In the light of new accelerators

Advancing superconducting cavities for present and future projects

| 22 July 2016

SRF technology in use around the world. Slide by Akira Yamamoto taken from Hasan Padamsee's closing talk.

SRF technology in use around the world. Slide by Akira Yamamoto taken from Hasan Padamsee’s closing talk.

At the TTC meeting (TESLA Technology Collaboration meeting) scientists from all over the world came together to present their advances for superconducting radiofrequency cavities and related topics. This summer meeting from 5 to 8 July 2016 was hosted by CEA in Saclay, near Paris in France, where the production of the European XFEL (X-ray free electron laser) cryomodules just finished. TTC remains a meeting were nuts and bolts, measurements and problems and many other hands-on topics are discussed openly between physicists, engineers and everyone else involved with the technology.

The meeting was opened by a talk from Hasan Padamsee, Chair of the TTC, who dedicated the talk to Helen Edwards, a recently deceased pioneer accelerator developer who worked at and with many laboratories like Fermilab and DESY. “Helen Edwards was a leader in the design, construction, commissioning and operation of the Tevatron. She pressed the start and shutdown buttons of this machine”, said Padamsee. “She made significant contributions to the TESLA technology as an active member of the TTC.”

The talk summarised the highlights of superconducting radio frequency (SRF) in already running projects: Big accelerator machines like the European XFEL at DESY, LCLS-II (Linac Coherent Light Source II) at SLAC, STF2 (Superconducting Test Facility 2) at KEK  and many more all around the planet are using SRF technology. Padamsee showed important developments from all of them, ranging from test results of the cavities to module assemblies. “The European XFEL needed 800 cavities for 100 cryomodules; many ongoing – already approved – projects like LCLS-II, FRIB, RAON, ESS, PIP-II and many more will require a further 1200 cavities for more than 200 cryomodules,” summarised Padamsee. “This means our community will be busy for at least the next decades. And this calculation does not include the projects which are still seeking for approval. We are in for a lot of fun and work!” He also hinted that his closing talk would be dedicated to future projects like the ILC.

A number of plenary talks followed about the status updates from ongoing projects. Here the progress and especially lessons learned from all these projects for future projects were important messages. The challenges and difficulties the projects had overcome and how they had handled them, were always part of the talks. “This is the spirit of TTC. It is not about the accomplishments alone. It is about understanding the problems and being open even across the whole planet,” explains Akira Yamamoto, Asian Director for the ILC. “This is also why this community advances so fast together – from project to project.”

Especially for the ILC this meeting had many highlights to offer. Nick Walker, scientist at DESY, showed a statistical analysis of European-XFEL cavity production comparing it to the ILC requirements. The ILC requirements in vertical test for cavities were defined in the Technical Design Report (TDR): an average accelerating gradient of 35 megavolts per metre (MV/m) and an assumed yield of 75% in the first pass. For the second pass – which means after an additional surface cleaning – the yield should be 90% with the same average of 35MV/m. The so called ILC recipe, which describes the surface treatment process, was used to build half of the European XFEL cavities. Nick Walker used the results from those cavities to extrapolate to a full-size ILC cavity production. “For the maximum gradient we got a yield of 94% with an average of 35 MV/m – for the first and second pass – which is better than we need,” explained Walker. “If we look at the useable gradient – meaning considering field emission, quality factors and so on – we have a yield of 82% and an average of 33.4 MV/m. Well, not perfect but really close. This is a very encouraging result.”

Fermilab presented new results for an improved nitrogen treatment of cavities. LCLS-II uses nitrogen doping to enhance the quality factor and therefore reduce the cryogenic costs of the machine. One price to be paid for this enhancement is a lower accelerating gradient which makes it not yet suitable for the ILC. This could change dramatically with the new “nitrogen infusion” technique Fermilab presented. Instead of doping the cavity with nitrogen gas at 800 degrees Celsius, the new infusion technique combines the exposure to nitrogen with the relatively mild 120-degree bake. This means that the nitrogen atoms penetrate the niobium less deep and only the first few nanometers below the surface are enriched with nitrogen atoms. What is more, the heat treatment is done in a way that no further surface chemistry needs to be done before the vertical test, which is usually necessary. “We have infused a number of different one-cell cavities with the result that each of them reached above 35MV/m and four out of five went up to 45 MV/m with a Q-factor of 2×1010.” explains Sebastian Aderhold. “So far we are still working on improving this technique to understand its possibilities and limits.” The big question after the talk was from Akira Yamamoto: Could this reduce the cost for the ILC? The answer to this is: possibly. If this technique could be well matured for TESLA-shape nine-cell cavities and if one chemistry step can be omitted, this might reduce the costs. So far it is hard to say by how much. Attempts to understand the infusion benefits were also presented.

The closing words for the meeting were given again by Hasan Padamsee about the future of the field. He also ventured into the topic of communication and motivated the need for outreach for the ILC. The physics case needs to be explained to the public and the benefits should be made clear to everyone. He finished with an overview of the future projects and their needed numbers of cavities and cryomodules: more than 1000 cavities for roughly 150 cryomodules are needed for projects which are seeking approval at the moment. “And these numbers don’t include the ILC and the FCC which would mean – both together – another 23000 cavities”, said Padamsee. “I guess SRF continues to be needed in the future and so TTC expertise and continuous exchange.”

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