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Because the undulator period is half the wavelength of the laser pulse, LDFELs can amplify X rays using lower electron energies and over shorter interaction lengths than a traditional free-electron laser. In LDFELs driven by conventional laser pulses, the undulator uniformity required for high gain necessitates large laser-pulse energies. Here, we show that a flying-focus pulse provides the undulator uniformity required to reach high gain with a substantially lower energy than a conventional pulse. The flying-focus pulse features an intensity peak that travels in the opposite direction of its phase fronts. This enables an LDFEL configuration where an electron beam collides head-on with the phase fronts and experiences a near-constant undulator strength as it co-propagates with the intensity peak. Three-dimensional simulations of this configuration demonstrate the generation of megawatts of coherent X-ray radiation with 20\u00a0\u00d7 less energy than a conventional laser pulse.<\/jats:p>","DOI":"10.1038\/s42005-025-02028-x","type":"journal-article","created":{"date-parts":[[2025,3,23]],"date-time":"2025-03-23T09:17:35Z","timestamp":1742721455000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["X-ray free-electron lasing in a flying-focus undulator"],"prefix":"10.1038","volume":"8","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-6556-5139","authenticated-orcid":false,"given":"D.","family":"Ramsey","sequence":"first","affiliation":[]},{"given":"B.","family":"Malaca","sequence":"additional","affiliation":[]},{"given":"T. T.","family":"Simpson","sequence":"additional","affiliation":[]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2704-6474","authenticated-orcid":false,"given":"M. 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