{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a156f517a28d5d9b367ec4f588853293a1ac701337c4e0e09894fda4f0a64c69","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["2590722c932c601bc46523cec65d6f0d3e0892d5481d07c0791e0d31ff5e7ce1","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"29757b8aa0556fceff91e6af4dc4070669ea593ee8071117d764a502303d5ed8","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"yes-no","assisted_by":[],"external_id":"beams.fel-physics.xfelo-gain","kind":"well-posed","literature_status":"partially-resolved","n":"2","parents":[],"plain":"An x-ray FEL oscillator traps x-rays between diamond crystal mirrors and amplifies them on every pass, which would give a fully coherent, very narrow-band beam; amplification over a few passes has been seen, but steady growth to a stable, saturated output has not.","posed_since":"2008","precise":"In an x-ray FEL oscillator, hard x-ray pulses of photon energy $\\sim 5\\text{-}15\\ \\mathrm{keV}$ circulate in a Bragg-crystal cavity with round-trip reflectivity above $\\sim 85\\%$ and are amplified by successive electron bunches synchronized with the round trip. Determine experimentally whether round-trip gain exceeding total cavity loss, sustained exponential growth over many passes, and saturation with relative bandwidth $\\sim 10^{-7}$ can be obtained with existing high-repetition-rate linac beams, given crystal heat load and cavity alignment stability.","problem_ref":"2590722c932c601bc46523cec65d6f0d3e0892d5481d07c0791e0d31ff5e7ce1","references":"","settled_by":"Observation of exponential growth of stored x-ray pulse energy over many successive passes, followed by saturation.","status_note":"Spectral narrowing and proof-of-concept multi-pass amplification were reported at the European XFEL demonstrator (Li, Rauer, Huang, Deng, review arXiv:2608.15244, https://arxiv.org/abs/2608.15244, 2026); sustained growth to saturation is not reported.","title":"Exponential growth to saturation in a hard x-ray FEL oscillator","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"a5496c3bb54f3df4ac2daba2a0f230d9c9e70dcc27fb878a526f478a1a22fb7c","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"963e8572c230bac84b8bc69143efe80b1ed02b13928d0df540750d3c4767da33","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"eXh05SskWqV0r_5Rd5EV77YI5PLowVvCG5w84pJeT4XyRB07ChWvr755wygm3KBlKNb0h3x_iw9g81SAV8i4DA"},"schema":"pubphys.envelope/1"},"record_hash":"a5496c3bb54f3df4ac2daba2a0f230d9c9e70dcc27fb878a526f478a1a22fb7c","leaf_index":2484}