CH Capsules Research Articles

X-ray source improvements for EXAFS measurement on SGIII prototype facility

A proper x-ray source is crucial to extended x-ray absorption fine structure (EXAFS) measurement as parameter diagnosis for compressed materials on large laser facilities. In this article, modification of the CH capsule as an x-ray source has been done through simulations and experiments for the SGIII prototype laser facility. On the one hand, simulation results show that the radiation intensity of the CH capsule increases largely while wall thickness decreases. On the other hand, chlorine atoms whose atomic number is higher than that of carbon were brought into the capsule by doping on the inner side wall to enhance x-ray radiation intensity. CH capsules were designed, and experiments were carried out on the SGIII prototype laser facility. The x-ray source spectrum, size, radiation intensity, radiation pulse, and Ti K-edge EXAFS spectrum under ambient conditions were compared and discussed. Experimental results show that, with the doping of chlorine atoms and decreasing the wall thickness from 9 µm to 6 µm, the x-ray radiation intensity increases by about 63.5 times, energy efficiency is improved a lot, and other properties are also improved. The chlorine atom doped CH capsule with 6 µm wall thickness is proved to be the best x-ray source in the experiments in this article. Its size is 57 ± 0.5 µm, pulse width is 240 ps, and radiation intensity is about 9.1 times the undoped one with the same wall thickness. The methods can be extended to other laser facilities for the EXAFS measurement of other materials.

Achieving 280 Gbar hot spot pressure in DT-layered CH capsule implosions at the National Ignition Facility

We are reporting on a series of indirect-drive 0.9-scale CH capsule implosions (inner radius = 840 μm) fielded in low gas-fill (0.6 mg/cm3) hohlraums of 6.72 mm diameter at the National Ignition Facility. Thanks to the 11%-reduction of the capsule size at a given hohlraum diameter compared to previously tested full-scale capsules, we achieved good hot spot symmetry control near 33% cone-fraction and without the need to invoke cross beam energy transfer. As a result, we achieved a hot spot pressure of 280 ± 40 Gbar, which is the highest pressure demonstrated in layered DT implosions with CH capsules to date. Pushing this design to higher velocity resulted in a reduction of neutron yield. Highly resolved capsule simulations suggest that higher Au M-shell preheat resulted in an increase in Atwood number at the ablator–ice interface, which leads to increased fuel-ablator instability and mixing. The results reported here provide important scaling information for next-generation CH designs.

Octahedral spherical Hohlraum for Rev. 6 NIF beryllium capsule

We have designed an octahedral spherical hohlraum with 6 laser entrance holes (LEHs) for the Rev. 6 Be ignition capsule [Simakov et al., Phys. Plasmas 21, 022701 (2014)]. With a Au spherical hohlraum of 4400 μm in radius and six LEHs of 1200 μm in radius, a laser pulse of 2.15 MJ energy and 630 TW peak power is required to deliver the radiation drive designed for the Rev. 6 Be ignition capsule. Both our 1D and 2D simulations have shown that the expansion of the Be capsule is very close to or even slightly slower than that of the CH capsule under the same radiation drive inside the spherical hohlraum, in spite of the higher ablation rate of Be. The reason why the Be and CH capsules have similar expansions is due to their similar exhaust velocities, and the reason why CH has a slightly faster expansion is that the CH capsule has a higher opacity which causes the hotter ablated plasmas and then a faster expansion of the CH ablated plasmas. The large volume of the octahedral spherical hohlraum, together with the incident angle of 55° in its laser arrangement, leaves enough room for the laser transportation, thus avoiding the laser being absorbed by Be ablated plasmas and, consequently, avoiding the high risk of laser plasma instabilities. It means that the higher mass ablation rate of Be does not affect the hohlraum energetics and the naturally high symmetry of radiation inside an octahedral spherical hohlraum. Thus, the superior ablation properties of the Be capsule can be fully exploited and hence have a higher opportunity to achieve ignition inside the octahedral spherical hohlraum.

Numerical Analysis of the Effect of Infrared Radiation on Cryogenic Inertial Confinement Fusion Targets

The present work applies the finite element method to calculate the maximum allowable time that cryogenic inertial confinement fusion (ICF) targets can be exposed to infrared radiation (IR). Hence, a 3-D numerical model integrated with discrete coordinate radiation model was developed to investigate the influence of transmittance of the laser entrance holes (LEHs) and boundary conditions on the temperature field distribution and the maximum DT layer deterioration time for CH, Be, and diamond capsules. Our study shows that introducing such a radiation model can accurately obtain more detailed spatial and temporal distribution information in the ICF targets. The simulation results demonstrate that the Be and diamond capsules provided much better temperature field homogenization than the CH capsule under equivalent boundary conditions, but the CH capsule was heated more by IR radiation than the Be and diamond. In addition, the maximum DT layer deterioration time was significantly increased to 3 s when decreasing the transmittance of the LEH from 0.2 to 0.01. However, either reducing the capsule IR absorption or increasing the inner hohlraum IR absorption demonstrated no conclusive increase in the maximum DT layer deterioration time. These results are expected to provide useful parameters in the design of cryogenic targets and shroud systems.

