Superconducting radio frequency (SRF) cavities represent an important application of cryogenics and superconductivity. SRF cavities are a technology for accelerating charged particle beams via the transfer of radiofrequency (RF) energy to the beams via resonant structures. As such, they are frequently found in large particle accelerators used for scientific research.

While this acceleration technique can be done with normal conducting materials, the use of superconductivity allows a much greater portion of the RF energy to accelerate the beam rather than be dissipated as heat in the cavity wall. SRF cavities also eliminate the need for extensive water cooling systems to remove the heat from the cavities, though they do require complex cryogenic systems to maintain the cavities at their operating temperatures. SRF cavities may also be used to change the orientation of the particle bunches that constitute the beam. While there are circumstances in which normally conducting systems are the better option, the vast majority of large particle accelerators in operation or under construction today use SRF cavities.

Examples of operating SRF systems include: the Large Hadron Collider at CERN, the CEBAF accelerator (recently upgraded) at Jefferson Lab and the Spallation Neutron Source at Oak Ridge National Lab. SRF systems that are under development or construction include the European Spallation Source (ESS) in Sweden, the XFEL accelerator at DESY in Germany, the LCLS II accelerator (SLAC) and the FRIB machine at Michigan State University. In many linear accelerators, SRF cavities represent the principal application of cryogenics and superconductivity in the accelerator.

SRF cavities are constructed from pure niobium and come in many different shapes depending on the particles being accelerated, the operating frequency and the desired energy. There are two key parameters for SRF cavities: the accelerating gradient, representing how much the beam is accelerated per unit length of the cavity, and the Q0, which represents the efficiency with which the RF energy is stored in the cavity.

In order to optimize these parameters, the cavities undergo a complex set of heat treatments and cleaning of the inner cavity surface. Once cleaned, the inner cavity surface must be kept clean and assembly of the cavity systems is carried out in clean room facilities. Finding the optimal sequence of heat treatment and cleaning steps has required significant R&D in laboratories throughout the world over the past several decades. This is an ongoing area of research, and new cavity designs involve a prototype stage in which the cavity design and processing steps are optimized. Figure 1 shows a prototype elliptical cavity for the ESS machine. Many CSA members and Corporate Sustaining Members have contributed to the development of SRF cavity technology over the years.

Most SRF systems today cool the cavities with saturated baths of He II (Cold Facts, Spring 2010) at temperatures at or below 2K. The lower temperature reduces the intrinsic wall heating found in SRF systems and takes advantage of the very efficient heat transfer mechanism in He II. A particular advantage is that He II does not support bulk boiling. A boiling bath can affect the tuning of the cavity, driving it off its resonant frequency. This effect is avoided by the use of He II.

SRF cavities are combined with other subsystems such as power couplers, tuners, instrumentation and cryogenic piping into cryomodules (Cold Facts, Summer 2011) which become the building blocks of the accelerator.

A very thorough review of SRF cavities and their technology is given in RF Superconductivity for Accelerators, H. Padamsee, J. Knobloch and T. Hayes (2008) and RF Superconductivity: Science, Technology and Applications, H. Padamsee (2009). Examples of SRF cavity systems can be found in “The Jefferson Lab 12 GeV Upgrade,” R. McKeown, Journal of Physics: Conference Series 312 (2011), “The ESS Elliptical Cavity Cryomodules,” C. Darve et al. Adv. Cryo. Engr. Vol. 59A (at press), “The Injector Cryomodule for e-Linac at TRIUMF,” M. Abammed et al., Adv. Cryo. Engr., Vol. 57A (2012).

An example of the use of SRF cavities to alter the orientation of the particle beam may be found in “Design Approach for the Development of a Cryomodule for Compact Crab Cavities for Hi-Lumi LHC,” S. Pattalwar et al., Adv. Cryo. Engr. Vol. 59A (at press). Additional information may also be found in the proceedings of the international conferences on RF Superconductivity. The most recent of these was held in Paris in 2013 (http://www.srf2013.fr).