Surface phenomena associated with thermal cycling of copper and their impact on the service life of particle accelerator structures

The performance of accelerating structures (AS) in the Compact LInear Collider (CLIC) is sensitive to a variety of parameters, including the surface quality of key elements of the AS. Processes which affect the surface quality are therefore of particular concern. The present work addresses surface modifications associated with thermal cycling during operation. This type of operating condition represents a specific type of fatigue loading. Four fatigue test procedures were used in the present study in order to investigate the fatigue behaviour of oxygen{free{electronic (OFE) copper, the candidate material of the CLIC-AS: conventional fatigue (CVF), ultrasonic swinger (USS), laser fatigue (LAF) and radio{frequency fatigue (RFF). During operation of the accelerator the material of the AS will be subjected to cyclic temperature changes of approx. Delta T = 56 K, from about 40° C to about 100° C. These temperature changes will result in cyclic biaxial strains in the surface of the order of epsilon(biax) = 9.2 x 10e4 at a macro equivalent stress of 110 MPa. Characterisation and quantification of roughening and hardening phenomena on cycled samples revealed a strong dependence on the orientation of individual grains of the polycrystalline material. Orientation imaging with a scanning electron microscope showed that roughness and hardness increased less in grains oriented in [1 0 0] direction than grains oriented in the [1 1 0] direc tion, with the [1 1 1] grains falling in between. A detailed analysis of the slip systems of the different orientations clarified the origin of the roughening phenomena and their dependence on crystallographic orientation. This dependence arises from the different number of active slip systems. Additionally, grain boundaries need to be considered in fine structures (grain sizes as fine as 1 um - 5 um). High local stresses related to elastic anisotropy in the polycrystal average out on the macroscopic scale. Analysis of the results obtained by different fatigue test techniques indicates that similar elementary processes govern the material response in all cases. The two mechanical loading methods CVF and USS have experimental advantages in terms of standardisation and in terms of the capability of addressing high cycle numbers. Unfortunately, the results obtained cannot be directly related to thermal fatigue data obtained by LAF and RFF. Given the importance of electrical conductivity for the CLIC AS material, copper is the only material that comes in to consideration. On the basis of the results obtained in the present study, the two possible candidates for the material are: 1) A strongly textured and fine grained OFE copper, e.g. equal-channel- angular-pressed (ECAPed) OFE copper. 2) A strongly [1 0 0] orientation textured pure copper thin film. Either material would offer fatigue characteristics that should be compatible with the demanding requirements of the CLIC-AS. The results obtained in the present study provide a framework for discussing material selection and material pretreatments, developing alternative engineering designs and working out monitoring strategies for long-term operation of the CLIC-AS.