ANL, Argonne
Microphonics measurements have been performed on the recently commissioned ATLAS upgrade cryomodule which holds seven new beta=0.145 quarter-wave cavities operating at 109 MHz. Tests have been performed at the full operational fields with an average gradient EACC=8.3 MV/m and VACC=2.1 MV/cavity, a record for cavities at this beta. In the commissioning run of the cryomodule with cavities at full gradient, RMS frequency jitter ranged from 1-2 Hz RMS. With a VCX fast tuner on each cavity configured for a tuning window of 35 Hz there is essentially no “out-of-lock” due to microphonics. Measurements were performed with the cryostat attached to the ATLAS 4.5 Kelvin liquid helium refrigeration system. The quarter-wave cavities themselves are equipped with a passive mechanical vibration damper so that low-lying mechanical modes which couple to the cavity RF fields contribute only a little to the total microphonics. Rather, at useful accelerating fields most of the modest frequency jitter is due to relatively low frequency pressure oscillations in the helium bath due to pool boiling. Future plans for fast tuning on the next ATLAS upgrade cryomodule are discussed.
