Observation by soft-X-ray laser interferometry of niobium cathode surface evolution during very high electric field application

We present the very first experiment allowing in-situ observation of surface evolution during very high electric field application. This has been achieved by soft X-ray laser (λ=21.2 nm) Fresnel bi-mirror interferometry. The surface under study was a niobium planar cathode opposite a stainless steel bladelike anode. The applied field range on the cathode was E≈9-50 MV/m. The interferograms where single shot recorded of 80 ps duration allowing to quasi-instantaneously probe the surface morphology. The delay between two shots was 20 min while electric field has been kept on. The comparison of successive interferograms gives the Nb surface evolution over several hours. Thus, we observed the appearance of defects a few nanometers high and their evolution with time. Such a technique makes it possible to detect the preliminary stages of a discharge