Temple University, Philadelphia, USA
LANL, Los Alamos, New Mexico, USA
Since SRF2013, there has been a remarkable progress in terms of sample measurement. Instead of measuring a flat film that allows magnetic field on both sides of the film, which does not simulate the situation on a SRF cavity correctly, an ellipsoidal bulk Nb (rugby-ball shape with ~8 mm long axis) was coated with a MgB2 film and its vortex penetration field has been measured with a SQUID magnetometer and compared with uncoated samples. After a number of measurements, vortex penetration field has been consistent with maximum critical RF field, superheating field. Here, we show that 100 nm and 200 nm thick MgB2 coating increases the vortex penetration field by up to ~70 mT, e.g., 240 mT (200 nm MgB2 coated Nb) vs. 170 mT (uncoated Nb) at 2.8 K (lowest measurement temperature) with the trend of increasing as temperature goes down. This is consistent with recent theoretical development saying that the increase is possible even without an insulation layer, which makes the coating easier. In this talk, the thickness dependence of the rise and comparison with theory will be shown.
