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

Direct-drive–ignition designs with plastic CH ablators create plasmas of long density scale lengths (L_n ≥ 500 μm) at the quarter-critical density (N_qc) 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 L_n approaching ∼400 μm have been created; (2) the density scale length at N_qc scales as $Ln(μm)≃(RDPP×I1/4/2);$ and (3) the electron temperature T_e at N_qc 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 (R_DPP) 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 f_hot is found to have a similar behavior for both configurations: a rapid growth [$fhot≃fc×(Gc/4)6$ for G_c < 4] followed by a saturation of the form, $fhot≃fc×(Gc/4)1.2$ for G_c ≥ 4, with the common wave gain is defined as $Gc=3 × 10−2×IqcLnλ0/Te,$ where the laser intensity contributing to common-wave gain I_qc, L_n, T_e at N_qc, and the laser wavelength λ₀ are, respectively, measured in [10¹⁴ W/cm²], [μm], [keV], and [μm]. The saturation level f_c is observed to be f_c ≃ 10^–2 at around G_c ≃ 4. The hot-electron temperature scales roughly linear with G_c. 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.