Plasma density jump for the control of coherent x-ray emission via BISER mechanism

We have discovered Burst Intensification by Singularity Emitting Radiation (BISER) [1], a new phenomenon which in particular results in bright coherent x-ray generation by relativistic plasma singularities driven by a multi-terawatt femtosecond laser focused onto a gas jet target [2], [3]. Singularities are produced in plasma by the laser pulse which pushes electrons creating an electron-free cavity and bow wave [4]. The singularities at the joining of the cavity wall and bow wave manifest themselves as point-like coherent x-ray emitters. A spontaneous BISER regime realized in previous experiments exhibits relatively large shot-to-shot fluctuations of the x-ray source position and low overall stability because the emission occurs after the relativistic self-focusing. However, stability is desirable for prospective applications of the new coherent x-ray source.\nA tailored plasma density profile can help to control the BISER, as shown by our simulations with the PIC REMP code [5]. Here, we present an experiment with the plasma profile tailoring, where a shock in supersonic gas flow generates a sharp density up-jump. We demonstrate the BISER control using this density jump scheme and the J-KAREN-P laser [6]-[7]. In contrast to the spontaneous regime, here we obtain low-jitter x-ray source position and higher x-ray yield. Moreover, these advantages are achieved simultaneously in an optimized case. Our results represent a crucial step towards implementation of the next-generation bright compact coherent x-ray source. Further, they demonstrate the new technique of relativistic self-focusing control and its diagnostics.\nWe thank the J-KAREN-P laser operation group. We acknowledge the financial support from JSPS Kakenhi JP 25390135 and JP 26707031.\n[1]A. S. Pirozhkov, T. Zh. Esirkepov, et al., "Burst intensification by singularity emitting radiation in multi-stream flows," Scientific Reports 7, 17968 (2017).[2]A.S.Pirozhkov, et al. "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes" Phys. Rev. Lett. 108, 135004 (2012).[3]A.S.Pirozhkov et al., "High order harmonics from relativistic electron spikes" New J. Phys. 16, 093003-30 (2014).[4]T. Zh. Esirkepov, Y. Kato, and S. V. Bulanov, "Bow Wave from Ultraintense Electromagnetic Pulses in Plasmas," Phys. Rev. Lett. 101, 265001-4 (2008).[5]T. Zh. Esirkepov, "Exact charge conservation scheme for Particle-in-Cell simulation with an arbitrary form-factor," Comput. Phys. Comm. 135, 144-153 (2001).[6]H. Kiriyama, et al., "High-Contrast, High-Intensity Petawatt-Class Laser and Applications," IEEE J.S.T.Quantum Electron. 21, 1601118-18 (2015).[7]A.S.Pirozhkov, et al. "Approaching the diffraction-limited, bandwidth-limited Petawatt" Opt. Express 25, 20486 (2017).JSAP Spring Meetin