High order harmonics from relativistic electron spikes

Pirozhkov, A. S.; Kando, Masaki; Esirkepov, T. Z.; Faenov, A. Y.*; Pikuz, T. A.*; Kawachi, Tetsuya; Sagisaka, Akito; Koga, J. K.; Mori, Michiaki; Kawase, Keigo*; Kameshima, Takashi*; Fukuda, Yuji; Chen, L.-M.*; Daito, Izuru*; Ogura, Koichi; Hayashi, Yukio; Kotaki, Hideyuki; Kiriyama, Hiromitsu; Okada, Hajime; Imazono, Takashi; Kondo, Kiminori; Kimura, Toyoaki*; Tajima, Toshiki*; Daido, Hiroyuki; Bulanov, S. V.; Nishimori, Nobuyuki; Kato, Yoshiaki* ; Gallegos, P.*; Green, J.*; Foster, P.*; Brenner, C.*; Symes, D. R.*; Rajeev, P.*; Neely, D.*; Ahmed, H.*; Dromey, B.*; Borghesi, M.*; Ragozin, E. N.*; Coury, M.*; McKenna, P.*

We provide a description of experiments performed with the J-KAREN and Astra Gemini lasers where we discovered a new regime of relativistic high-order harmonic generation by multi-terawatt femtosecond lasers in gas targets. The results were explained using particle-in-cell simulations and catastrophe theory. Our work paves the way towards a bright coherent X-ray source based on compact lasers and accessible, repetitive, and debris-free gas jet targets. Such a source will be crucial for fundamental research and numerous applications requiring pumping, probing, imaging of microscopic objects and for attosecond science.