A Proof-of-Principle Demonstration of 805 MHz High Gradient SRF Cavities
An accelerating gradient of 50 MV/m at an unloaded quality factor (Q0) of 1.4 x 1010 was achieved with a single-cell 805 MHz superconducting radio-frequency (SRF) cavity. This accelerating gradient is more than twice as high as the previous maximum gradient at 805 MHz, and the result demonstrates that 805 MHz cavities, which have a larger beam aperture of 10 cm than 6-7 cm for 1.3 GHz high-gradient cavities, can be used, in principle, as high-gradient cavities for various purposes, especially suited for protons, light ions and other machines that require lower beam impedances due to a larger beam aperture of 10 cm compared to 6-7 cm with 1.3 GHz high-gradient cavities. One remarkable fact is that this cavity was only treated with a usual buffered chemical polishing of ~150 microns and an ultra-high vacuum baking at 115-130 ˚C for 48 hours, and it did not have without high temperature treatment except for a vacuum baking at 115-130 ˚C for 48 hours, and without or any costly electro-polishing, that which has been required for 1.3 GHz cavities to get to very high gradients.
SRF cavities have been used successfully for particle accelerators since 1970s . A large fraction of them are is made of bulk niobium (Nb) due to the fact that it niobium has the highest superconducting transition temperature of ~9.2 K as a single metalamong pure metals. The benefits of SRF cavities compared to normal conducting structures are 1) lower losses due to the intrinsic nature of superconductivity, 2) they can run at higher gradients forin long pulses orand CW operations, and 3) lower beam impedances due to larger beam apertures. However, Nb SRF cavities are fundamentally limited by the RF critical field of Nb, which is thought to be approximately 200 mT. If the ratio of the peak magnetic field to the accelerating gradient Bpk/Eacc, is 4, then the maximum achievable accelerating gradient (Eacc) in Nb SRF cavities is 50 MV/m. The fundamental limitation of Nb SRF cavities, however, is its relatively low achievable accelerating gradient (Eacc) due to the RF critical magnetic field of Nb, which is proportional to Eacc and has been thought to be approximately 200 mT that corresponds to an Eacc of ~50 MV/m if the ratio of peak magnetic field to accelerating gradient, Bpk/Eacc, is 4.
Historically, the design Eacc was 5-10 MV/m until mid 1990s, but withwhen a global effort called TESLA Technology Collaboration (TTC), which started early 1990s aiming at TeV Energy Superconducting Linear Accelerator (TESLA), lead to a significant increase in the achievable Eacc has increased significantly and now the design Eacc is often above has increased to >20 MV/m, e.g., 23.5 MV/m for European X-ray Free Electron Laser (XFEL) and 35 MV/m for the International Linear Collider (ILC). These TTC high-gradient studies have been focused at 1.3 GHz due to the fact that the ILC decided to use...