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Description
The shock ignition approach (SI) [1] to inertial confinement fusion (ICF) [2] relaxes the ignition requirements by splitting the compression phase, led by a low-intensity laser pulse, from the ignition phase, driven by laser pulses of around 10$^{16}$ W/cm$^2$. Albeit more robust against hydrodynamics instabilities, SI is more vulnerable to laser-plasma instabilities (LPI) [3], which produce undesired scattering of laser light and hot-electron (HE).
Here we report on the preliminary analysis of a planar experiment carried out on the National Ignition Facility (NIF) laboratory aimed to investigate LPI at SI conditions to understand the origin of HE and characterise the light scattering processes. In the experiment, the laser pulse irradiates a planar CH target and reaches an on-target spike intensity of 1$\times10^{16}$ W/cm$^2$, with beams arranged in two cones.
The data analysis shows an angular dependence of Raman scattering: at 50$^{\circ}$ Raman signal displays side-scattered-like behaviour [4]. The average HE temperature measured is around 50 keV, with hot electron energy conversion efficiency of around 12$\%$. This does not match the measured time averaged low Raman reflectivity. From the near-backscattered imager (NBI), we estimate that only 30$\%$ of the scattered light is recorded by the light station. In order to understand the processes involved, we perform two-dimensional particle-in-cell simulations at relevant laser and plasma conditions.
Reference
[1] R. Betti, et al., Phys. Rev. Lett. 98, 155001 (2007) - K. S. Anderson, et al., Phys. Plasmas 20 056312 (2013);
[2] J. D. Lindl, ``Inertial Confinement Fusion'', AIP-Press (1998).
[3] W. Kruer, The Physics Of Laser Plasma Interactions", CRC Press (2003).
[4] M. J. Rosenberg, ,et al., Phys. Plasmas 27, 042705 (2020);