Hydrodynamic simulations of long-scale-length two-plasmon-decay experiments at the Omega Laser Facility

Hu, S. X.; Michel, D. T.; Edgell, D. H.; Froula, D. H.; Follett, R. K.; Goncharov, V. N.; Myatt, J. F.; Skupsky, S.; Yaakobi, B.
March 2013
Physics of Plasmas;Mar2013, Vol. 20 Issue 3, p032704
Academic Journal
Direct-drive-ignition designs with plastic CH ablators create plasmas of long density scale lengths (Ln ≥ 500 μm) at the quarter-critical density (Nqc) region of the driving laser. The two-plasmon-decay (TPD) instability can exceed its threshold in such long-scale-length plasmas (LSPs). To investigate the scaling of TPD-induced hot electrons to laser intensity and plasma conditions, a series of planar experiments have been conducted at the Omega Laser Facility with 2-ns square pulses at the maximum laser energies available on OMEGA and OMEGA EP. Radiation-hydrodynamic simulations have been performed for these LSP experiments using the two-dimensional hydrocode draco. The simulated hydrodynamic evolution of such long-scale-length plasmas has been validated with the time-resolved full-aperture backscattering and Thomson-scattering measurements. draco simulations for CH ablator indicate that (1) ignition-relevant long-scale-length plasmas of Ln approaching ∼400 μm have been created; (2) the density scale length at Nqc scales as Ln(μm)≃(RDPP×I1/4/2); and (3) the electron temperature Te at Nqc scales as Te(keV)≃0.95×I, with the incident intensity (I) measured in 1014 W/cm2 for plasmas created on both OMEGA and OMEGA EP configurations with different-sized (RDPP) distributed phase plates. These intensity scalings are in good agreement with the self-similar model predictions. The measured conversion fraction of laser energy into hot electrons fhot is found to have a similar behavior for both configurations: a rapid growth [fhot≃fc×(Gc/4)6 for Gc < 4] followed by a saturation of the form, fhot≃fc×(Gc/4)1.2 for Gc ≥ 4, with the common wave gain is defined as Gc=3 × 10-2×IqcLnλ0/Te, where the laser intensity contributing to common-wave gain Iqc, Ln, Te at Nqc, and the laser wavelength λ0 are, respectively, measured in [1014 W/cm2], [μm], [keV], and [μm]. The saturation level fc is observed to be fc ≃ 10-2 at around Gc ≃ 4. The hot-electron temperature scales roughly linear with Gc. Furthermore, to mitigate TPD instability in long-scale-length plasmas, different ablator materials such as saran and aluminum have been investigated on OMEGA EP. Hot-electron generation has been reduced by a factor of 3-10 for saran and aluminum plasmas, compared to the CH case at the same incident laser intensity. draco simulations suggest that saran might be a better ablator for direct-drive-ignition designs as it balances TPD mitigation with an acceptable hydro-efficiency.


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