Liquid helium-free SRF cavities can make industrial use practical


The Copper / Niobium / NiobiumTin cavity is connected to the cryocooler. Credit: US Department of Energy.

The building blocks of superconducting accelerators are superconducting radio frequency (SRF) cavities made primarily of niobium, which are combined in a vessel and immersed in liquid helium to achieve superconducting temperatures. Although a large plant for the production of liquid helium may be useful for a large research facility, it can become an obstacle to new applications of this accelerator technology.


At present, advances in cavity technology, materials and cryocooler development can reduce this barrier for industrial and medical applications of SRF technology. After completing more than 5,000 cavity tests in a vertical test zone (VTA) using liquid heliumThis year, a team from the research and development department of the SRF Institute in Jefferson's laboratory first cooled and successfully tested the SRF cavity in one of the VTA's vertical cryostats without any liquid helium.

How was this achieved? A number of ongoing initiatives have come together to make this possible.

The first critical component is the use of a cryocooler to cool the SRF cavity. Cryocooler is a closed-cycle refrigerator that requires only a small amount of gaseous helium, and it has several advantages – ease of use, compactness, reliability and commercial finished products. Cryocoolers are already used to cool superconducting magnets in magnetic resonance MRI devices in hospitals, and there is currently an industrial interest in accelerator Developing technology, Jefferson Lab was interested in further developing SRF technology to meet this need.

New coatings, new features

The next element was advances in the use of the niobium-tin compound Nb.3Sn having a higher superconducting transition temperature for SRF cavities. Jefferson Lab Develops High Performance Nb3Sn cavities since 2013, based on the work of Grigory Yeremeyev, who received the Ministry of Energy's 2016 Early Career Award. The key advantage offered by these niobium and tin cavities is that they remain superconducting at twice the temperature needed for accelerating cavities of pure niobium and can work more efficiently at higher temperatures than niobium. Using this technology can provide significant operational cost savings for future accelerators. Research at Jefferson Labs Delivers Excellent Nb Quality3Thin film coatings of Sn on several types of SRF resonators. The specific volume cavity Nb with a frequency of 1.5 GHz, on which Nb is located3Sn film was grown, was selected for integration with cryocooler.

When using a cryocooler, the surface of the cavity is not directly cooled by liquid helium, which makes the cavity more susceptible to thermal damage, especially in the presence of defects. Therefore, the outer surface of the cavity was coated with a high-purity copper layer several millimeters thick. Copper (Cu), which has a higher thermal conductivity than Nb, enhances heat transfer to the cryo-cooler. This was achieved by applying a layer of Cu to the cavity using standard methods from a commercial supplier.

The Jefferson Lab team then designed and built a test bench containing a cavity and a cryo-cooler to fit into one of the existing VTA cryostats serving as a vacuum test vessel. The RF test results were close to those measured in liquid helium. “We were able to reach the peak of the surface a magnetic field 29 mT, which corresponds to an accelerating gradient of 6.5 MV / m, and we could operate the cavity at 5 W of dissipated power without any thermal instability, ”says Gigi Ciovati, the accelerator scientist conducting this study. These results are similar to those recently achieved at Fermilab using a different conductivity cooling installation.

Industrialization of SRF technology

What is the significance of this work? While maintaining and operating a liquid helium cryoplant for operating with SRF cavities is standard in a national laboratory such as Jefferson Lab for companies engaged in industrial or medical applications of effective SRF technology, this is a serious obstacle. One such application is a low energy high power electron accelerator for wastewater or flue gas treatment. Already at Jefferson Lab, one such accelerator based on the cooled conductivity of the SRF cavity was developed (G. Ciovati et al., Phys. Rev. Accel. Beams 21, 091601 (2018)), and the experimental results achieved both at Jefferson Lab and at Fermilab, they put the design on a much more solid basis.

“The next step over the next two and a half years is the demonstration that we can achieve a peak surface field corresponding to an increase in energy of 1 MeV, the beam energy needed for the accelerator to restore the environment that we developed in SRF with cooling conductivity cavity inside the horizontal cryomodule, ”says Chiovati, who received a grant from the DOE Accelerator Stewardship program to carry out this work. The industry will actively participate in this project, and the final RF test will be conducted at General Atomics, industry partner of Jefferson Lab.


Cryocooler cools the cavity of the accelerator


citation:
Liquid helium-free SRF cavities can make industrial use practical (March 13, 2020)
retrieved March 13, 2020
from https://phys.org/news/2020-03-liquid-helium-free-srf-cavities-industrial.html

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