STFC/DL/ASTeC, Daresbury, Warrington, Cheshire, United Kingdom
Cockcroft Institute, Warrington, Cheshire, United Kingdom
ODU, Norfolk, Virginia, USA
Lancaster University, Lancaster, United Kingdom
Cockcroft Institute, Lancaster University, Lancaster, United Kingdom
Superconducting RF cavities made of bulk Nb has reached a breakdown field of about 200 mT which is close to the superheating field for Nb. As it was theoretically shown* a multilayer coating can be used to enhance the breakdown field of SRF cavities. The simple example is a superconductor-insulator-superconductor (S-I-S), for example bulk niobium (S) coated with a thin film of insulator (I) followed by a thin layer of a superconductor (S) which could be a dirty niobium**. To verify such an enhancement in a presence of a DC magnetic field at 4.2 K a simple experimental facility was designed, built and tested in ASTeC. The details of experimental setup and results of the measurements will be shown at the conference.
*A. Gurevich, APL 88, 012511 (2006)
**A. Gurevich, AIP Advances, 5, 017112 (2015)
ODU, Norfolk, Virginia, USA
University of Wisconsin-Madison, Madison, USA
JLab, Newport News, Virginia, USA
Results of RF tests of Nb3Sn thin film samples related to the superconducting multilayer coating development are presented. We have investigated thin film samples of Nb3Sn/Al2O3/Nb with Nb3Sn layer thicknesses of 50 nm and 100 nm using a Surface Impedance Characterization system. These samples were measured in the temperature range 4 K-19 K, where significant screening by Nb3Sn layers was observed below 16-17 K, consistent with the bulk critical temperature of Nb3Sn.
WEA1A01
ODU, Norfolk, Virginia, USA
The talk gives a theoretical overview of recent progress in our understanding of the field dependence of the SRF surface resistance Rs(H). A theory of microwave suppression of Rs(H) in the Meissner state of alloyed superconductors under a low-frequency, strong RF field at low temperatures is presented*. Solving equations for Rs(H) derived from a non-equilibrium BCS theory in the dirty limit shows that Rs(H) has a minimum as a function of H, mostly due to field-induced temporal oscillations of the quasiparticle density of states. The calculated field dependence Rs(H) is in good agreement with recent experiments on Ti and N alloyed Nb resonator cavities. The theory suggests that the extended Q(B) rise can be further enhanced by reducing subgap density of states by materials modifications. Applying superimposed dc and ac fields can be used to reduce the rf dissipation by tuning Rs(H0) with the dc field and also to probe the mechanism of microwave suppression of Rs(H) under equilibrium conditions. Ways of increasing the SRF performance by an optimized dirty layer at the Nb surface, which can withstand magnetic field exceeding the superheating field of Nb is discussed **.
*A. Gurevich, PRL, 113, 087001 (2014).
**A. Gurevich, AIP Advances, 5, 017112 (2015)
