Diagnosing and controlling mix in National Ignition Facility implosion experiments

Abstract

High mode number instability growth of “isolated defects” on the surfaces of National Ignition Facility [Moses et al., Phys. Plasmas 16, 041006 (2009)] capsules can be large enough for the perturbation to penetrate the imploding shell, and produce a jet of ablator material that enters the hot-spot. Since internal regions of the CH ablator are doped with Ge, mixing of this material into the hot-spot results in a clear signature of Ge K-shell emission. Evidence of jets entering the hot-spot has been recorded in x-ray images and spectra, consistent with simulation predictions [Hammel et al., High Energy Density Phys. 6, 171 (2010)]. Ignition targets have been designed to minimize instability growth, and capsule fabrication improvements are underway to reduce “isolated defects.” An experimental strategy has been developed where the final requirements for ignition targets can be adjusted through direct measurements of mix and experimental tuning.

Analysis of mix experiments on Omega

“…Until recently, simulations did not include the effects of mix and even now, the mix prescriptions are either approximate or are applicable only in one dimension (1-D) [4][5][6][7]. We have not seen any reference to 2-D calculations with a subgrid turbulent mix model in the literature, although Hammel et al [6] uses a large eddy model in their simulations. The simulations described in this paper are 2-D and use the BHR-2 turbulent mix model [8].…”

Non-LTE modeling for the National Ignition Facility (and beyond)

“…In addition, the presence of Ge in this material raises the possibility of a spectroscopic diagnostic for the mixed material. This is discussed in detail elsewhere, for simulations [2] and for application to experimental data [8]. Here we consider the implications and requirements for NLTE modeling of observed spectrum.…”

“…Here we consider the implications and requirements for NLTE modeling of observed spectrum. Figure 3 shows the innermost portion of the results of a 2D simulation of a Rev 5 capsule (see [2] for details). The simulation follow the evolution of a perturbation due to the fill tube, shown here at a time several ps before peak compression.…”

“…For this time-resolved spectrum, the emission is primarily due to these transitions, but in time-integrated spectra the red components dominate and mostly come from Li-like and Be-like satellites. These emission features, corrected for optical depth effects, can be used to approximately infer the amount of hot Ge in the fuel region [2,9]. A large emission feature also appears at energies between 9.5 and 10.0 keV.…”

“…Current capsule designs avoid excessive preheat from high-energy X-rays by shielding the fuel with a mid-Z dopant, which affects the capsule dynamics. The dopant also mixes into the hot spot through hydrodynamic instabilities, providing diagnostic possibilities but potentially impacting the energy balance of the capsule [2]. Looking beyond the NIF, a proposed design for a fusion reactor chamber depends on lowdensity high-Z gas absorbing X-rays and particles to protect the first wall [3].…”