XFEL SASE: micro-bunching instability in an undulator gives coherent X-rays

Kondratenko-Saldin 1980 / Pellegrini 1992 self-amplified spontaneous emission (SASE): a relativistic electron bunch (γ ≈ 17000 for 9 GeV, LCLS) traversing a long undulator (periodic transverse magnetic field, period λ_u, strength K =…