Fatigue Testing of Materials by UV Pulsed Laser Irradiation

The energy dissipated by the RF currents in the cavities of pulsed high-power linacs induces cycles of the surface temperature. In the case of the CLIC main linac the expected amplitude of the thermal cycles is above fifty degrees, for a total number of pulses reaching 1011. The differential thermal expansion due to the temperature gradient in the material creates a cyclic stress that can result in surface break-up by fatigue. The materials for cavity fabrication must therefore be selected in order to withstand such constraints whilst maintaining an acceptable surface state. The fatigue behaviour of Cu and CuZr alloy has been tested by inducing larger surface peak temperatures, thus reducing the number of cycles to failure, irradiating the surface with 40 ns pulses of UV light (308 nm) from an excimer laser. Surface break-up is observed after different number of laser shots as a function of the peak temperature. CuZr appears to withstand a much larger number of cycles than Cu, for equal peak temperature. The characterization of the surface states and possible means of extrapolating the measured behaviour to the expected number of pulses of CLIC are discussed in detail.