ON LINE PERFORMANCE AND UPGRADING OF ALPI RESONATORS

The first 44 Pb/Cu, β=0.11 QWRs have been in operation for more than 5 years accelerating ion species ranging from 28 Si to 90 Zr. The resonators have been subject to many thermal cycles without showing any degradation (average Ea = 2.7 MV/m). The possibility to set automatically a He gas pressure of 5x10 -5 mbar inside the cryostat during the RF conditioning, implemented in 1994, has been extensively used to recover and stabilize the resonator performance. Other 12 (β= 0.055 bulk Nb) and 4 (β = 0.13 sputtered Nb/Cu) resonators have been installed. Both Nb and Nb/Cu resonators sustain more than twice the accelerating field of Pb /Cu resonators. The Nb/Cu resonators maintain the excellent frequency stability of Pb resonators. The sensitivity of the Nb cavities to changes of He bath pressure is successfully controlled by a computer program that adjusts continuously the mechanical tuner and an innovative mechanical damper allows their operation without a fast tuner. Two cryostats, housing resonators in which sputtered Nb substitutes the Pb layer, have been installed during this year. Even if some difficulties arose because of the not perfectly suitable copper base, accelerating fields of about 4.5 MV/m could be obtained, thus suggesting a low cost program of ALPI upgrading.