Theory of Thermal Fatigue Caused by RF Pulsed Heating

The normal conducting electron-positron Linear Collider projects imply that accelerating structures and other RF components will undergo an action of extremely high RF fields. Except for the RF breakdown threat, there is an effect of the copper surface being damage due to multi-pulse mechanical stress caused by Ohmic losses in the skin layer. In this paper we would like to introduce a new “grain” model of the processes responsible for the fatigue of the metal surface. This model is based on the quasi-elastic interaction between neighboring grains in the metal due to the thermal expansion of the skin layer. This mechanism of fatigue is compared with another, where stresses are generated by the temperature gradient towards the bulk of the material. With the proposed formalism one can estimate the total number of the RF pulses required to fracture the surface depending on the temperature rise, pulse length and steady state temperature. The parameters necessary to finalize the proposed approach were found through the comparison of experimental data obtained at 11.424 GHz.