Title: Intrinsic limitations of Niobium under RF
Author: Tobias Junginger
Abstract: Calorimetric measurements on bulk niobium samples aim to reveal its intrinsic limitations for RF accelerator application. We tested the surface resistance vs. the RF magnetic field at 400 and 1200 MHz for the same sample and cooldown cycle. The ambient DC magnetic field was less than 0.1 ?T. The surface resistance shows an increase of 50 % when the field level is raised from 0 to 35 mT, independent of frequency and temperature. This is consistent with measurements on film cavities at 1500 MHz. We show that the residual surface resistance depends on frequency. The maximum field under RF exposure is consistent with the superheating field and does not depend on frequency. Our device has been commissioned using bulk niobium samples and studies of thin film samples are already being prepared.