COMMISSIONING OF THE 72 MHz QUARTER- WAVE CAVITY CRYOMODULE AT ATLAS*

A cryomodule of seven 72 MHz superconducting (SC) quarter-wave cavities optimized for ions with v/c=0.077 has been commissioned in the ATLAS heavy-ion accelerator at Argonne. The new module, with the new CW RFQ injector [1], provide ATLAS the capability to deliver much higher beam currents with low beam losses for nuclear physics experiments using stable or rare isotope beams or neutron rich beams from the Californium Rare Isotope Breeder. The goal for the cryomodule, to provide a large accelerating voltage of 17.5 MV (2.5 MV/cavity) with no detectable beam losses, was been met during the first month of commissioning. To date, cavities and primary subsystems, including 4 kW nominal RF couplers and pneumatic tuners, are operating as designed with full availability. For VACC=17.5 MV (ave. EPEAK=40 MV/m) field emission is small and RF losses to 4.5 K helium are 5 Watts/cavity, about half of the planned value. Cavity fields continue to be gradually increased beyond the nominal design values. The limit due to cavity quench is at least VACC=3.75 MV per cavity. The good RF performance stems primarily from combination of RF design and cavity processing. Effective voltages are 2½ times higher than those of other operational cavities for this v/c. We report here on the recent online test results and important technical features.