Thermally Induced Ultra High Cycle Fatigue of Copper Alloys of the High Gradient Accelerating Structures

In order to keep the overall length of the compact linear collider (CLIC), currently being studied at the European Organization for Nuclear Research (CERN), within reasonable limits, i.e. less than 50 km, an accelerating gradient above 100 MV/m is required. This imposes considerable demands on the materials of the accelerating structures. The internal surfaces of these core components of a linear accelerator are exposed to pulsed radio frequency (RF) currents resulting in cyclic thermal stresses expected to cause surface damage by fatigue. The designed lifetime of CLIC is 20 years, which results in a number of thermal stress cycles of the order of 2.33•1010. Since no fatigue data existed in the literature for CLIC parameter space, a set of three complementary experiments were initiated: ultra high cycle mechanical fatigue by ultrasound, low cycle fatigue by pulsed laser irradiation and low cycle thermal fatigue by high power microwaves, each test representing a subset of the original problem. High conductivity copper alloys in different temper states and several techniques to improve their fatigue life were investigated. The results obtained by the three techniques are presented and the relations between them are determined. The RF fatigue experiments had conditions similar to the CLIC application, but the achievable number of cycles was limited. The data obtained by RF is extrapolated with the ultrasonic fatigue experiments which show a similar relative merit for the candidate alloys. Based on the results, a precipitation hardened copper zirconium alloy and an aluminum oxide dispersion strengthened copper alloy fulfill the CLIC requirements, the former being slightly better in terms of fatigue, but more sensitive to the temper state. The ultra-high cycle ultrasound data showed surface roughening, which appeared at stress amplitudes lower than the fatigue strength. Different crack growth rates were observed between the precipitation strengthened and the dispersion hardened copper alloys. Compressive mean stresses were studied at ultra high number of cycles regime and they were not found to have an effect on fatigue performance under mechanical loading. Surface damage, due to RF induced fatigue, was observed to be anisotropic.