  We demonstrate an x-ray beam splitter with high performances for
multi-kilo-electron-volt photons. The device is based on diffraction on
kinoform structures, which overcome the limitations of binary diffraction
gratings. This beam splitter achieves a dynamical splitting ratio in the range
0-99.1% by tilting the optics and is tunable, here shown in a photon energy
range of 7.2-19 keV. High diffraction efficiency of 62.6% together with an
extinction ratio of 0.6% is demonstrated at 12.4 keV, with angular separation
for the split beam of 0.5 mrad. This device can find applications in beam
monitoring at synchrotrons, at x-ray free electron lasers for online
diagnostics and beamline multiplexing and, possibly, as key elements for delay
lines or ultrashort x-ray pulses manipulation.
