Apparatus and Technique for Measuring Low RF Resistivity of Tube Coatings at Cryogenic Temperatures

An in-situ technique for coating stainless steel vacuum tubes with Cu was developed to mitigate the problems of wall resistivity that leads to unacceptable ohmic heating of the cold bore of superconducting magnets and generation of electron clouds in RHIC that can limit future machine luminosity enhancement. Room temperature RF resistivity measurements indicated that 10 μm Cu coated stainless steel RHIC beam tube has a conductivity close to copper tubing. Before coating the RHIC beam pipe with copper, it is imperative to test the Cu coating’s conductivity at cryogenic temperatures to ensure coating effectiveness in lowering resistivity. A folded quarter wave resonator structure has been designed and built to be inserted inside a cryogenic system to measure the RF resistivity of copper coated RHIC tubing at liquid helium temperatures. The design is based on making the resonator structure out of a superconducting material such that the copper coating is the most lossy material. RHIC tubing samples prepared with different magnetron sputtering deposition modes are to be measured by the apparatus. Cu coating is to be optimized by iterative processes. Additionally, this device can also be used for the development of better, cheaper superconducting radio frequency (SRF) cavities and electron guns. The apparatus and its design details will be presented.