by Dr. Ram C. Dhuley Staff Engineer, Michael Geelhoed Engineering Physicist, and Aaron Sauers Patent and Licensing Agent. All are from the Illinois Accelerator Research Center at Fermi National Accelerator Laboratory, a CSA CSM.
Particle accelerators have been essential tools of particle physics since initial development in the early twentieth century, with commercial applications soon following each successive advancement. Particle accelerators today are found in diverse use, ranging from ion implantation of consumer electronics to both imaging and treatment of cancers.
Superconducting radio frequency (SRF) cavities are the workhorse of modern-day scientific linear particle accelerators. Developers of large-scale science machines often strive to push SRF cavities to higher and higher accelerating gradients, but many industrial applications can take advantage of the high wall power efficiency and continuous wave capability of SRF for medium gradients (~10 megavolts/meter or MV/m).
However, transitioning SRF technology to the commercial linac market has its own set of challenges. Niobium cavities—with superconducting transition temperature of ~9.3 K—require operation at liquid helium temperatures to sustain superconductivity. These cavities are operated in a bath of liquid helium (He II) at 2 K to reduce the heat dissipation, in turn reducing the cryogenic load on the liquid helium plant.
But operation of a liquid helium plant and the accompanying cryomodule infrastructure isn’t feasible for industrial settings. The large liquid-to-gas volumetric expansion ratio of helium carries the risk of overpressurization-related accidents and asphyxiation.The liquid helium installation requires a large helium inventory and an efficient recovery system, making the accelerator a permanent fixture rather than a mobile device. Moving away from liquid helium operation is therefore a crucial step toward deploying the SRF technology to industrial accelerators.
Growing a thin layer of niobium-tin (Nb3Sn) on the RF surface of a bulk niobium cavity increases the acceptable operating temperature of the cavity  because the Nb3Sn RF surface has a superconducting transition temperature of nearly 18 K.
Moreover, Nb3Sn coated cavities can sustain gradients of 10 MV/m while dissipating comparable heat that bulk niobium cavities would dissipate at the same gradient in 2 K He II. Thus, a meter-long chain of Nb3Sn coated cavities will dissipate only a few watts of heat at 4.2 K while realizing gradients as high as 10 MV/m. This configuration can produce electron beams of 10 MeV energy.
The Illinois Accelerator Research Center (IARC) team at Fermilab is pioneering an exciting method of operating Nb3Sn coated cavities via closed-cycle 4 K cryocoolers. The novel approach eliminates liquid helium by conductively coupling the cavities to the cryocooler . A test cavity is depicted in Figure 1, wherein a high-purity aluminum thermal link connects a niobium cavity  to a pulse tube cryocooler. The concept of an electron-beam accelerator based on conduction cooling is illustrated in Figure 2.
The 4 K system can be housed in a simple vacuum vessel rather than a complex liquid helium pressure vessel. A high-permeability magnetic shield inside the vacuum vessel, running at ambient or cryogenic temperatures, reduces the magnetic background to less than 10 milliGauss.
The requisite thermal shield is coupled to the first stage of the cryocooler to cut down on the ambient-to-4 K radiation heat load. This simple cryostat yields a mobile, completely cryogen-free (no liquid helium or liquid nitrogen) SRF accelerator system ideal for deploying SRF technology to industrial accelerators.
Conduction cooling of SRF cavities is an enabling feature of IARC’s compact SRF accelerator, a mobile and high-powered industrial electron linac under heavy development. Because it is compact, yet powerful and free of liquid cryogens, this new class of SRF accelerators enables new industrial applications for linacs.
These applications include special purpose coatings, thick film coatings for a variety of industrial applications, in situ environmental remediation, sludge and wastewater treatment, medical device sterilization, food preservation and additive manufacturing.
 S. Posen, M. Liepe and D.L. Hall, “Proof-of-principle demonstration of Nb3Sn superconducting radiofrequency cavities for high Q0 applications,” Applied Physical Letters Vol. 106, 2015.
 R. Kephart, US Patent No. US 9,642,239 B2. Washington, DC: US Patent and Trademark Office, 2017.
 R.C. Dhuley, M.I. Geelhoed and J.C.T. Thangaraj, “Thermal resistance of pressed contacts of aluminum and niobium at liquid helium temperatures,” Cryogenics, Vol. 93, 2018. ■