The past four decades have seen many successes in using superconducting RF (SRF) technology in a variety of accelerator applications. These are the result of steady progress in understanding the science behind the gradient limitations in SRF cavities and developing effective countermeasures.
In the 1970s research groups at Stanford, Siemens and Cornell demonstrated spectacular results with available niobium, with surface fields corresponding to accelerating gradients of 25–35 MV/m in single-cell cavities at X-band frequencies (8–10 GHz). However, these performance levels fell apart at the frequencies used in accelerator applications, i.e. below 3 GHz. The primary roadblock was multipacting (the spontaneous resonant production of electrons) limiting the performance of 1.3 GHz cavities to 2–4 MV/m. The physics of multipacting clearly shows that the limiting field levels scale with the RF frequency, so that the X-band cavities of the 1970s had been fortuitously exempt.
Steady progress
The next three decades saw several layers of gradient problems uncovered, the underlying physics understood and solutions developed. Performance then ratcheted up at a steady pace, as did applications. The development of the anti-multipacting spherical (and elliptical) cavities in the 1980s was a breakthrough moment. With multipacting overcome, thermal breakdown of superconductivity became the next limiting mechanism, at 4–6 MV/m. Local heating at surface imperfections led to thermal runaway and a quench of superconductivity. The cure was to switch to high-purity – high residual resistance ratio (RRR) – niobium. With the co-operation of industry, the RRR improved by an order of magnitude and cavity gradients rose on average by a factor of three. Another cure for thermal breakdown was to sputter a film of niobium only a few microns thick onto a copper cavity substrate of high thermal conductivity, which also had the benefit of reduced material costs – especially for low-frequency (0.35 GHz) cavities.
Major applications of RF superconductivity then took off and pushed the energy frontier in storage rings, with TRISTAN at KEK, HERA at DESY and LEP-II at CERN. At the cutting edge of nuclear physics, Jefferson Lab installed the recirculating linac, CEBAF, while at the luminosity frontier CESR at Cornell and KEK B applied niobium cavities to store ampere-sized beams. Superconducting linacs powered FELs at Jefferson Lab and at JAERI in Japan. Thus, 1 km of SRF cavities provided a total 5 GV of acceleration.
With the corresponding rise in surface electric fields, electron emission became the next significant limit to gradients, at 10–15 MV/m. Global R&D revealed microparticle contamination to be the dominant source of field emission, which demanded better preparation techniques such as powerful surface scrubbing with high-pressure water and assembly in Class 100 clean rooms. With these breakthroughs cavity gradients in accelerator assemblies climbed to 20 MV/m. For any given preparation protocol, the probability of encountering field emitters and quench-producing defects grows with cavity area, providing favourable conditions for the X-band cavities of the 1970s, with areas of square centimetres compared to multicell gigahertz-accelerator cavities with surface areas in the order of square metres.
These advances spurred new applications of accelerators for materials science, with the FLASH light source in Hamburg and the Spallation Neutron Source in Oak Ridge adding another 2 GV of acceleration.
The quest for higher gradients continued into the new millennium. At levels above 20 MV/m, RF losses rise exponentially with electric field. The physics of increased losses is still under active investigation worldwide, but pragmatic countermeasures are already in place. Electro-polishing has replaced standard chemical etching to obtain a smoother surface with reduced microscopic field enhancements, followed by mild baking at 120 °C for two days. Research continues to understand the physics of these cures. One alternative may be to use large-grain niobium, cut directly from the ingot to avoid defects, combined with standard chemical etching to produce a smooth surface – again followed by baking at 120 °C, but for 12 hours. Niobium cavities with surface areas of square metres now reach 30–40 MV/m with tolerable losses.
The next frontier
These successes demonstrate how R&D in SRF science and technology has pushed towards ever higher gradients. New applications continue to benefit from the steady progress. Nearly 20 GV of SRF cavities are foreseen for the European XFEL at DESY. The worldwide SRF community has expanded with re-invigorated efforts, encouraged by the successes in unravelling the physics of gradient limitations and the invention of effective countermeasures.
The next frontiers with bulk niobium lie in achieving consistent high-Q performance above 35 MV/m and pushing towards 50 MV/m – already achieved in 1-cell cavities operating at gigahertz frequencies. High-Q values are vital for continuous-wave applications. To accomplish this and push beyond it, materials scientists and process engineers are eager to join SRF enthusiasts in exploring new techniques and new materials to produce more powerful and compact accelerators for physical research.