Status of very high-gradient cavity tests

Accelerating gradients in excess of 100 MV/m are essential for future multi-TeV linear colliders in order to limit their overall length and cost. Experience has shown that to obtain such high gradients with normal-conducting (nc) structures requires operation at a relatively high frequency. A brief review of the very high gradient test work being carried out around the world will be presented with special emphasis being placed on the effect of frequency on the maximum achievable gradient. In the framework of the CLIC study at CERN, evidence has been found during high gradient tests at 30 GHz that part of the limitation on gradient is due to the choice of copper as the standard cavity fabrication material. To support this hypothesis, a breakdown mechanism leading to material destruction is presented together with a review of results and experience obtained with alternative materials.