[13p-2G-1~15] 3.6 超高速・高強度レーザー

▲ [13p-2G-1] Laser Requirements for Efficient X-Ray Generation by Relativistic Electron Spikes

〇Alexander Pirozhkov¹, Masaki Kando¹, Timur Esirkepov¹, Tatiana Pikuz², Anatoly Faenov², Koichi Ogura¹, Yukio Hayashi¹, Hideyuki Kotaki¹, Eugene Ragozin³, David Neely⁴, Hiromitsu Kiriyama¹, James Koga¹, Yuji Fukuda¹, Akito Sagisaka¹, Masaharu Nishikino¹, Takashi Imazono¹, Noboru Hasegawa¹, Tetsuya Kawachi¹, Hiroyuki Daido¹, Yoshiaki Kato⁵, Sergei Bulanov¹, Kiminori Kondo¹ (1.JAEA, 2.Osaka Univ., 3.Lebedev Phys. Inst., 4.CLF RAL, 5.GPI)

キーワード：Relativistic Laser Plasma,High-Order Harmonics from Relativistic Electron Spikes,High-Power Lasers

High-order harmonics from relativistic electron spikes [PRL 108, 135004 (2012); NJP 16, 093003 (2014)] are a source of bright coherent x-rays produced by multi-terawatt relativistic-irradiance (>10¹⁸ W/cm²) femtosecond lasers focused onto gas jet targets. Our experiments with the J-KAREN laser [JSTQE 21, 1601118 (2015)] demonstrate that the harmonics generation efficiency strongly depends on laser pulse quality and in particular on the focal spot shape. Here we establish requirements for the high-power laser needed for efficient harmonics generation. Specifically, the focal spot should be close to a diffraction-limited shape with the Strehl ratio greater than 0.5. This requires a rms wavefront error of much smaller than 100 nm, assuming a noise-like high-frequency wavefront error distribution. In addition, the angular dispersion must be kept smaller than a fraction of the diffraction divergence. For typical 100 to 300 mm diameter high-power laser beams, this requires angular dispersion below the μrad level, i.e. < 10^-2 μrad/nm angular chirp for typical 50 nm bandwidths.