Observation by soft X-ray laser interferometry of Nb 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 (λ = 21.2 nm) X-ray laser Fresnel two-mirror interferometry. The surface under study was a niobium planar cathode opposite to a stainless steel blade-like anode. The applied field range on the cathode was F≈0 to 50 MV/m. The interferograms were single shots of 80 ps duration, allowing to probe the surface morphology quasi-instantaneously. The delay between two shots was 20 min while the electric field was kept on. The comparison of successive interferograms gives the Nb surface evolution over approximately 2 h. Thus, we observed the appearance of defects of approximately 3 nm high and their evolution with time. These may be the precursors of the minute breakdown which occurred later on. This technique thus makes it possible to detect the preliminary stages of a discharge.