@misc{oai:repo.qst.go.jp:00073306,
 author = {ピロジコフ, アレキサンダー and Timur, Esirkepov and A., Pikuz T. and Ya., Faenov A. and Ogura, Koichi and Hayashi, Yukio and Kotaki, Hideyuki and N., Ragozin E. and Neely, D. and Kiriyama, Hiromitsu and Kevin, Koga James and Fukuda, Yuuji and Sagisaka, Akito and Nishikino, Masaharu and Imazono, Takashi and Hasegawa, Noboru and Kawachi, Tetsuya and Daido, Hiroyuki and Kato, Yoshiaki and Bulanov, Sergey and Kondo, Kiminori and Kando, Masaki 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 = {We have recently discovered high-order harmonics from relativistic electron spikes [1, 2] produced by multi-terawatt relativistic-irradiance (>10^18 W/cm^2) femtosecond lasers focused onto gas jet targets with typical electron densities ~ a few x10^19 cm^-3. These harmonics represent a new source of bright coherent x-rays. Our experiments with the J KAREN laser [3] demonstrated a strong dependence of the harmonics generation efficiency on the laser pulse energy and quality, in particular on the focal spot shape.
\nHere we discuss laser requirements for efficient harmonics generation. In addition to the obvious need for a higher power, the focal spot quality should approach the diffraction limit, so that the Strehl ratio exceeds 0.5. We consider two typical reasons of the spot degradation. First, it is the noise-like high-frequency wavefront distortions. The above stated quality requirement corresponds to the rms wavefront error of much smaller than 100 nm. Second, it is the angular dispersion, which must be kept smaller than a fraction of the diffraction divergence. For typical 100 to 300 mm beam diameters and 50 nm bandwidths, the angular dispersion should not exceed a urad level and the angular chirp should be less than ~10^-2 urad/nm. We show our recent progress in satisfying these requirements with the J-KAREN-P laser.
\n[1] A. S. Pirozhkov, et al., "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes," Phys. Rev. Lett. 108, 135004-5 (2012).
\n[2] A S. Pirozhkov, et al., "High order harmonics from relativistic electron spikes," New J. Phys. 16, 093003-30 (2014).
\n[3] H. Kiriyama, et al., "High-Contrast, High-Intensity Petawatt-Class Laser and Applications," IEEE J. Sel. Topics Quantum Electron. 21, 1601118-18 (2015)., The Optics & Photonics International Congress OPIC-2016, International Conference on High-Energy Density Sciences HEDS-2016},
 title = {Required Laser Properties for Efficient High-Order Harmonic Generation from Relativistic Electron Spikes},
 year = {2016}
}