Engineered Surface Treatments for ILC Cavities

The superconducting radio-frequency cavities being tested for use in the International Linear Collider are made from high purity niobium sheet metal, which has an inherent, performance-hindering "damaged layer." This surface layer is removed by acid treatments, which result in hydrogen contamination and necessitate high-temperature, vacuum heat treatments. This project will develop technology both to characterize and to remove the ¿damaged layer.¿ For characterization, the effect of a ¿damaged layer¿ of un-recrystalized grains at the sheet surface will be measured. For removal, acid-free electro-polishes will be evaluated to eliminate the hydrogen contamination problem. Phase I will use electron microscopy techniques and the TE011 cavity at Thomas Jefferson Laboratory to examine the relationship between the unrecrystallized surface layer and the radio-frequency superconductivity of the niobium sheet. Procedures for avoiding the ¿damaged¿ layer during sheet metal rolling will be identified. Current-density vs. voltage plots will be used to determine optimum acid-free polishing conditions, and polished samples will be tested for high-field Q droop. Commercial Applications And Other Benefits as described by the Applicant: The International Linear Collider will use over 20,000, meter-long, nine-cell, superconducting radio frequency (SRF) cavities made of RRR niobium. The technology developed in this project should reduce the time and greatly lower the cost of the surface treatment of these cavities. Lower cost, higher performance accelerators also would have application in many other fields: over 10,000 accelerators are now used, worldwide, for medical diagnostics, advanced materials characterization, communications, military applications, condensed matter physics, nuclear and heavy ion research, structural biology, and environmental studies