Noninvasive monitoring of x-ray beam at a windowless undulator beamline using x-ray induced drain current of a focusing mirror

An x-ray induced electric current of an x-ray mirror (drain current) was measured as a function of the incident x-ray photon flux and the mirror angle at the Soft Matter Interfaces (SMI) beamline at NSLS-11. The mirror was a horizontal focusing bimorph mirror operated in vacuum. The electric current was found to be an approximately linear function of the incident photon flux. The drain current as a function of the mirror angle has a maximum in vicinity of the critical angle of total external reflection. In reflection from the Si stripe of the mirror, an additional signal enhancement was found at the critical angle corresponding to the the 3rd radiation harmonic under high-power load conditions of the double-crystal Si 111 monochromator operated upstream of the mirror (at the primary photon energy of 2.15 keV). The drain current as a function of angle of the monochromator's second crystal (rocking curve) measured at the critical angle revealed a harmonic contamination due to higher order Bragg reflections. The observations validate the utility of the x-ray induced current for beamline alignment and diagnostics.