Cold Copper Accelerator Technology and Applications Workshop.
The first ICFA Beam Dynamics workshop on Cold Copper Accelerator Technology and Applications was held at Cornell University from 31 August to 1 September 2023. Nearly 100 people came together to discuss the technology and explore next directions for R&D. Originally conceived at SLAC as an attractive approach to a linear-collider Higgs factory (dubbed the Cool Copper Collider, C3), interest in the technology has expanded to other areas.
Following opening presentations by Julia Thom-Levy (Cornell associate vice provost for research and innovation) and Jared Maxson (who leads the cold copper programme at Cornell), Emilio Nanni (SLAC) presented an overview of radio-frequency (RF) breakthroughs using cold copper cavities. He described three major advantages over conventional materials such as superconducting niobium: increased material conductivity at cryogenic temperatures (a reduction in resistance by a factor of three), significant reduction in pulsed heating, and improved yield strength and thermal diffusion. Combined, these lead to a high potential acceleration gradient of 70–120 MV/m, and an estimated 8 km footprint for a 550 GeV Higgs factory.
The optimised C-band cavity design enables a novel coupling of RF signals into each of the 40 cells along the cavity. A 9 m-long cryomodule would provide 1 GeV of acceleration. Some challenges identified for future R&D in the coming years are vibration control, meeting linac alignment specifications of 10 microns, and reducing the cost via optimised RF. Other applications of cold-copper technology include an ultra-compact free-electron laser (FEL) with 10–100 fs timing resolution as well as synergies with other proposed colliders such as ILC and FCC, where it could be used for positron production or as an injector, respectively. Walter Wuensch (CERN) summarised the extensive work over the past two decades on high-field limitations to copper performance. Breakdowns, field emission current and pulsed heating are fundamental limitations to performance, along with some practical ones such as limited RF power, conditioning time, small-aperture requirements, wakefields, power feeds and cooling capacity. Wuensch concluded that the community has a reasonably good understanding of copper, but that the demands for higher gradients and more performant cavities require careful optimisation.
The accelerator R&D community has a reasonably good under-standing of copper, but the demands for higher gradients and more performant cavities require careful optimisation
The workshop also delved into the details of cryomodule design, fabrication and damping, as well as the progress of relevant developments at LANL and INFN Frascati. Numerous industry participants gave presentations, including researchers from Radiabeam, Scandinova, Canon, EEC Permanent Magnets and Calabazas Creek.
Day two started with Caterina Vernieri (SLAC) presenting the C3 ambition for a Higgs factory based on extensive, recently published studies. Jamie Rosenzweig (UCLA) presented the design for an ultra-compact FEL and Paul Gueye (Michigan State) provided an overview of a potential high-gradient linac at the Facility for Rare Isotope Beams. Sami Tantawi (SLAC) presented potential medical applications of the technology, aimed at FLASH and very-high-energy-electron treatment modalities. Xi Yang (BNL) reviewed ultrafast electron diffraction devices and how moving from keV to MeV energies using compact copper accelerators could open new research opportunities. A session devoted to sustainability at CERN was covered by Maxim Titov (CEA Saclay), while Sarah Carsen (Cornell) presented the renewable programme at Cornell, which includes lake-source cooling of the campus and CESR accelerator complex, 28 MW of installed solar power, as well as geothermal plans. The successful mini-workshop concluded with a request to complete a report summarising the R&D discussions and post them on the Indico workshop site.
The accelerator R&D community awaits the P5 report (see p7) and the resulting strategies of the Department of Energy and National Science Foundation for accelerator research over the next decade.
