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Soft-x-ray harmonic comb from relativistic electron spikes

Pirozhkov, A. S. and Kando, M. and Esirkepov, T. Zh and Gallegos, P. and Ahmed, H. and Ragozin, E. N. and Faenov, A. Ya and Pikuz, T. A. and Kawachi, T. and Sagisaka, A. and Koga, J. K. and Coury, M. and Green, J. and Foster, P. and Brenner, C. and Dromey, B. and Symes, D. R. and Mori, M. and Kawase, K. and Kameshima, T. and Fukuda, Y. and Chen, L. and Daito, I. and Ogura, K. and Hayashi, Y. and Kotaki, H. and Kiriyama, H. and Okada, H. and Nishimori, N. and Imazono, T. and Kondo, K. and Kimura, T. and Tajima, T. and Daido, H. and Rajeev, P. and McKenna, P. and Borghesi, M. and Neely, D. and Kato, Y. and Bulanov, S. V. (2012) Soft-x-ray harmonic comb from relativistic electron spikes. Physical Review Letters, 108 (13). -. ISSN 0031-9007

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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 mu J/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.