Inelastic X-ray scattering (IXS) is an important tool for studies ofequilibrium dynamics in condensed matter. A new spectrometer recently proposedfor ultra-high-resolution IXS (UHRIX) has achieved 0.6~meV and 0.25~nm$^{-1}$spectral and momentum transfer resolutions, respectively. However, furtherimprovements down to 0.1~meV and 0.02~nm$^{-1}$ are required to close the gapin energy-momentum space between high and low frequency probes. We show thatthis goal can be achieved by further optimizing the X-ray optics and byincreasing the spectral flux of the incident X-ray pulses. UHRIX performs bestat energies from 5 to 10 keV, where a combination of self-seeding and undulatortapering at the SASE-2 beamline of the European XFEL promises up to ahundred-fold increase in average spectral flux compared with nominal SASEpulses at saturation, or three orders of magnitude more than possible withstorage-ring based radiation sources. Wave-optics propagation shows that about$7\times 10^{12}$~ph/s in a $90$-$\mu$eV bandwidth can be achieved on thesample. This will provide unique new possibilities for dynamics studies by IXS.
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