ANL, Argonne
The ATLAS facility provides beams of essentially all stable isotopes at energies above the Coulomb barrier for nuclear physics research. We have developed a two-stage ATLAS upgrade plan which includes the replacement of aging split-ring cavities by high-performance quarter-wave resonators (QWR) capable of accelerating ~100 pμA ion beams. The first stage of the upgrade project funded through the American Recovery and Reinvestment Act includes accelerator efficiency increase by adding a new RFQ injector, development and construction of a new cryomodule containing up to 3 SC solenoids and 8 QWRs. A new 72.75 MHz resonator is designed for an optimum ion velocity β=0.075. To achieve record high accelerating voltage ~2.5 MV at this very low velocity range, EM properties of the resonator are highly optimized to reduce peak surface fields. The resonator will be equipped with a piezoelectric fast tuner and capacitive coupler to transmit several kilowatts of RF power. The vast experience gained during the development, commissioning and operation of the ATLAS energy upgrade cryomodule [1] will be applied for the design of the new cryomodule.
[1] J. Fuerst. ATLAS Energy Upgrade Cryomodule, these Proceedings.
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.
