Capture Cavity II — Superconducting RF at Fermilab
One of Tim Koeth’s formative experiences as a young accelerator physicist was his work on Capture Cavity II (CCII) at Fermilab under the mentorship of legendary accelerator physicist Helen Edwards. Begun in the early 2000s, CCII was Tim’s first intended Ph.D. research project and immersed him in virtually every aspect of building, commissioning, and operating a modern superconducting radio-frequency (SRF) accelerator system.
Capture Cavity II centered on a nine-cell, 1.3-GHz TESLA-style superconducting niobium RF cavity, prepared for high-gradient operation and supplied by DESY in Germany. Its original mission was to upgrade Fermilab’s A0 Photoinjector, increasing the electron-beam energy from approximately 15 MeV to 40 MeV. Before that could happen, however, Fermilab first had to turn the cavity into a complete, functioning accelerator system.
For Tim, CCII was therefore far more than a cavity test. He oversaw and deeply participated in the project from its earliest stages through full high-gradient operation. The work began with such fundamental matters as acquisition of the cryovessel, preparation of the high-gradient superconducting cavity, and coordinating its shipment from DESY to Fermilab. From there, the project expanded into the assembly and integration of an extraordinarily complex collection of accelerator technologies.
Tim coordinated closely with specialists across Fermilab in cryogenics, low-level RF (LLRF), high-level RF (HLRF), controls, vacuum, mechanical engineering, instrumentation, rigging, shielding, and radiation safety, among many other areas. Components had to be acquired or fabricated, transported, assembled, aligned, connected, tested, commissioned, and ultimately made to function together as a single machine.
Although still a graduate student, Tim was fortunate to help lead an exceptional multidisciplinary team of more than 40 scientists, engineers, technicians, and specialists whose collective expertise made CCII possible. Helen Edwards provided the scientific mentorship and accelerator experience behind the effort, while Tim found himself learning one of the most important lessons of large experimental physics: a sophisticated accelerator is rarely the work of one person or even one discipline. Success depends upon assembling the right people, understanding how their individual systems interact, coordinating their work, and then being present at the machine when those systems finally have to operate together.
This made CCII an unusually comprehensive education in experimental accelerator physics. Tim was not confined to analyzing data after someone else had built the apparatus. He was involved with the physical machine itself—from hardware acquisition and cavity preparation through assembly, cryogenic operation, RF commissioning, controls integration, troubleshooting, and high-gradient testing. The experience required him to move constantly between physics and engineering, between planning and hands-on work, and between the detailed requirements of individual subsystems and the performance of the accelerator as a whole.
Operating CCII was itself demanding. The niobium cavity had to be cooled to temperatures approaching 1.8 K, maintained under high vacuum, mechanically tuned, driven with high-power RF, and stabilized against mechanical vibrations and other sources of detuning. Following extensive preparation, assembly, and commissioning, the system was successfully operated in Fermilab’s Meson Building and demonstrated accelerating gradients as high as 33 MV/m.
That achievement was both a technical success and, unexpectedly, the reason CCII did not ultimately become Tim’s Ph.D. thesis experiment.
CCII had originally been intended for relocation to the A0 Photoinjector, where it would serve as a superconducting booster cavity and provide the higher-energy electron beam needed for the planned research program. But during the years in which Tim and the Fermilab team brought CCII into operation, superconducting RF had assumed much greater strategic importance to the laboratory. Fermilab intended to become a major participant in SRF technology, particularly in support of the proposed International Linear Collider (ILC).
Once CCII had successfully demonstrated 33 MV/m, the assembled installation in the Meson Building had become much more than a component awaiting installation elsewhere. It was now a valuable, all-encompassing SRF development facility through which Fermilab could gain experience with high-gradient cavities, cryogenics, RF power, LLRF controls, tuners, instrumentation, and the practical problems of operating an integrated superconducting accelerator system.
Moving the cavity to A0 would have meant dismantling that capability. Consequently, CCII remained in the Meson Building rather than becoming the A0 booster.
Thus, in an unusual twist, the success of Tim’s first Ph.D. project helped make the original dissertation plan impossible. After years spent helping bring CCII from a superconducting cavity and collection of components into a successful 33-MV/m accelerator system, Tim now needed another experiment on which to complete his doctorate.
And he found one.
Working again under Helen Edwards at the A0 Photoinjector, Tim turned to the experimental demonstration of transverse-to-longitudinal emittance exchange (EEX)—an ambitious technique for deliberately rearranging the phase space of an electron beam. The experiment required a purpose-built beamline and, at its heart, a specialized TM₁₁₀ deflecting-mode RF cavity situated between magnetic doglegs. Tim would design the beamline and RF cavity, oversee their construction and commissioning, and carry out the experiments that produced the first demonstrations of transverse-to-longitudinal emittance exchange.
That work became Tim’s Ph.D. dissertation and ultimately earned him the Richard Plano Dissertation Prize.
In retrospect, losing CCII as his intended dissertation experiment proved to be an extraordinarily productive detour. The project had already given Tim something arguably just as important as a thesis result: an apprenticeship in how large experimental physics actually gets done. Under Helen Edwards, and working alongside a remarkable Fermilab team of more than 40 people, he had followed a sophisticated superconducting accelerator project from acquisition and preparation through integration, commissioning, troubleshooting, and successful high-gradient operation.
CCII taught Tim how to bring together people and technologies to make a complex accelerator work. Emittance Exchange then gave him the opportunity to apply that experience to an experiment of his own—and to do something in accelerator physics that had never been experimentally demonstrated before.
Tim Koeth with Capture Cavity II (CCII) during a 2019 visit to Fermilab. Years after Tim led the cavity’s original assembly and commissioning in the Meson Building, CCII had finally been moved to the New Muon Lab, where it became part of the front end of the ASTA accelerator facility. For Tim, it was a satisfying reunion with a machine he had helped bring to life—now installed and serving a lasting role in Fermilab’s accelerator program.