The first observation of niobium surface evolution under applied electric field has been achieved by means of a x-ray laser interferometry. Single shots of 80 ps pulse X-ray laser duration allowed record of interferograms leading to quasi instantaneously probe the surface morphology. Delay between shots was 20 mn while applied voltage between the Nb sample and a stainless steel blade-like anode has been kept constant. Comparison of successive interferograms shows Nb surface evolution over several hours. Prior any measurable current apparition a defect has grown up for electric field of 25 MV/m. At 35 MV/m a micro-breakdown appeared leading to a strong modification of the surface morphology. Keeping this field during 20 mn has generated the evolution of the defect produced by the micro-breakdown. These results open a new way to a better understanding of surface emission field effect and electric breakdown precursors.
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