The Laboratory for Laser Energetics Completes Two National Ignition Campaign Level-2 Milestones

June 2007

LLE Infers 200 mg/cm2 Fuel Areal Density in a Cryogenic Direct-Drive Implosion on OMEGA

Figure 1: Optical shadowgraph of the target for shot 47206, showing a 2.4μm rms inner-ice-surface roughness.

On April 17 2007, LLE scientists completed a NIC Level-2 milestone when a neutron-burn-averaged areal density of 202▒7 mg/cm2 was inferred from the measured secondary proton spectrum, averaged over multiple directions.

The target for shot 47206 was a ~10-Ám thick CH shell enclosing a 95-Ám-thick D2 ice layer. An optical shadowgraph of the target is shown in Fig. 1. Full 3-D optical characterization showed that the target had a 2.4-Ám-rms inner-ice-surface roughness.

Figure 2: Measured and designed 16.5-kJ pulse shape for shot 47206.

The target was imploded with 16.5 kJ of 351-nm, UV laser light using a highly shaped laser pulse. The pulse shape shown in Fig. 2 included a picket prepulse to provide adiabat shaping. This is a high-contrast pulse shape with a designed fuel adiabat of 1.8 (ratio of the pressure to the Fermi-degenerate pressure). Fig. 2 also plots the designed pulse shape in gray. The width of the gray band is the specified pulse-shape tolerance. The measured pulse shape falls within this bound.

The areal density of the target was determined using the energy downshift of secondary D3He protons produced during the implosion. The spectra are measured using wedged range filters, positioned around the target chamber. Figure 3 shows the measured proton spectrum, averaged over the multiple detectors. The burn-weighted areal density (ρR) is 202▒7 mg/cm2, completing the milestone. This measurement provides a lower limit for the areal density because:

Figure 3: Secondary D³He proton spectrum showing a neutron averaged areal density of 202±7 mg/cm²
  • The detector has a lower energy threshold (cut-off) of 4 MeV (See Fig. 3). Any protons not measured because of this cutoff would lead to higher areal densities than quoted.
  • The areal density is measured during the period where nuclear reactions occur, i.e., before peak compression. Thus, the peak areal density achieved in the implosion is higher than that inferred from the proton spectrum.

From the areal density and x-ray pinhole camera images of the compressed core, the peak D2 density is estimated to be 100 g/cm2, roughly 500 times liquid deuterium density.

LLE Validates Polar-Direct-Drive Concept for Ignition on the NIF

Figure 4: Optical shadowgraph of the target for shot 47206, showing a 2.4-micron-rms inner-ice-surface roughness.

In 2006, LLE began studying the use of the National Ignition Facility (NIF) in the x-ray-drive configuration for direct-drive-ignition experiments. The study focused on implementing the standard polar-direct-drive (PDD) illumination scheme, which uses judicious re-pointing of the NIF beams to minimize illumination perturbations that arise from the absence of equatorial beams in the x-ray-drive laser configuration.

The study was completed in early 2007 and validated the polar-direct-drive concept using numerical simulations and experiments on OMEGA for ignition on the NIF.

Figure 5: PDD pointing configuration for the 4 cones of NIF beams.

The study used a DT, wetted-foam ignition design to take advantage of the increased laser absorption (compared to an all-DT design) obtained by the presence of carbon ions in the ablator material. The boost in absorbed energy compensates for refractive losses from the oblique, re-pointed NIF beams in the equatorial regions of the target. This design has undergone significant 2-D modeling that examines the effects of the PDD illumination and all other main perturbation sources (inner- and outer-surface roughness, power imbalance, and laser imprint). In all cases, ignition was obtained with gains ranging from 15 to 20. Fig. 4 shows a schematic of the target, as well as density and temperature contours near the time of ignition, from a 2-D simulation that includes all sources of nonuniformity. The simulations use 2-D SSD beam smoothing (1-THz, 2-color cycles) and take into consideration all current NIF direct-drive system specifications for sources of perturbations including 8% beam-to-beam power imbalance, 100-nm outer-surface roughness, and a 1-Ám-rms inner-surface roughness with less than 0.25-Ám rms contained in modes greater than 10.

Figure 6: The pulse shape and beam intensity profile (obtained from the requisite phase plate) for each ring of NIF beams

The beams are repointed to the positions shown in Fig. 5, and are well within the current NIF specification for positioning beams on target.

Future ignition experiments with PDD will require that each ring of beams uses a unique pulse shape and distributed phase plate. The combination of these two components provides adequate drive uniformity to achieve ignition conditions. Fig. 6 illustrates the pulse shape and beam-intensity profile (obtained from the requisite phase plate) for each ring of NIF beams. The pulse shapes adhere to current NIF specifications of peak fluence, and the phase plates are well within current manufacturing guidelines.

Results of these simulations are very encouraging especially considering that previous reports detailed the good correlation between the experimentally observed and theoretically predicted shell deformations of PPD implosions on OMEGA. Some of these results are shown in Fig. 7, which shows that DRACO provides a good theoretical description of such experiments.

Figure 7: Good correlation is seen between the experimentally-observed and theoretically-predicted shell deformations of PPD implosions on OMEGA Figure 8: Comparison of DRACO and OMEGA experimental shell positions showing good agreement

Fig. 8 shows a comparison of the measured and calculated shell positions at a single time during this implosion. It demonstrates the capability of DRACO in correctly predicting the hydrodynamic assembly of the implosion.

LLE will continue its work to optimize these designs, as well as optimizing an alternative PDD approach to NIF ignition using the SATURN concept. Such simulations pave the way for the routine use of PDD implosions on the NIF not just in the ignition area, but also in areas of low-mode-growth verification and high-yield diagnostic development.