Speaker
Description
With the recent outstanding progress towards Inertial Confinement Fusion (ICF) ignition at the National Ignition Facility, the interest for high gain Inertial Fusion Energy (IFE) is rapidly expanding. Shock ignition (SI) is based on direct drive and relies on a strong shock wave (>300 Mbar) to be launched by means of a short laser spike (300-500 ps) irradiation at intensities around 1016 W cm-2 [1,2] at the end of the compression phase to initiate ignition. The success of the SI concept depends mainly on the coupling of the laser spike with the plasma surrounding the imploding shell, where the onset of parametric instabilities, including Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Two-Plasmon Decay (TPD), can lead to a degradation of the laser-plasma coupling. Moreover, TPD and SRS generate electron plasma waves (EPW) that give rise to hot electrons (HE) which, depending on their energy, may affect the shock pressure or preheat the pre-compressed fuel. The onset of laser-driven instabilities is, in turn, affected by the growth of micrometer-scale filamentation, which produces a local enhancement of laser intensity and a modification of plasma density profile. Modelling of the interaction of the ignition shock laser pulse [3] with the coronal plasma is made complex by the presence of highly non-linear kinetic effects - notably collective speckles effects and ponderomotive electron dynamics - and by the competition of the various processes, whose modelling is beyond the current full-scale kinetic simulations capabilities.
In a recent experimental campaign we investigated the impact of parametric instabilities – in particular SRS and TPD – and the generation of HE on the interaction of a laser pulse at SI intensity (~1016 W cm-2) with a preformed, long scale-length plasma mimicking the ICF corona. In the experiment [4] a long scale-length, hot plasma was generated by using 4 driver heating beams (250 J, 1053 nm, 3 ns) at an intensity on target of ~3x1013 W/cm2 per beam, simultaneously focused with a F/10 optics at angles of 25° and -25° in the vertical plane (Fig.1). Random Phase Plates on the beams gave a focal spot of 800 μm (FWHM) on a flat foil targets consisting of a 50 µm thick plastic to mimic the low-Z ablating capsule material in a direct drive ICF compression. A separate interaction beam (85 J, 527 nm, 500 ps) was focused on the approximately 1D expanding plasma [4] with an F/2.5 optics and with p-polarization, to mimic the shock driving beam in the SI scheme. A Random Phase Plate was used which resulted in a FWHM ≈ 30 μm spot and in a laser intensity of ~1016 W/cm2. The interaction beam was delayed by 0.3 ns to 3 ns with respect to the rise time of the driver beams. Simulations with the 2D hydro-code DUED show that density scale-length in the interaction region (0.04 nc < ne < 0.25 nc) can be in this way tuned in the range 90-500 μm at the beginning of the interaction and electron temperatures range between 1 keV and 5 keV in the underdense plasma. Diagnostics included time resolved optical spectroscopy for SRS, TPD and SBS, and broadband X-ray emission for hot electron generation [5] using recently developed analysis tools [12] to compare HE temperatires expected from SRS and TPD. The interaction of the narrow-band interaction pulse was compared with the stretched laser pulse obtained by the amplification of a larger bandwidth oscillator, resulting in a laser bandwidth Δλ/λ≈0.3% but maintaining the same intensity on target and the same pulse duration as the narrow-band pulse. Remarkably, as shown in Fig.1(right), the shots with broad-band stretched pulse yielded a strong spectral signature of SRS driven at higher densities, with a strong reduction of the SRS driven at lower density compared with the narrow-band interaction. This can be explained by the short coherence time τp of the laser light in this configuration (~ 1 ps), inhibiting filamentation at low plasma densities and reducing the plasma-smoothing of the laser propagating in denser regions. In fact it is well known that in classical Direct-Drive interaction regime, at intensities around 1E14 W cm–2, a reduction of laser coherence time results in the inhibition of both SBS and filamentation, and consequently also of SRS which is – in those conditions - under threshold [7,8]. This motivated [9] the use of Induced Spatial Incoherence (ISI) or Smoothing by Spectral Dispersion (SSD) on the laser beams, which reduces the SRS to negligible levels [10]. The picture, however, could dramatically change at the higher laser intensities of the SI conditions. Here, as suggested by the preliminary new results given above, the inhibition of filamentation at low densities could produce a stronger LPI (SRS, and possibly TPD) in denser regions and in principle also the generation of more energetic HE. We stress that such coherence time of the pulse with broadband oscillator, τp ≈ 1 ps, is of the same order of magnitude of the filamentation growth time and, based on the above results, can account for an on/off effect on the filamentation of speckles at low density.
Acknowledgements
The authors acknowledge financial support from the LASERLAB-EUROPE Access to Research Infrastructure activity within the EC’s seventh Framework Program (Application No. 18110033).
References
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[3]O. Klimo et al., Plasma Phys. Control. Fusion 56, 055010 (2014).
[4]G. Cristoforetti et al., arXiv:2108.13485, submitted to HPLSE
[5]C. D. Chen et al., Rev. Sci. Instrum. 79, 10E305 (2008)
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