title: Soft-x-ray harmonic comb from relativistic electron spikes.
abstract: We demonstrate a new high-order harmonic generation mechanism reaching the "water window" spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving μJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.
authors.affiliation: Advanced Beam Technology Division, JAEA, 8-1-7 Umemidai, Kizugawa, Kyoto 619-0215, Japan.
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