSA CSM). The scientists used this meeting to arrive at some common ground and to hash out differences regarding the theoretical side of this new technique.
Similar discussion swirled around magnetic flux expulsion. As a cavity is cooled to the superconducting state, magnetic field lines can be trapped in the surface, therein reducing its quality factor. “We have been working on a way to prevent this decrease of quality factor with specific preparations of the cavity, and by cooling it down with a large thermal gradient,” said Sam Posen, associate scientist at Fermilab (CSA CSM). “This way we can push magnetic field lines out, so that they go around the cavity, like a river flowing around a rock.”
“Especially for nitrogen-doped cavities, the preparation and cool-down methods are important,” added Dan Gonnella, PhD-student at Cornell. “Since nitrogen doped cavities generally show a higher sensitivity to trapped flux than non-doped cavities.”
The cryomodule working group featured different labs presenting designs, assembly procedures and worklines for cryomodule assembly, including quality control. The assembly of cavities into cryomodules is a vital step for each accelerator and the goal is always to closely maintain the quality of each cavity. Hence the focus of these sessions was to improve already existing procedures and workflows. A balance needs to be found between risk minimization, requiring frequent interruptions to test the quality and faster assembly. This ultimately results in fewer tests. “Even though the big picture of the assembly is important, we also learned that the details cannot be overlooked in all areas,” said Elvin Harms, a scientist at Fermilab working on cryomodule testing for LCLS-II. “Quality control and assurance are absolutely critical.”
Further sessions focused on couplers and tuners, vitally important components. Many different tuner and coupler designs are in use and key details from different designs were exchanged to improve future versions. As the name suggests tuners tune the cavity within the cryomodule to the correct frequency, whereas couplers feed power into the cavity. Several vendors of couplers were present at the meeting and actively participated.
The close cooperation between scientists and industry is well established within the field of SRF. All working groups had industry present. Mass production is the only way to build as many cavities, couplers, tuners and cryomodules as are needed for recent and possible future accelerator projects, and therefore the industry must be involved. The first steps were taken with European XFEL, and now LCLS-II and other projects are following suit. “The TTC supports and encourages free and open exchange of scientific and technical knowledge, expertise, engineering designs, and equipment,” its mission states. “It is also encouraging to see a strong cooperation between these two projects, European XFEL and LCLS-II, to help build two of the most advanced particle accelerators the world has ever seen,” said Posen.
During the conference, SLAC organized a tour of its facilities. Usually TTC members visit one of the accelerator facilities to get a closer look at SRF technology in use, but this time was a little different. The TTC members visited the experimental halls of the already running light source LCLS. “It was rather delightful to learn what kind of experiments the beam is being used for, since we put so much effort into creating the beam in the first place,” said Sebastian Aderhold, a postdoc at Fermilab.
In the closing session of the meeting summary talks about each working group were given. Hasan Padamsee gave the closing presentation, saying he was looking forward from the golden era of SRF to possible new accelerators like the ILC which would use this technology. “ILC is the best path to advancing the energy frontier because the technology is highly mature, the Global Design Effort has put together a strong technical design, a world-wide industrial base is established, and the exciting discovery of the Higgs boson at the LHC points the way to improved precision via ILC experiments,” said Padamsee. “The TTC community will continue to push gradients towards 50 MV/m so that one TeV will eventually become possible to upgrade the ILC.”