Accelerator physics: alternative material investigated for superconducting radio-frequency cavity resonators
The photomontage shows a sample of solid, pure niobium before coating (left), and coated with a thin layer of Nb3Sn (right). © HZB
In modern synchrotron sources and free-electron lasers, superconducting radio-frequency cavity resonators are able to supply electron bunches with extremely high energy. These resonators are currently constructed of pure niobium. Now an international collaboration has investigated the potential advantages a niobium-tin coating might offer in comparison to pure niobium.
At present, niobium is the material of choice for constructing superconducting radio-frequency cavity resonators. These will be used in projects at the HZB such as bERLinPro and BESSY-VSR, but also for free-electron lasers such as the XFEL and LCLS-II. However, a coating of niobium-tin (Nb3Sn) could lead to considerable improvements.
Coatings may save money and energy
Superconducting radio-frequency cavity resonators made of niobium must be operated at 2 Kelvin (-271 degrees Celsius), which requires expensive and complicated cryogenic engineering. In contrast, a coating of Nb3Sn might make it possible to operate resonators at 4 Kelvin instead of 2 Kelvin and possibly withstand higher electromagnetic fields without the superconductivity collapsing. In the future, this could save millions of euros in construction and electricity costs for large accelerators, as the cost of cooling would be substantially lower.
Experiments in the USA, Canada, Switzerland and HZB
A team led by Prof. Jens Knobloch, who heads the SRF Institute at HZB, has now carried out tests of superconducting samples coated with Nb3Sn by Cornell University, USA, in collaboration with colleagues from the USA, Canada, and Switzerland. The experiments took place at the Paul Scherrer Institute, Switzerland, at TRIUMF, Canada, and the HZB.
“We measured the critical magnetic field strengths of superconducting Nb3Sn samples in both static and radio-frequency fields”, says Sebastian Keckert, first author of the study, who is doing his doctorate as part of the Knobloch team. By combining different measurement methods, they were able to confirm the theoretical prediction that the critical magnetic field of Nb3Sn in radio-frequency fields is higher than that for static magnetic fields. However, the coated material should display a very much higher critical magnetic field level in a radio-frequency field. Thus, the tests have also shown that the coating process used currently for the production of Nb3Sn might be improved upon in order to more closely approach the theoretical values.
The publication has been mentioned on the Cover of „Superconductor Science and Technology“ , (2019): Critical fields of Nb3Sn prepared for superconducting cavities; S. Keckert, T. Junginger, T. Buck, D. Hall, P. Kolb, O. Kugeler, R. Laxdal, M. Liepe, S. Posen , T. Prokscha, Z. Salman, A. Suter and J. Knobloch.
doi:10.1088/1361-6668/ab119e
- Ultrafast dissociation of molecules studied at BESSY IIFor the first time, an international team has tracked at BESSY II how heavy molecules – in this case bromochloromethane – disintegrate into smaller fragments when they absorb X-ray light. Using a newly developed analytical method, they were able to visualise the ultrafast dynamics of this process. In this process, the X-ray photons trigger a "molecular catapult effect": light atomic groups are ejected first, similar to projectiles fired from a catapult, while the heavier atoms - bromine and chlorine - separate more slowly.
- Protons against cancer: New research beamline for innovative radiotherapiesTogether with the University of the Bundeswehr Munich, the HZB has set up a new beamline for preclinical research. It will enable experiments on biological samples on innovative radiation therapies with protons.
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.