Liepe Group: Research

Superconducting radio frequency

Superconducting radio frequency (RF) cavities are feet-long structures, providing extremely high electric field gradients (tens of MV/m) for the acceleration of particle beams. The electric field inside these cavities oscillates at GHz frequencies, with exceptional high quality factors of 1E10 to 1E11. By using superconducting materials operated at temperatures between 1.5K and 4K for the walls of the cavities, we can achieve such high efficiency. The evolution in the performance of superconducting cavities has revolutionized the performance and scientific reach of particle accelerators for a variety of science applications, including high-energy physics, nuclear physics, synchrotron radiation based research, and high power lasers. Future particle accelerators like the International Linear Collider, the LCLS-II x-ray FEL at SLAC, a muon accelerator, and the Energy Recovery Linac Light Source planned here at Cornell University all rely on the performance we hope to achieve in next generations of superconducting cavities.

Cornell’s Superconducting Radio Frequency (SRF) group is a world leader in the application of superconductivity for accelerating cavities in high-energy particle accelerators. We have an extensive, state of the art infrastructure for the design, fabrication, preparation and test of superconducting cavities. Our research program is multi-faceted and interdisciplinary, and therefore ideal suited for graduate research. It ranges from studying the fundamental behavior of superconductors in high GHz fields to complex multi-parameter optimizations of RF cavities to studying the non-linear beam dynamics in superconducting linacs.

Current and Future Research Activities

SRF cavities not only enable accelerator-based sciences, but they also allow the measurement of superconducting response under extreme conditions with very high sensitivity. They allow the study of surface resistance, critical fields, superconducting magnetic microwave shielding, and the metastability of the superheating-field barrier. They are a testing ground for the science of disorder and defects, coupling superconductivity and high fields to grain boundaries, surface anisotropy, surface oxides, and crystal orientation at microwave fields. My current research concentrates on the following areas:

• Superheating fields in superconductors: The highest gradient niobium SRF cavities are operated with peak magnetic fields beyond Hc1, the field where vortices (that would cause massive losses) would penetrate in equilibrium. Operation is possible until a higher superheating field Hsh because of a surface barrier to flux penetration. Our group has done a first measurement of the full temperature dependence of the superheating field of niobium using SRF cavities, and has shown that it depends strongly on the preparation of the niobium surface. Much remains unknown. How does the superheating field depend on the Fermi surface anisotropy, i.e. could optimally oriented superconducting surfaces offer higher fields in SRF cavities? How do strong-coupling effects, as present in many of the higher-temperature traditional superconductors, impact Hsh? Can defects bypass the metastable superheating-field barrier for large-kappa materials?
• Processing, characterization and microwave surface resistance of the RF surface penetration layer of superconductors: The surface resistance of a superconductor in microwave fields is determined by a highly complex surface layer of a few 100 nm thickness (roughly the penetration depth of the field), with oxides, grain boundaries, impurities, and defects present. This surface resistance, and is observed strong field dependence, strikingly depend on the surface treatment protocol (etching, polishing, annealing). Open questions include: What is the physics underlying the residual surface resistance at the lowest temperatures? What are the effects of surface oxides on the surface electronic structure of materials and their impact on RF cavity performance characteristics? What surface morphology and (likely) mixed metallic phases arise from electrochemical polishing of niobium? What are the sources of the observed strong field dependence of the microwave surface resistance?
• Synthesis, characterization, and microwave surface resistance of new superconducting compound materials for RF cavity applications: Current SRF cavities exclusively use niobium as superconductor, and are approaching theoretical limits. However, new potentially game-changing materials (e.g. Nb3Sn, MgB2, NbN) have the potential for fields and cavity quality factors far above the niobium limit. Moving to higher-kappa, compound superconductors brings new questions: What alternative superconductors with critical temperatures higher than that of niobium can open the path towards a new generation of SRF cavities with even lower RF surface resistance and higher accelerating fields? Does the small coherence length of these superconductors limit their usefulness due to grain boundary losses or defects? How can these more complex compound superconductors be synthesized with ideal stoichiometry and defect free?
• Electron beam emittance preservation and beam dynamics in superconducting RF linacs: When a particle beam passes though a superconducting linac, it interacts with the cavity environment. This can lead to excessive fields (Higher-Order-Modes) excited by the beam in the cavities, degradation of the beam quality (emittance growth) and beam instability. Our Cornell ERL injector prototype gives us a unique tool for studying questions like: What is the spectrum of electromagnetic fields excited by the beam? Where is the excited high frequency (10 GHz – 100 GHz) power absorbed? What effects contribute to emittance growth in an SRF linac, and do measurements agree with numerical simulations of these various effects?
• Developing the superconducting linac technology for future particle accelerators: In addition to designing and optimizing the cavities for future superconducting accelerators, we are developing related and technologically challenging components like RF input couplers, Higher-Order-Mode dampers and frequency tuners. For the Cornell ERL, we are designing, building and testing entire, complex SRF cryomodules. This work relates to a wide breath of scientific and engineering questions.