The feasibility of generating X-ray pulses in the 4-8 keV fundamental photon energy range with 0.65 TW peak power, 15 fs pulse duration and 9 × 10-5 bandwidth using the LCLS-II copper linac and hard X-ray (HXR) undulator is shown. In addition, third-harmonic pulses with 8-12 GW peak power and narrow bandwidth are also generated. High-power and small-bandwidth X-rays are obtained using two electron bunches separated by about 1 ns, one to generate a high-power seed signal, the other to amplify it through the process of the HXR undulator tapering. The bunch delay is compensated by delaying the seed pulse with a four-crystal monochromator. The high-power seed leads to higher output power and better spectral properties, with more than 94% of the X-ray power within the near-transform-limited bandwidth. Some of the experiments made possible by X-ray pulses with these characteristics are discussed, such as single-particle imaging and high-field physics.
Keywords: FEL; fresh-bunch FEL; terawatt FEL.