Applications of terawatt-class lasers can enormously benefit from pulse trains with kHz repetition rates. The associated unprecedented combinations of peak and average powers require the development of new concepts for scalable ultrashort pulse generation. Through comprehensive simulations, we analyze spectral broadening of 300-ps, 100-mJ pulses in multi-pass cells and anti-resonant hollow-core fibers towards the fs regime. We introduce an 11-mirror cell geometry that can deliver kilometer-scale nonlinear interaction lengths. We experimentally demonstrate nearly 300 passes in such a cell. This enables at least four times larger B-integral to peak power ratios than in previously reported spectral broadening experiments with air-filled multi-pass cells. These results highlight the strong potential of advanced multi-pass cell designs to reach unprecedented broadening factors. The proposed scheme can efficiently transform industrially mature high-power, high-energy lasers into unique ultrafast sources.
