A series of indirectly driven capsule implosions has been performed on the National Ignition Facility to assess the relative contributions of ablation-front instability growth vs. fuel compression on ...implosion performance. Laser pulse shapes for both low and high-foot pulses were modified to vary ablation-front growth & fuel adiabat, separately and controllably. Two principal conclusions are drawn from this study: 1) It is shown that an increase in laser picket energy reduces ablation-front instability growth in low-foot implosions resulting in a substantial (3-10X) increase in neutron yield with no loss of fuel compression. 2.) It is shown that a decrease in laser trough power reduces the fuel adiabat in high-foot implosions results in a significant (36%) increase in fuel compression together with no reduction in neutron yield. These results taken collectively bridge the space between the higher compression low-foot results and the higher yield high-foot results.
A wide range of targets with laser energies spanning two orders of magnitude have been shot at the National Ignition Facility (NIF). The National Ignition Campaign (NIC) targets are cryogenic with Si ...supports and cooling rings attached to an Al thermo-mechanical package (TMP) with a thin (30 micron) Au hohlraum inside. Particular attention is placed on the low-energy shots where the TMP is not completely vaporized. In addition to NIC targets, a range of other targets has also been fielded on NIF. For all targets, simulations play a critical role in determining if the risks associated with debris and shrapnel are acceptable. In a number of cases, experiments were redesigned, based on simulations, to reduce risks or to obtain data. The majority of these simulations were done using the ALE-AMR code, which provides efficient late-time (100-1000X the pulse duration) 3D calculations of complex NIF targets.
A detailed simulation-based model of the June 2011 National Ignition Campaign (NIC) cryogenic DT experiments is presented. The model is based on integrated hohlraum-capsule simulations that utilize ...the best available models for the hohlraum wall, ablator, and DT equations of state and opacities. The calculated radiation drive was adjusted by changing the input laser power to match the experimentally measured shock speeds, shock merger times, peak implosion velocity, and bangtime. The crossbeam energy transfer model was tuned to match the measured time-dependent symmetry. Mid-mode mix was included by directly modeling the ablator and ice surface perturbations up to mode 60. Simulated experimental values were extracted from the simulation and compared against the experiment. The model adjustments brought much of the simulated data into closer agreement with the experiment, with the notable exception of the measured yields, which were 15-45% of the calculated yields.
The reconstruction algorithm used for a new spatially resolved plasma optical emission spectroscopy sensor is described. The sensor has a wedge shaped field of view which is rotated horizontally ...across the plasma. The resulting signal as a function of angle defines an ill-posed linear problem that must be solved to determine the emissivity as a function of radius in the plasma. This problem is solved by modified Tikhonov regularization using a finite difference regularizer which discourages rapid variation in the reconstructed emission. The optimal regularization parameter is determined by minimizing the product of the norm of the residual and the norm of this regularizer. The robustness of the algorithm against noise introduced into idealized data is demonstrated, both for a standard Abel inversion problem and for test data based on the sensor model. The algorithm is used to reconstruct emitted power density profiles and to perform actinometry in a Lam TCP 9400 plasma processing tool; these reconstructions show qualitatively correct behaviors.
Electrons in a standing electromagnetic wave--an optical lattice--tend to oscillate due to the quiver and ponderomotive potentials. For sufficiently intense laser fields (Ilamda2 approximately < or = ...5 x 10(17) W cm(-2) microm2) and in plasmas with sufficiently low electron densities (n approximately < or = 10(18) cm(-3)), these oscillations can occur faster than the plasma can respond. This paper shows that these oscillations result in Thomson scattering of light at both the laser and ponderomotive bounce frequencies and their harmonics as well as at mixtures of these frequencies. We term this mixing ponderomotive intermodulation. Here, the case of counterpropagating laser beams creating a one-dimensional (1D) optical lattice is analyzed. The near-equilibrium electron orbits and subsequent Thomson scattering patterns are computed in the single-particle limit. Scaling laws are derived to quantify the range of validity of this approach. Finally, collective plasma and laser focusing effects are included by using particle-in-cell (PIC) techniques. This effect resulting in light-frequency conversion has applications both as an infrared light source and as a means to diagnose high laser intensities inside dense plasmas.
NIF experiments with Be capsules have followed a path of the highly successful "high-foot" CH capsules. Several keyhole and ConA targets preceeded a DT layered shot. In addition to backscatter ...subtraction, laser drive multipliers were needed to match observed X-ray drives. Those for the picket (0.95), trough (1.0) and second pulse (0.80) were determined by VISAR measurements. The time dependence of the Dante total x-ray flux and its fraction > 1.8 keV reflect the time dependence of the multipliers. A two step drive multiplier for the main pulse can match implosion times, but Dante measurements suggest the drive multiplier must increase late in time. With a single set of time dependent, multi-level multipliers the Dante data are well matched. These same third pulse drive multipliers also match the implosion times and Dante signals for two CH capsule DT. One discrepancy in the calculations is the X-ray flux in the picket. Calculations over-estimate the flux > 1.8 keV by a factor of ~100, while getting the total flux correctly. These harder X-rays cause an expansion of the Be/fuel interface of 2-3 km/s before the arrival of the first shock. VISAR measurements show only 0.2 to 0.3 km/s. The X-ray drive on the DT Be capsule was further degraded by a random decrease of 9% in the total picket flux. This small change caused the capsule fuel to change from an adiabat of 1.8 to 2.3 by mistiming of the first and second shocks. With this shock tuning and adjustments to the calculation, the first NIF Be capsule implosion achieved 29% of calculated yield, comparable to the CH DT capsules of 68% and 21%. Inclusion of a large M1 asymmetry in the DT ice layer and mixing from instability growth may help explain this final degradation. In summary when driven similarly the Be capsules performed like CH capsules. Performance degradation for both seems to be dominated by drive and capsule asymmetries.
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