Accelerator Physics: Alternative Material Tested For Superconducting High Frequency Cavities

Superconducting high frequency cavities can provide electron packets in modern synchrotron sources and free electron lasers with extremely high energy. At the moment they are made of pure niobium. An international cooperation has now investigated the benefits of coating with niobium-tin compared to pure niobium.

At present, niobium is the material of choice for building superconducting high frequency cavities. They are used for projects such as bERLinPro and BESSY-VSR, but also for free electron lasers such as the XFEL or the LCLS-II.

Coating promises savings
But a coating with niobium tin (Nb 3 Sn) could lead to significant improvements. For superconducting high-frequency cavities made of niobium must be operated at 2 Kelvin (-271 degrees Celsius), which requires elaborate cryogenics. On the other hand, by coating with Nb 3 Sn, cavities could also be operated at 4 instead of 2 Kelvin, and could possibly withstand higher electromagnetic fields without the superconductivity breaking down. In the future, this could save millions of euros in construction and electricity costs for large accelerators, since the cost of cooling is significantly lower.

Experiments in USA, Canada, Switzerland and HZB
A team around Prof. dr. Jens Knobloch, who heads the SRF Institute at HZB, has now carried out tests with superconducting samples in collaboration with colleagues from the USA, Canada and Switzerland, which were coated with Nb 3 Sn at Cornell University, USA . The experiments took place at the Paul Scherrer Institute, Switzerland, at the TRIUMF, Canada, and at the HZB.

Coated sample is more effective
"We measured the critical magnetic field strengths of superconducting Nb 3 Sn samples in static and high-frequency fields," says Sebastian Keckert, lead author of the study, a PhD student in the Knobloch team. By combining different methods of measurement, they were able to confirm the theoretical prediction that the critical magnetic field of Nb 3 Sn in radio-frequency fields is higher than that for static magnetic fields. However, the coated material in the high frequency field should still have a much higher critical magnetic field.

Thus, the tests have also shown that the currently used coating process for the production of Nb 3 Sn could be further developed to come even closer to the theoretical values.