Studies on thin film MgB2 for applications to RF structures for particle accelerators

Abstract

Niobium (Nb) superconducting radio-frequency (SRF) cavities have become the technology of choice for many recent and future particle accelerators. Increasing their accelerating gradient beyond the theoretical limit of Nb (∼50 MV/m) by coating multilayer thin film superconductors is our ultimate goal. This idea proposed by Gurevich in 2005 is based on the assumption that the lower critical magnetic field parallel to the surface (Bc1//) increases with very thin films having a thickness close to its magnetic penetration depth. We measured the magnetic field Bpen at which a large number of vortices or fluxons start to penetrate into the material using a SQUID magnetometer. While we found that the measurements for the films thinner than ∼200 nm is difficult due to insufficient signal strength and alignment accuracy, we also found that even the films with a thickness a few times the penetration depth show significant increase in Bpen, e.g., ∼135 mT and ∼210 mT for 500 nm and 300 nm films at 4.5 K, respectively, as compared to ∼46mT for bulk MgB2. RF measurements of 50.8 mm diameter MgB2(100 nm)/Al2O3(20 nm)/Nb sample using 11.4 GHz pulsed power and a TE013-mode cavity showed quenches at ∼42 mT at 4 K and ∼34 mT at 10 K. These results were significantly lower than the numbers predicted from the low-power DC measurement results mentioned earlier, but these were found to be thermal quenches due to a high RF surface resistance caused by the inter-diffusion of coated components. © 2012 American Institute of Physics.