C-band superconducting structure for a electron linac

We have built a 5-cell superconducting structure, for the main section (β=1) of an e- linear accelerator, forming half cavities, "door-bell" shaped, by deep drawing a 2 mm thick niobium sheet (Kawecki). Running the structure in the π mode, (the accelerating one) accelerating field in excess of 7 MV/m was achieved at a fairly low Qovalue of 3× 10⁸. We report a comparison between the results obtained in a single cell cavity of the same shape ( mT) and that ones previously obtained in the 1mm thick cavities operating at the same frequency (BP= 105 mT). We introduce a simple model for the heat transfer to He bath which explain the worse behaviour of the low Qocavities.