A team at Cornell University in the US has demonstrated that high-frequency superconducting radio-frequency (SRF) cavities made from niobium–tin alloy can be operated more efficiently than conventional niobium designs, representing a step towards smaller and more economical particle accelerators.
SRF cavities are the gold standard for the acceleration of charged-particle beams and are used, for example, in the LHC at CERN and the upcoming LCLS-II free-electron-laser X-ray source at SLAC. Currently, the material of choice for the best accelerating cavities is niobium, which frequently has to be operated at a temperature of around 2 K and requires costly cryogenic equipment to cool the cavity in a bath of superfluid liquid helium. The technology is only heavily used at large-scale accelerators, and not at smaller institutions or in industry due to its complexity and costs.
Researchers around the world are striving to remove some of the barriers prohibiting broader uptake of SRF technology. Two major obstacles still need to be overcome to make this possible: the temperature of operation, and the size of the cavity.
Earlier this year, a team at Cornell led by Matthias Liepe demonstrated that small, high-frequency triniobium-tin (Nb3Sn) cavities can be operated very efficiently at a temperature of 4.2 K. While seemingly only slightly warmer than the 2 K required by niobium cavities, this small rise in temperature omits the need for superfluid-helium refrigeration.
The size of the cavity is inversely related to the frequency of the oscillating radio-frequency electromagnetic field within it: as the frequency doubles, the necessary transverse size of the cavity is halved. A smaller cavity with a higher frequency also demands a smaller cryomodule; what was once 1 m in diameter, the typical size of an accelerating SRF cryomodule, can now be roughly half that size.
The vast majority of SRF cavities currently in use operate at frequencies of 1.5 GHz and below – a region favoured because RF power losses in a superconductor rapidly decrease at lower frequency. But this results in large SRF accelerating structures. Cornell graduate student Ryan Porter successfully made and tested a considerably smaller proof-of-principle Nb3Sn cavity at 2.6 GHz with promising results. “Niobium cannot operate efficiently at 2.6 GHz and 4.2 K,” Porter explains. “But the performance of this 2.6 GHz Nb3Sn cavity was just as good as the 1.3 GHz performance. Compared to a niobium cavity at the same temperature and frequency, it was 50 times more efficient.”
“This is really the first step that shows that you can get good 4.2 K performance at high frequency, and it is quite promising,” adds Liepe. “The dream is to have an SRF accelerator that can fit on top of the table.”