Atomic inner-shell x-ray laser pumped by an x-ray free-electron laser

We discuss possibilities of pumping an atomic inner-shell x-ray laser with an x-ray free-electron laser (XFEL). Self-consistent gain calculations show that with the first available XFEL, the Linac Coherent Light Source at Stanford, it will become possible to produce subfemtosecond x-ray pulses at intensities reaching 6 X 10(16) W/cm(2). Small-signal gain calculations indicate that saturation of more than one lasing line is possible, resulting in temporally separated femtosecond x-ray pulses of different wavelengths. The presented lasing scheme creates broad capability for advancing the field of high-intensity ultrashort x-ray physics.