LANL, Los Alamos, New Mexico, USA
SLAC, Menlo Park, California, USA
KEK, Ibaraki, Japan
NIMS, Tsukuba, Ibaraki, Japan
STI, Santa Barbara, California, USA
ANL, Argonne, USA
TU, Philadelphia, USA
JLAB, Newport News, Virginia, USA
Demonstrating the idea of enhancing achievable surface magnetic field by coating multilayer thin film superconductors proposed by Gurevich is the main objective. DC magnetization measurements of 500 nm and 300 nm MgB2 films coated on Sapphire showed an increase in the lower critical magnetic field (Bc1) compared to that of bulk. Also, the Bc1 of a 300 nm film showed >200 mT at 4.5 K, which is >25 % higher than that of Nb (~145 mT). RF measurements using a 11.4 GHz pulsed Klystron and a TE013-like mode hemispherical copper cavity with a 2-inch (50.8 mm) diameter sample, however, have shown a low quenching field of 42 mT at 4 K. From detailed data analyses together with the data on Nb quench fields these quenches were found to be thermal, not magnetic, due to a high RF resistance caused by inter-diffusion of coated materials at the interfaces. Additionally, recent results of RF surface resistance at 7.5 GHz using a calorimetric technique at JLab will also be shown.
NSWC, West Bethesda, Maryland, USA
STI, Santa Barbara, California, USA
MIT, Lexington, Massachusetts, USA
We have explored magnesium diboride films for applications in SRF cavities. MgB2 has a high Tc = 40 K and low surface resistance that, even in polycrystalline films, is comparable to niobium. It also shows the potential for higher power handling than niobium because of the higher critical fields. We report the results of measurements of the surface resistance and power handling in films deposited by the reactive-evaporation method. The measurements were made using both a stripline-resonator at 2 GHz and with a dielectric resonator at 10.7 GHz. The best results for surface resistance are 14 micro-ohm at 10.7 GHz, which scales to 0.5 micro-ohm at 2 GHz. The maximum rf magnetic field of the best film has been measured to be 300 Oe, limited by the available amplifier. We have also demonstrated a successful surface-passivation method of atomic-layer-deposited (ALD) films of Al2O3 and ZrO2. We have also demonstrated evidence that one of the two energy gaps shows unconventional symmetry with a six-fold-symmetric nodal order parameter. The implications for applications of these findings of the basic physics of the material will be discussed.