Surface defect induced by strong electric field probed by soft x-ray laser interferometry

We use X-ray laser interferometry to probe defects induced by a strong electric field on niobium surface. Niobium has been chosen on account of its frequent use in superconductive cavities of particle accelerators. The X-ray laser emits bright, 50ps-duration pulses at λ = 21.2 nm. The beam is reflected on the niobium surface under grazing incidence. The interferometer is of the wave-front division type. Interferograms are single shot recorded, which enables to probe 'instantaneous' defect morphology. We observed appearance and evolution of defects between 14 MV/m and 35 MV/m. The vertical set amplitude is of 10-20 nm. The defect structure has been observed to shift by 500 μm along the metal surface under a constant 35 MV/m electric field, during the 20 minutes time interval between two laser shots.