Superconducting Radio Frequency - SRF Cavity Application in Particle Accelerators

SRF Cavity Application in Particle Accelerators

A large variety of RF cavities are utilized in particle accelerators. Historically they have been made of copper, a good electrical conductor, and operated near room temperature with water cooling. The water cooling is necessary to remove the heat generated by the electrical loss in the cavity. In the past two decades, though, there has been a growing number of accelerator facilities for which superconducting cavities were deemed more suitable, or necessary, for the accelerator than normal-conducting copper versions. The motivation for using superconductors in RF cavities is not to achieve a net power savings. Though superconductors have very small electrical resistance, the little power that they do dissipate is done so at very low temperatures, typically in a liquid helium bath at 1.6 K to 4.5 K. The refrigeration power to maintain the cryogenic bath at low temperature in the presence of heat from small RF power dissipation is dictated by the Carnot efficiency, and can easily be comparable to the normal-conductor power dissipation of a room-temperature copper cavity. The motivations for using superconducting RF cavities, are instead, the following:

• High duty cycle or cw operation. SRF cavities allow the excitation of high electromagnetic fields at high duty cycle, or even cw, in such regimes that a copper cavity's electrical loss could melt the copper, even with robust water cooling.
• Low beam impedance. The low electrical loss in an SRF cavity allows their geometry to have large beampipe apertures while still maintaining a high accelerating field along the beam axis. Normal-conducting cavities need small beam apertures to concentrate the electric field as compensation for power losses in wall currents. However, the small apertures can be deleterious to a particle beam due to their spawning of larger wakefields, which are quantified by the accelerator parameters termed "beam impedance" and "loss parameter".
• Nearly all RF power goes to the beam. The RF source driving the cavity need only provide the RF power that is absorbed by the particle beam being accelerated, since the RF power dissipated in the SRF cavity walls is negligible. This is in contrast to normal-conducting cavities where the wall power loss can easily equal or exceed the beam power consumption. The RF power budget is important since the RF source technologies, such as a Klystron, Inductive output tube (IOT), or solid state amplifier, have costs that increase dramatically with increasing power.

When future superconducting material advances occur to obtain higher superconducting critical temperatures T_c and consequently higher SRF bath temperatures, then the better efficiencies of the refrigerator could yield a significant net power savings by SRF over the normal conducting approach to RF cavities. There are other issues that would have to be considered with a higher bath temperature, though, such as the absence of superfluidity that is presently exploited with liquid helium that would not be present with, e.g., liquid nitrogen. At present, none of the "high T_c" superconducting materials are suitable for RF applications. Shortcomings of these materials arise due to their underlying physics as well as their bulk mechanical properties not being amenable to fabricating accelerator cavities. However, depositing films of promising materials onto other mechanically amenable cavity materials could provide a viable option for exotic materials serving SRF applications. At present, the de facto choice of SRF material is still pure niobium, which has a critical temperature of 9.3 K and functions as a superconductor nicely in a liquid helium bath of 4.2 K or lower.