Strong-gain water-window X-ray laser driven by plasma photocathode wakefield acceleration

X-ray free-electron lasers are large and complex machines, limited by electron beam brightness. Here we show through start-to-end simulations how to realise compact, robust and tunable X-ray lasers in the water window, based on ultra-bright electron beams from plasma wakefield accelerators. First, an ultra-low-emittance electron beam is released by a plasma photocathode in a metre-scale plasma wakefield accelerator. By tuning the beam charge, space-charge forces create a balance between beam fields and wakefields that reduces beam energy spread and improves energy stability – both critical for beam extraction, transport, and focusing into a metre-scale undulator. Here, the resulting ultra-bright beams produce wavelength-tunable, coherent, femtosecond-scale photon pulses at strong gain. This regime enables reliable generation of multi-gigawatt-class X-ray laser pulses across the water window, offering tunability via the witness beam charge and robustness against variations in plasma wakefield strength and plasma photocathode laser jitter. Our findings help democratise access to coherent, high-power, soft X-ray radiation.