US researchers recently proved their ability to process and test world-class superconducting radiofrequency, SRF, cavities.
In preparing two dressed, high-gradient nine-cell ILC-type cavities for use in the S1-global effort, a prototype at KEK of the International Linear Collider main linac, researchers had to climb multiple technical hurdles.
SRF cavities enable accelerators to increase particle beam energy levels while minimizing the use of electrical power by all but eliminating electrical resistance. Future experiments into the origins of the universe and nature of matter, including the proposed ILC and Project X, both of which Fermilab would like to host, will require advanced SRF technology.
The shipped cavities show the US is well on its way to meeting those technology needs.
The processing and assembly of the cavities depended heavily on the contribution of scientists and engineers across the globe, including at Argonne National Laboratory, Thomas Jefferson National Accelerator Facility, SLAC National Accelerator Laboratory, Cornell University, INFN and DESY.
Initial testing of the cavities involved dipping them in liquid helium andsupplying a low-power RF source. Cavities that reached the needed accelerating field got sealed into a helium vessel and outfitted, or dressed, with auxiliary components necessary for high-power operation in an accelerator. These components include input couplers to feed RF power into the cavity and tuners to adjust the cavity's resonant frequency.
The first cavity, dubbed AES004, was fabricated by Advanced Energy Systems in New Jersey and is the first dressed cavity assembled and tested at Fermilab. The fact that the cavity’s power gradient did not degrade between the processing steps shows that the US can operate world-class SCRF cavity facilities. Jefferson National Laboratory oversaw the chemical processing of the cavity, including etching and baking; Fermilab welded the helium tank and connection components onto the cavity, and then Argonne National Laboratory conducted high-pressure rinses to eliminate microscopic dust particles inside the cavity.
The second cavity, dubbed ACCO11, was fabricated by a German manufacturer but is the first cavity fully prepared—dressed and chemically tested – in the US to operate above 30 megavolts per meter, or MV/M. The cavity will be able to accelerate particles to 33 MV/M, exceeding the ILC’s design gradient of 31.5 MV/M. The AES004 cavity successfully passed a horizontal test with a gradient of 28 MV/M, similar to the best vertical test of this cavity type.
Because of a time crunch to get the cavities to Japan, Fermilab essentially combined quality testing steps and for the first time put a dressed cavity through vertical testing. The successful test opens the door to quicker turnaround of cavities, though with some limitations.
This pair of successful tests of two dressed ILC-type cavities with Blade tuners demonstrates the viability of the US as a provider of finished cavities for international research projects, says Tug Arkan, Fermilab project engineer for the cryomodule assembly facilities group.