Gonnella, D. Kaufman, J.J. Liepe, M. JACoW Geneva, Switzerland 978-3-95450-178-6 10.18429/JACoW-SRF2015-MOBA03 English 34-36 MOBA03 cavity vacuum niobium impedance SRF Contribution to a conference proceedings 2015 2015-12 http://dx.doi.org/10.18429/JACoW-SRF2015-MOBA03 http://srf2015.vrws.de/papers/moba03.pdf One important characteristic of nitrogen-doped cavities is their very high sensitivity to increased residual surface resistance from trapped ambient magnetic flux. We have performed a systematic study on the losses by trapped flux, and their dependence on the mean-free-path (MFP) of the niobium RF penetration layer. Cavities with a wide range of MFP values were tested in uniform ambient magnetic fields to measure trapped magnetic flux and resulting increase in RF surface resistance. MFP values were determined from surface impedance measurements. It was found that larger mean free paths lead to lower sensitivity to trapped magnetic flux.