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.
The College of William and Mary, Williamsburg, USA
JLAB, Newport News, Virginia, USA
LANL, Los Alamos, New Mexico, USA
The Surface Impedance Characterization (SIC) system in Jefferson Lab can presently make direct calorimetric RF surface impedance measurements on the central 0.8 cm2 area of 5 cm diameter disk samples from 2 to 20 K exposed to RF magnetic fields up to 14 mT at 7.5 GHz. MgB2 thin films from STI/LANL were deposited on 5 cm diameter Nb disks using reactive evaporation technique. We will report the results of measurements on these samples using the SIC system. The data will be interpreted based on BCS theory as the temperature-dependent properties suggest evaluation of the Tc, energy gap, penetration depth, mean free path and coherence length.
