  Plasma-based accelerators are a promising approach for reducing the size and
cost of future particle accelerators, making them a viable technology for
constructing and upgrading X-ray free-electron lasers (FELs). Adding an energy
booster stage to the linear accelerator of an operational X-ray FEL is
recognised as a realistic near-term application of plasma accelerators, with a
significant impact on the scientific reach of these facilities. Here, we
discuss potential use cases of such a plasma-based energy booster and apply
particle-in-cell simulations to estimate its ability to enhance the performance
of existing X-ray FEL facilities.
