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.
INFN/LNL, Legnaro
Recently we developed a novel measurement apparatus that simplify the tests of superconducting cavities. A few commercial electronic boards, mounted in a devoted chassis and controlled by a PC, operate most of the functions usually carried out by standard RF instrumentation. The set up allows the measurements of resonators in the 80-700 MHz frequency range and we used it to characterize resonators both in the ALPI vault and in off-line tests. Upgraded control program carries out all the typical procedures, related to the cavity measurements in classical VCO-PLL system. It allows to adjust and to measure the RF forward power, to find and update the cavity resonant frequency, to calibrate the pick-up signal, to monitor the transmitted power, to adjust the coupler position. The implemented automatic procedures permit to measure the cavity decay time, to trace the Q-curve, to perform CW and pulse RF conditioning, to calibrate cables and measurement instruments. The same software applies to the other two measurement systems routinely used at Legnaro to test resonators up to 6 GHz frequency.