Experimental arrangement of the soft x-ray laser and the interferometric arm (not to scale). The XRL beam is switched between the footprint monitor and the interferometer by a retractable multilayer mirror working at 45°. Another multilayer mirror at 45° sends the beam to the interferometer, polarizing it vertically.
Schematic of the niobium-coated flat substrate and an iron knife electrode system, showing the principle of the probing technique (not to scale).
Setup of the HV electric circuit. Both positive and negative voltages up to may be applied to the system Nb electrode-knife edge.
Fresnel bimirror interferometer, analyzing the x-ray beam reflected from the vertically orientated Nb cathode (not to scale); the probing XRL beam is vertically polarized. The cathode is located at a distance of from the XRL exit aperture and a few centimeters before the Fresnel bimirror; the optical distance between the bimirror and the CCD camera is . The grazing incidence of the CCD is 6° in order to enhance the apparent fringe shift.
Field-emission current measured at the negative polarity of the niobium cathode (Nb acts as an emitter of electrons), as a function of the applied dc voltage.
(Color) A sequence of interferograms of the Nb surface at negative potential, subjected to dc field between 0 (reference) and . The scale bar corresponds to the distance on the probed surface; fringe shift by one period corresponds to surface elevation of . The red quadrangles correspond to regions converted to the surface maps in Fig. 7.
(Color) Surface maps of the Nb layer at the individual values of the electric field, reconstructed from the corresponding interferograms shown in Fig. 6.
Field-emission current with the Nb layer at positive potential (Nb acts as collector of electrons emitted from the knife electrode), accompanying the increasing bias voltage: value of the current, ▵, and maximal values detected.
(Color) A sequence of recorded interferograms for the niobium layer at positive potential, with electric field gradually increasing from 0 (reference) to . The scale corresponds to the distance across the Nb surface; fringe shift by one period corresponds to surface elevation of about . The red quadrangles correspond to regions converted to the surface maps in Fig. 10.
(Color) Surface maps of the Nb layer at the individual values of the electric field, reconstructed from the corresponding interferograms from Fig. 9.
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