@misc{oai:repo.qst.go.jp:00073305,
 author = {ピロジコフ, アレキサンダー and エシロケポフ, ティムル and A., Pikuz T. and Ya., Faenov A. and 小倉, 浩一 and 林, 由紀雄 and 小瀧, 秀行 and N., Ragozin E. and Neely, D. and 桐山, 博光 and コーガ, ジェームズ and 福田, 祐仁 and 匂坂, 明人 and 錦野, 将元 and 今園, 孝志 and 長谷川, 登 and 河内, 哲哉 and Daido, Hiroyuki and 加藤, 義章 and ブラノフ, セルゲイ and 近藤, 公伯 and 神門, 正城 and ピロジコフ アレキサンダー and エシロケポフ ティムル and 小倉 浩一 and 林 由紀雄 and 小瀧 秀行 and 桐山 博光 and コーガ ジェームズ and 福田 祐仁 and 匂坂 明人 and 錦野 将元 and 今園 孝志 and 長谷川 登 and 河内 哲哉 and 加藤 義章 and ブラノフ セルゲイ and 近藤 公伯 and 神門 正城},
 month = {May},
 note = {*Abstract:* We discuss the new regime of high-order harmonic generation by relativistic-irradiance multi-terawatt femtosecond lasers focused onto gas jet targets [PRL 108, 135004 (2012); NJP 16, 093003 (2014)]. The laser induces multi-stream relativistic plasma flow resulting in the formation of density singularities: structurally stable, oscillating electron spikes coherently emitting high-frequency radiation. In this presentation we analyze the dependence of the harmonic yield on the laser power and focal spot quality, and derive the required laser parameters for efficient harmonics generation. We show the status of the J-KAREN-P laser and report on the progress towards satisfying these requirements.
\nIn the new regime of high-order harmonic generation, intense (>10^18 W/cm^2) high-power (multi-TW) femtosecond (30-50 fs) laser pulses focused onto gas targets induce multi-stream relativistic flows in underdense plasma (ne ~ 10^19 cm^-3). This results in the formation of density singularities, which are structurally stable, oscillating electron spikes coherently emitting high-frequency radiation with spectra comprising high-order harmonics [1, 2]. The experiments with the J-KAREN laser [3] demonstrated strong dependence of the harmonics yield on the laser pulse energy and focal spot quality.
\nIn this presentation we analyze the obtained dependences and derive laser parameters required for efficient harmonics generation. Apart from higher power, it turns out that the focal spot quality should approach the diffraction limit, i.e. the Strehl ratio should exceed 0.5. The focal spots of high-power lasers typically suffer from wavefront distortions and angular dispersion. For noise-like wavefront distortions, the above stated requirement corresponds to an rms wavefront error <100 nm. The angular dispersion should be kept smaller than a fraction of the diffraction divergence, i.e. urad level for 100 to 300 mm beam diameters. The corresponding angular chirp should be <10^-2 urad/nm for 50 nm bandwidth. We show the status of the J-KAREN-P laser and report on the progress towards satisfying these requirements.
\n1.A. S. Pirozhkov, et al., "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes," Phys. Rev. Lett. 108, 135004-5 (2012).
\n2.A. S. Pirozhkov, et al., "High order harmonics from relativistic electron spikes," New J. Phys. 16, 093003-30 (2014).
\n3.H. Kiriyama, et al., "High-Contrast, High-Intensity Petawatt-Class Laser and Applications," IEEE J. Sel. Topics Quantum Electron. 21, 1601118-18 (2015)., The 15th International Conference on X-Ray Lasers (ICXRL2016)},
 title = {High-order harmonic generation by relativistic plasma singularities},
 year = {2016}
}