Fermilab attains unprecedented quality factor for superconducting cavity

Fermi National Accelerator Laboratory (Fermilab, CSA CSM) announced on November 6 that researchers had recorded a record-high quality factor while performing integrated tests on a new superconducting radio frequency cavity for the SLAC-headed Linac Coherent Light Source II (LCLS-II) project. Quality factor (Q) is a measure of a particle acceleration cavity’s efficiency. A higher Q means a cavity is losing less energy and is therefore more cost-effective.

Fermilab is responsible for designing, developing, building and testing about 150 nine-cell cavities for the LCLS-II superconducting accelerator. The R&D process began over a year ago and includes criteria ensuring the cavities meet certain Q values during testing. The fully dressed 1.3-gigahertz cavity’s quality factor during integrated testing was 3.1 x 1010 at 2K and at a 16-megavolt-per-meter peak surface electric field. This Q exceeds LCLS-II’s goal of 2.7 x 1010 and far surpasses current state-of-the-art standards, according to a statement issued by Fermilab.

“This is the first integrated test we did,” said Nikolay Solyak, project support group leader. In an integrated test, everything is checked under real conditions. A fully dressed cavity is outfitted with all the components it will wear in the LCLS-II accelerator. This includes a titanium jacket filled with liquid helium chilled to 2K, a superconducting temperature for the niobium used inside cavities. The cavity is also furnished with power-providing couplers, cavity-squeezing tuners to control frequency and magnetic shielding.

Dubbed nitrogen doping, the new technique involves infusing nitrogen into a cavity’s inner niobium surface. Nitrogen doping and other Fermilab discoveries that led to this Q value, such as the removal of magnetic flux through rapid cooling, could become the new standards for achieving highly efficient accelerators worldwide.

“This record Q is really the sum, the final point, of many years of research,” said Anna Grassellino, a Fermilab Technical Division scientist who leads cavity testing and processing for LCLS-II. “It’s really a miracle of science and technology and engineering coming together and producing an unprecedented quality factor. It opens up a way for machines to operate much more efficiently at a much lower cost.”

The scientists will next test to show that a full cryomodule with eight cavities meets specifications. SLAC physicist Marc Ross, LCLS-II cryogenics systems manager, said he’s pleased with the results coming out of Fermi. “It’s definitely a victory. These are some of the highest-quality-factor practical resonators ever built.”