RF-loss measurements in an open coaxial resonator for characterization of copper plating

An experiment has been conducted to measure differences in cavity Q caused by various cavity surface treatments. A requirement of the experiment was that it show little sensitivity to the reassembly of the apparatus with various test pieces. We chose a coaxial half-wave resonator, with an outer conductor extending significantly beyond the length of the inner conductor. At 700 MHz the outer conductor acts as a cut-off tube, eliminating the need for electric termination and thus any RF-contacts that can influence the accuracy of the Q-measurements. The experiment is aimed at qualifying the performance of cyanide-copper plated GlidCop Al-15 LO-OX in comparison with that of a machined GlidCop Al-15 LO- OX surface. To maximize the sensitivity of the measurement we use a fixed outer conductor made of oxygen free electronic (OFE) copper and only replaced the inner conductor, which is mounted on a low-loss Teflon pedestal located in the low electric field region of the half-wave TEM mode. The Q-values of machined GlidCop Al-15 and cyanide-copper plated GlidCop Al-15 inner conductors are measured against the reference Q of the annealed OFE co-axial cavity. This extremely simple configuration allows a statistically significant number of repetitions of measurements and should provide accurate comparative measurements. The results will be useful for the community in deciding the surface treatments for high-Q copper accelerators. While outside of the scope of the qualification of the cyanide copper plating, two UBAC plated OFE copper pieces have been added to the comparison.