An optical spectrometer system based on 60 channels of fiber has been designed and employed to diagnose scattered lights from laser-plasma instabilities. The 60 fiber collectors cover an integrated solid angle of π, enabling the measurement of global energy losses in a symmetrical configuration. A detecting spectral range from ultraviolet to near-infrared, with angular distribution, allows for the understanding of the physical mechanisms involving various plasma modes. Experimental measurements of scattered lights from a conical implosion driven by high-energy nanosecond laser beams at the Shenguang-II Upgrade facility have been demonstrated, serving as reliable diagnostics to characterize laser absorption and energy losses from laser-plasma instabilities. This compact diagnostic system can provide comprehensive insights into laser energy coupling in direct-drive inertial confinement fusion research, which is essential for studying the driving asymmetry and improving the implosion efficiencies.
This system is closely integrated with the layout of the laser beams and the geometric structure of the target chamber in the Shenguang-II Upgrade facility. Sixty reflective collectors are installed above the equatorial plane and fixed on the inner wall of the spherical target chamber, covering one-quarter of the spherical surface area (Fig. 1a). The global scattered-light spectral information is extrapolated based on the symmetry of the incident laser and the target.
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Fig. 1 Schematic diagram of the 60-channel optical spectrometer. 60 reflective collectors are set on the inner side of the chamber wall, as shown in (a), collecting the lights emitted from the laser-target interactions. The coordinates of the collectors in the 2D angular map, with Beams #1, #3 and #7 incident angles are shown in (b). The collectors reflect the lights to a fiber bundle, which is extended for 20 m outside the chamber to a spectrometer, as show in (c), and the spectra are recorded by a CCD camera.
This system was used to measure the laser energy absorption during the conical implosion compression stage of the DCI scheme at the Shenguang-II Upgrade facility. In the 60-channel scattered light spectral image (Fig. 2a), the signals near 351 nm contain the direct reflection of the laser, stimulated Brillouin scattering (SBS), and crossed-beam energy transfer (CBET) (Fig. 2b). The continuous spectrum from 500 - 650 nm originates from stimulated Raman scattering (SRS), and its energy dominates the side scattering (Fig. 2c). The secondary scattering branch of two-plasmon decay (TPD) at ω/2 (680 - 720 nm) can be observed (Fig. 2d). Combining the measurement results of this system and the full-aperture backscatter system, in the DCI-R7 experiment, the peak power density of the overlapped lasers was 5.7×10¹⁴ W/cm², and the laser energy absorption rate was approximately 94.6%.
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Fig. 2 (a) Raw image for the multi-channel spectra of the scattered lights from the laser-driven shell-in-cone target implosion. Here the longitudinal axis represents collectors at different orientations, and the wavelength in spectra indicates different processes of LPIs (spectral response need to be taken into account for absolute energy calculation). The corresponding angular distributions of the 351-nm lights, SRS and ω/2 re-scattering TPD are shown in (b), (c) and (d), using linear interpolation for the directions in between two adjacent collectors. The incident angles for the laser beams are shown in the angular distribution maps.
The thorough report of the design, calibration and experimental application has been published in High Power Laser Science and Engineering, vol. 12, Issue 6 (Yihang Zhang, Zhe Zhang, Xu Zhao, Kevin Glize, Yufeng Dong, Xiaohui Yuan, Yutong Li, Jie Zhang, "Global scattered-light spectrography for laser absorption and laser–plasma instability studies," High Power Laser Sci. Eng. 12, 06000e84 (2024)).
