Fast-ignition design transport studies: realistic electron source, integrated PIC-hydrodynamics, imposed magnetic fields

Transport modeling of idealized, cone-guided fast ignition targets indicates the severe challenge posed by fast-electron source divergence. The hybrid particle-in-cell [PIC] code Zuma is run in tandem with the radiation-hydrodynamics code Hydra to model fast-electron propagation, fuel heating, and thermonuclear burn. The fast electron source is based on a 3D explicit-PIC laser-plasma simulation with the PSC code. This shows a quasi two-temperature energy spectrum, and a divergent angle spectrum (average velocity-space polar angle of 52 degrees). Transport simulations with the PIC-based divergence do not ignite for > 1 MJ of fast-electron energy, for a modest 70 micron standoff distance from fast-electron injection to the dense fuel. However, artificially collimating the source gives an ignition energy of 132 kJ. To mitigate the divergence, we consider imposed axial magnetic fields. Uniform fields ~50 MG are sufficient to recover the artificially collimated ignition energy. Experiments at the Omega laser facility have generated fields of this magnitude by imploding a capsule in seed fields of 50-100 kG. Such imploded fields are however more compressed in the transport region than in the laser absorption region. When fast electrons encounter increasing field strength, magnetic mirroring can reflect a substantial fraction of them and reduce coupling to the fuel. A hollow magnetic pipe, which peaks at a finite radius, is presented as one field configuration which circumvents mirroring.Comment: 16 pages, 17 figures, submitted to Phys. Plasma

The Comparative Effects of 0.12% Chlorhexidine and Herbal Oral Rinse on Dental Plaque-Induced Gingivitis

PURPOSE: The purpose of this study was to determine the effects of two oral rinses-one 0.12% chlorhexidine rinse (CHX) and one herbal rinse (HBR)-on gingival health status over time. METHODS: Sixty-three participants were randomly assigned to one of three treatment groups: CHX, HBR, or placebo. For three months, participants rinsed twice daily (morning and evening) with (1/2) ounce of allocated rinse after brushing and flossing. Individuals were given the same type of soft bristle toothbrush and whitening toothpaste. No attempt was made to modify participants\u27 routine oral care, except they were advised to refrain from use of any other oral rinse for the duration of the study. Data were collected at baseline (B), month one (1), two (2), and three (3) utilizing the Gingival Index (GI), Plaque Index (PI), and bleeding on probing (BOP). A full mouth periodontal probing was performed at baseline and at the completion of the study. A soft tissue oral assessment was completed at each visit. CHX, HBR, and placebo data were compared between three time intervals, B-1, B-2, and B-3. Statistical analysis was conducted by means of multiple regression using generalized linear models. Paired comparison tests--ANOVA followed by a post hoc Tukey test--were used to confirm results. RESULTS: CHX was the only oral rinse to demonstrate a statistically significant effect on the reduction of mean GI, BOP, and PI scores when compared to placebo. CHX demonstrated a 31% reduction in the proportion of GI scores between B-2 and a 29% reduction between B-3 (p=.003 and p=.012, respectively). CHX demonstrated a 19% reduction of BOP sites between B-1, 32% reduction between B-2, and 29% reduction between B-3 (p=.028, p=.000, and p=.005, respectively). CHX demonstrated a 20% reduction in PI scores between B-1, and a 28% reduction between B-2 (p=.005 and p=.032, respectively). The effects of HBR on reducing mean GI, BOP, and PI scores were not statistically greater than placebo at any time during the study

Role of Hydrodynamics Simulations in Laser-Plasma Interaction Predictive Capability

Efforts to predict and control laser-plasma interactions (LPI) in ignition hohlraum targets for the National Ignition Facility [G. H. Miller et al., Optical Eng. 43, 2841 (2004)] are based on plasma conditions provided by radiation hydrodynamic simulations. Recent experiments provide compelling evidence that codes such as hydra [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] can accurately predict the plasma conditions in laser heated targets such as gas-filled balloon (gasbag) and hohlraum platforms for studying LPI. Initially puzzling experimental observations are found to be caused by bulk hydrodynamic phenomena. Features in backscatter spectra and transmitted light spectra are reproduced from the simulated plasma conditions. Simulations also agree well with Thomson scattering measurements of the electron temperature. The calculated plasma conditions are used to explore a linear-gain based phenomenological model of backscatter. For long plasmas at ignition-relevant electron temperatures, the measured backscatter increases monotonically with gain and is consistent with linear growth for low reflectivities. These results suggest a role for linear gain postprocessing as a metric for assessing LPI risk

Ion Beam Heated Target Simulations for Warm Dense Matter Physics and Inertial Fusion Energy

Hydrodynamic simulations have been carried out using the multi-physics radiation hydrodynamics code HYDRA and the simplified one-dimensional hydrodynamics code DISH. We simulate possible targets for a near-term experiment at LBNL (the Neutralized Drift Compression Experiment, NDCX) and possible later experiments on a proposed facility (NDCX-II) for studies of warm dense matter and inertial fusion energy related beam-target coupling. Simulations of various target materials (including solids and foams) are presented. Experimental configurations include single pulse planar metallic solid and foam foils. Concepts for double-pulsed and ramped-energy pulses on cryogenic targets and foams have been simulated for exploring direct drive beam target coupling, and concepts and simulations for collapsing cylindrical and spherical bubbles to enhance temperature and pressure for warm dense matter studies are described

Update on Specifications for NIF Ignition Targets and Their Rollup Into an Error Budget

Targets intended to produce ignition on NIF are being simulated and the simulations are used to set specifications for target fabrication. Recent design work has focused on designs that assume only 1.0 MJ of laser energy instead of the previous 1.6 MJ. To perform with less laser energy, the hohlraum has been redesigned to be more efficient than previously, and the capsules are slightly smaller. The main-line hohlraum design now has a SiO2 foam fill, a wall of U-Dy-Au, and shields mounted between the capsule and the laser entrance holes. Two capsule designs are being considered. One has a graded doped Be(Cu) ablator, and the other graded doped CH(Ge). Both can perform acceptably with recently demonstrated ice layer quality, and with recently demonstrated outer surface roughness. Smoothness of the internal interfaces may be an issue for the Be(Cu) design, and it may be necessary either to polish partially coated shells or to improve process control so that the internal layers are smoother. Complete tables of specifications are being prepared for both targets, to be completed this fiscal year. All the specifications are being rolled together into an error budget indicating adequate margin for ignition with the new designs

Experimental Studies of ICF Indirect-Drive Be and High Density C Candidate Ablators

To validate our modeling of the macroscopic and microscopic hydrodynamic and equation of state response of these candidate ablators to NIC-relevant x-ray drive, a multi-lab experimental program has been verifying the behavior of these new ablators. First, the pressures for onset and termination of melt for both Be and HDC under single or double shock drive has been measured at the Z and Omega facilities. Second, the level and effect of hard x-ray preheat has been quantified in scaled experiments at the Omega facility. Third, a long planar x-ray drive has been developed to check 2D and 3D perturbation growth at the ablation front upon acceleration. The concept has been extended to study growth at and near the ablator-ice interface upon deceleration. In addition, experimental designs for validating the expected low level of perturbation seeding due to possible residual microstructure after melt during first and second shock transit in Be and HDC have been completed. Results so far suggest both Be and HDC can remain ablator choices and have guided pulse shaping designs