@misc{oai:repo.qst.go.jp:00085966,
 author = {Shatokhin, A. and Kotov, A. and M. Jeong, T. and M. Grittani, G. and Dzelzainis, T. and Hull, G. and Dann, S. and Akito, Sagisaka and Vishnyakov, E. and Kolesnikov, A. and Masato, Koike and Timur, Esirkepov and Masaki, Kando and Koichi, Ogura and A. Pikuz, T. and Kevin Koga, James and Hiromitsu, Kiriyama and Soloviev, A. and Ragozin, E. and Bulanov, Sergey and Kiminori, Kondo and Tetsuya, Kawachi and Symes, D. and Neely, D. and Pirozhkov, Alexander and Akito, Sagisaka and Masato, Koike and Timur, Esirkepov and Masaki, Kando and Koichi, Ogura and Kevin Koga, James and Hiromitsu, Kiriyama and Bulanov, Sergey and Kiminori, Kondo and Tetsuya, Kawachi and Pirozhkov, Alexander},
 month = {Mar},
 note = {Burst Intensification by Singularity Emitting Radiation (BISER) [1] is the phenomenon of extremely bright coherent wave emission (constructive interference, N^2 effect) by singularities of multi-stream flows. High-power femtosecond lasers can produce such flows and corresponding relativistic plasma singularities in underdense plasma; these singularities emit very bright coherent x-rays [2]-[3]. We have recently performed an experiment on BISER coherent x-ray generation with the Astra Laser in the CLF RAL, UK. We observed BISER coherent soft x-rays with two spectrographs: a three-channel flat field spectrograph [4] in the forward direction (channels at -0.6, 0, and 0.8°) operating in the 17-34 nm range and a normal-incidence imaging spectrograph [5] in a 10° off-axis direction operating in the 12.4-20 nm range. Both spectrographs, in addition, provided data on the angular distribution of the BISER radiation. We took advantage of an extensive experiment term and long daily laser operation hours to optimize the laser (in particular, focal spot, which is a necessary prerequisite for BISER [6], and pulse width) and the experimental conditions (in particular, the density profile). These resulted in ~2 orders of magnitude enhancement in BISER photon number, providing >10^11 photons and >μJ energies per pulse in each channel at 7 TW laser power, the result which could previously be obtained at >100 TW laser power only.

We acknowledge support from the Astra Laser Group and the CLF Target Fab, Mechanical, and Electrical. Financial support: JSPS Kakenhi JP 19KK0355 and 19H00669, CLF, Russian Science Foundation (20-62-46050), IAP RAS, ELI-Beamlines, the Ministry of Education, Youth and Sports of the Czech Republic by the project "Advanced Research Using High Intensity Laser Produced Photons and Particles" (CZ.02.1.01/0.0/0.0/16_019/0000789), and the QST Director Fund 創成的研究 #20.
    [1] A. Pirozhkov, T. Esirkepov et al., "Burst intensification by singularity emitting radiation in multi-stream flows," Sci. Rep. 7, 17968 (2017).
    [2] A. S. Pirozhkov et al. "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes" PRL 108 135004 (2012)
    [3] A. S. Pirozhkov et al., "High order harmonics from relativistic electron spikes" New J. Phys. 16, 093003 (2014).
    [4] D. Neely et al., "A multi-channel soft X-ray flat-field spectrometer," AIP Conf. Proc. 426, 479 (1998).
    [5] A. Shatokhin et al., "High-resolution stigmatic spectrograph for a wavelength range of 12.5-30 nm," Opt. Express 26 19009 (2018).
    [6] A. S. Pirozhkov, et al. "Laser Requirements for High-Order Harmonic Generation by Relativistic Plasma Singularities," Quantum Beam Sci. 2, 7 (2018)., Annual meeting of the JSAP},
 title = {BISER enhancement with Astra laser},
 year = {2022}
}