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.
ANL, Argonne
PKU/IHIP, Beijing
A simple technique based on an in-situ moveable germanium resistance thermometer is used to measure the quench location in a superconducting (SC) cavity in superfluid liquid helium. SRF cavities are very often limited in operating field level by thermal instability manifesting as a transient "quench" of the electromagnetic field. The field energy is transferred into the superfluid helium bath as a heat pulse and may be detected as a wave of phonons at a thermometer. The germanium thermometer technique was developed at Argonne three decades ago and used to measure time-of-flight of the second-sound to locate defects in split-ring resonators for the ATLAS SC linac at Argonne. The present goal is to extend and adapt the second-sound diagnostic technique in a simple, easy-to-use and cost effective way for use with cavities under development today. These include for example, the 9-cell, 1.3 GHz SC cavities, as well as, reduced beta superconducting cavities such as half-wave and quarter-wave structures.
ANL, Argonne
A system based on a pair of mass flow controllers has been used to evacuate and vent a clean cavity rf space. The mass-flow system is used in both single cavity testing and with the ATLAS upgrade cryomodule at Argonne. It is similar schematically to that already in use at DESY, however, it is very compact and maintains the capability to precisely control both the pump out and venting rates. Initial tests of the system with both the ATLAS single cavity test cryostat and the ATLAS upgrade cryomodule show that pump down and venting cycles may be performed without introducing substantial particulates into the cavity rf space. The system, together with the ANL top loading cryomodule design with easy access to individual cavities, will allow an individual cavity to be removed and replaced in a cryomodule string without the need to re-clean the entire string. This capability would also remove the need to test every cavity individually before installation into the string, constituting a major savings for large projects.
ANL, Argonne
A new large 2 Kelvin test cryostat is being commissioned at Argonne National Laboratory. This system will have a full time connection to the 4.5 Kelvin ATLAS refrigerator and, with integrated J-T heat exchanger, will allow continuous 2 Kelvin operation. The large diameter was chosen to accommodate essentially all of today’s superconducting cavities and the top loading design facilitates clean room assembly. The commissioning run will be with a coaxial half wave cavity to be followed by testing with 1.3 GHz single-cell elliptical cavities. Details of the initial engineering cool down on the cryostat are presented.
ANL, Argonne
Fermilab, Batavia
A system for electropolishing of 1.3 GHz elliptical single- and nine-cell cavities is in operation at the joint ANL/FNAL cavity processing facility located at Argonne. The system is one peice of a larger 200 m2 complete single cavity processing and assembly facility which also includes clean rooms and high-pressure rinsing. Recently, the electropolishing system has been used to process a series of single and nine-cell cavities. For single cell cavities a good set of EP parameters has been demonstrated based on more than a half dozen complete processing and cold test cycles at ANL/FNAL. The lastest six single cell cavities each exceed EACC=35 MV/m and, at this gradient, have Q in the range 6 109 - 1 1010. The first nine cell cavities electropolished at ANL have not yet reached similar fields (~23 MV/m-26 MV/m) and ongoing activities are focussed on demonstrating >30 MV/m in these cavities. Suitable nine cell EP parameters using the ANL/FNAL EP system including acid/water temperatures, flow rates, current, voltage, air flow etc. are all substantially different than for single-cell cavities and are discussed here.