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Superconducting niobium film cavities deposited on copper substrates (Nb/Cu)have suffered from strong field-dependent surface resistance, often referred toas the Q-slope problem, since their invention. We argue that the Q-slope maynot be an intrinsic problem, but rather originates from a combination offactors which can be revealed in appropriate environmental conditions. In thisstudy, extrinsic effects were carefully minimized in a series of experiments ona seamless cavity. The origin of the Q-slope in low frequency cavities istraced back to two contributions with different temperature and magnetic fielddependences. The first component of Q-slope, affecting the residual resistance,is caused by trapped magnetic flux which is normally suppressed by a magneticshield for bulk niobium cavities. The second, temperature dependent componentof Q-slope, is similar to the medium-field Q-slope which is well known in bulkniobium cavities. These results are compared with theoretical models andpossible future studies are proposed.

Two different origins of the Q-slope problem in superconducting niobium film cavities for a heavy ion accelerator at CERN