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 mu m) 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-scalelength 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 similar to 400 mu m have been created; (2) the density scale length at N-qc scales as L-n(mu m) similar or equal to (R-DPP x I-1/4 / 2); and (3) the electron temperature T-e at N-qc scales as T-e(keV) similar or equal to 0.95 x root I, with the incident intensity (I) measured in 10(14) W/cm(2) 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 f(hot) is found to have a similar behavior for both configurations: a rapid growth [f(hot) similar or equal to f(c) x (G(c)/4)(6) for G(c) < 4] followed by a saturation of the form, f(hot) similar or equal to f(c) x (G(c)/4)(1.2) for G(c) >= 4, with the common wave gain is defined as G(c) = 3 x 10(-2) x IqcLn lambda(0)/T-e; where the laser intensity contributing to common-wave gain I-qc, L-n, T-e at N-qc, and the laser wavelength lambda(0) are, respectively, measured in [11(14) W/cm(2)], [mu m], [keV], and [mu m]. The saturation level fc is observed to be f(c) similar or equal to 10(-2) at around G(c) similar or equal to 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. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794285]