What is SRF?
Superconducting radiofrequency (SRF) technology is a means of accelerating subatomic or atomic particles like electrons, protons, or ions. As in any radiofrequency (RF) accelerator, electromagnetic fields in the microwave frequency regime are built up inside of a metallic resonant structure (a cavity). The fields are synchronized with the arrival of the bunches of charged particles so that the particles interact with the fields and are accelerated forward. With superconducting RF (SRF), the metallic structure is almost completely lossless. This allows very high field levels to be sustained in the resonator without dissipating power that would otherwise melt the structure itself.
Niobium is the current superconducting material of choice. It has excellent superconducting properties at liquid helium temperatures (1.8-4.2 K). Cryomodules that contain these superconducting resonators are designed to provide the required insulated cryogenic environment and precisely locate the cavities with respect to the beam that passes through it.
What is SRF technology used for?
Superconducting radiofrequency (SRF) technology is a means of accelerating subatomic or atomic particles like electrons, protons, or ions. Acceleration via SRF offers the advantage of immense power savings, though at the cost of the ultra-low-temperature refrigeration that is required for superconducting operation. In general, after accounting for refrigeration, particle acceleration with SRF nets better than two orders of magnitude in power savings—and also yields important benefits in the quality of the accelerated beams.
As both a science and a technology, SRF is a complex multidisciplinary field that is still advancing. Not all of SRF’s limits or applications are yet known, and it has not reached a technological plateau. To engage SRF in all its research-and-development dimensions requires work in overlapping areas that include solid-state physics, surface science, low-temperature physics, electromagnetism, materials science, RF and microwave technologies, feedback and control systems, interactions between radio waves and the beams they accelerate, vacuum science, mechanical engineering, and cryogenics.
Worldwide, SRF is used in or planned for a number of existing, impending, or envisioned facilities for research in high-energy physics, nuclear physics, nuclear astrophysics, life sciences, and materials science, as well as in facilities or equipment for applied research, industrial processing, and directed-energy weapons.
Most of SRF’s best-known actual or prospective applications fall into four categories:
- Low- to medium-current continuous-wave (CW) accelerators like CEBAF (the Continuous Electron Beam Accelerator Facility at Jefferson Lab) and the envisioned U.S. Rare-Isotope Accelerator ( RIA) for nuclear astrophysics.
- Pulsed high-current proton or ion accelerators like the proton accelerator that Jefferson Lab built at the heart of the new Spallation Neutron Source ( SNS) at Tennessee’s Oak Ridge National Laboratory. The SNS will provide the most intense pulsed neutron beams in the world for scientific research and industrial development.
- Pulsed high-energy linear accelerators for electrons and other leptons, including linear colliders like the International Linear Collider ( ILC), muon colliders, and neutrino factories.
- Energy-recovering linear accelerators (ERLs) in electron coolers and in light sources, including the SRF ERL that drives Jefferson Lab’s free-electron laser ( FEL)—the world’s most powerful source of tunable laser light. Cornell University and England’s Daresbury Laboratory are building SRF ERLs based on the Jefferson Lab FEL’s proof of the energy-recovery principle.
How did SRF technology develop at JLab?
When the Continuous Electron Beam Accelerator Facility (CEBAF) project began in the mid-1980s at what came to be called the Thomas Jefferson National Accelerator Facility (Jefferson Lab), there was an immediate need to gear up to meet challenges for implementing the then-novel SRF technology on which CEBAF was to be based. CEBAF’s 7/8-mile, racetrack-shaped underground accelerator tunnel was to contain in each “straightaway” a microwave superconducting linac operating at 2 K. This represented an order-of-magnitude increase in the scale of SRF accelerators.
SRF is a complex multidisciplinary field that is still advancing. Not all of its limits or applications are yet known, and it has not reached a technological plateau. SRF includes solid-state physics, surface science, low-temperature physics, electromagnetism, materials science, rf and microwave technologies, feedback and control systems, interactions between radio waves and the beams they accelerate, vacuum science, mechanical engineering, and cryogenics. In the mid-1980s, when the CEBAF project began applying SRF at an industrial scale for the first time anywhere in the world, the challenges were technological, industrial, and in some cases scientific. SRF was a newly developed technology, and reliable SRF components needed to be manufactured, processed, assembled, tested, and installed on a production basis. In some cases, subcomponents had to be researched and developed.
As a result, Jefferson Lab’s SRF R&D capabilities were born, together with limited, purpose-built production capabilities targeted on the task of building CEBAF. These SRF capabilities were based in large part on the importation of a contingent of people from the successful SRF R&D program at Cornell University and Stanford. A pre-existing high-bay facility on CEBAF’s site, which had been inherited from NASA, was converted largely to SRF purposes. The CEBAF project went forward, and as of 2004, CEBAF has been reliably serving nuclear and particle physics experimental users with SRF-accelerated beams of electrons having outstanding characteristics for nearly a decade. Because CEBAF’s scientific successes have led directly to a new generation of questions about the quark structure of nuclei, and because CEBAF itself has performed so well, an upgrade that will double CEBAF’s energy is a near-term component of the Office of Science’s road map, Facilities for the Future of Science: A Twenty Year Outlook.
However, it is not only CEBAF that underlies the growing international recognition of Jefferson Lab’s strengths in SRF science and technology. Working in partnership with other national labs, Jefferson Lab has also contributed design and engineering of the SRF linac and the associated cryogenics system at the heart of the Spallation Neutron Source (SNS), currently the largest SC project. Moreover, Jefferson Lab’s IR Demo Free-Electron Laser (FEL)—the world’s highest-average-power source of tunable coherent IR (infrared) light—was based on CEBAF’s SRF technology, as is the FEL upgrade. Jefferson Lab’s SRF expertise, experience, and facilities combine to position the laboratory to contribute usefully to a host of future accelerator and accelerator-based projects.