INFN/LNL, Legnaro
The average accelerating field of the ALPI 160 MHz sputtered QWRs has been improving with time up to reach, after the last conditioning cycle, the average accelerating field of 4.8 MV/m @ 7 W. Such value can be effectively sustained in operation due to the intrinsic mechanical stability of the sputtered cavity whose frequency is practically not influenced by fluctuations in the bath He pressure. The present average cavity performance approaches the maximum average accelerating field obtainable in the presently installed cavities, most of which were produced by replacement of Pb with Nb in the previously installed substrates. A higher average value can be obtained in ALPI replacing the less performing units; it is instead necessary to sputter on appropriately built substrates to produce QWRs which can reliably exceed 6 MV/m @7W. The cavity Q-curves, which were recently measured in ALPI, show a wide range of Q0 and Q-drop, mainly associated with the substrate characteristics, but in some cases also influenced, as discussed in the paper, by cryostat assembling procedures and by cavity production and conditioning.