Influence of Au M-band flux asymmetry on implosion symmetry

AbstractIn indirect-drive inertial confinement fusion, the radiation symmetry must be controlled for the achievement of hotspot ignition. The radiation symmetry is of great importance. In this paper, we investigate the drive asymmetry of the M-band (2–5 keV) radiation emitted from an Au holhraum wall by using the three-dimensional view-factor code IRAD3D. Analysis of the M-band flux drive at the Shenguang-III laser facility shows that it is asymmetric and that the asymmetry varies with time. For a given cross section over the pole, the initial M-band flux asymmetries are P2 = 11.59, P4 = 1.41, and P6 = −0.64%. When the asymmetries are artificially added to a symmetric radiation drive, the position of the deuterium-tritium (DT) ice/gas interface is asymmetric for a National Ignition Facility capsule in 1D simulation. This means that M-band flux asymmetry can lead to implosion asymmetry even if the total radiation is symmetric. Pure CH and Si-doped CH capsules are considered. The results show that a mid-Z dopant can partly reduce the asymmetry. However, the asymmetry is still very large. Thus, it is necessary to study the M-band flux asymmetry and its influence on the implosion symmetry.

Tumble Polishing of Faster-Coated Glow Discharge Polymer Capsules

CH capsules fabricated by glow discharge plasma polymerization have been used as the capsule point design for the National Ignition Facility (NIF). The coating time ranges from 6 to 8 weeks. We have reduced the fabrication time by improving the coating rate. The faster coating rate results in an increased surface roughness in high-spatial-frequency modes >100. We employed the tumble finishing process developed for NIF capsules to improve the high mode roughness. Material properties such as surface finish, density, and relative oxygen uptake rate were examined for the faster-coated plastic. The reduction of glow discharge polymer (GDP) capsule production time without significant deviation from benchmarked capsule properties is discussed.

First beryllium capsule implosions on the National Ignition Facility

The first indirect drive implosion experiments using Beryllium (Be) capsules at the National Ignition Facility confirm the superior ablation properties and elucidate possible Be-ablator issues such as hohlraum filling by ablator material. Since the 1990s, Be has been the preferred Inertial Confinement Fusion (ICF) ablator because of its higher mass ablation rate compared to that of carbon-based ablators. This enables ICF target designs with higher implosion velocities at lower radiation temperatures and improved hydrodynamic stability through greater ablative stabilization. Recent experiments to demonstrate the viability of Be ablator target designs measured the backscattered laser energy, capsule implosion velocity, core implosion shape from self-emission, and in-flight capsule shape from backlit imaging. The laser backscatter is similar to that from comparable plastic (CH) targets under the same hohlraum conditions. Implosion velocity measurements from backlit streaked radiography show that laser energy coupling to the hohlraum wall is comparable to plastic ablators. The measured implosion shape indicates no significant reduction of laser energy from the inner laser cone beams reaching the hohlraum wall as compared with plastic and high-density carbon ablators. These results indicate that the high mass ablation rate for beryllium capsules does not significantly alter hohlraum energetics. In addition, these data, together with data for low fill-density hohlraum performance, indicate that laser power multipliers, required to reconcile simulations with experimental observations, are likely due to our limited understanding of the hohlraum rather than the capsule physics since similar multipliers are needed for both Be and CH capsules as seen in experiments.

Platform for spectrally resolved x-ray scattering from imploding capsules at the National Ignition Facility

We present a new experimental platform to perform spectrally resolved x-ray scattering measurements of ionization, density and temperature in imploding CH or beryllium capsules at the National Ignition Facility. Scattered x-rays at 9 keV from a zinc He-alpha plasma source at a scattering angle of 120 degrees are highly sensitive to K-shell ionization, while at the same time constraining density and temperature. This platform will allow for x-ray scattering studies of dense plasmas with free electron densities up to 1025 cm-3 giving the possibility to investigate effects of pressure ionization and Pauli blocking on the ablator ionization state right before or shortly after stagnation of the implosion.

X-ray drive of beryllium capsule implosions at the National Ignition Facility

National Ignition Facility experiments with beryllium capsules have followed a path begun with “high-foot” plastic capsule implosions. Three shock timing keyhole targets, one symmetry capsule, a streaked backlit capsule, and a 2D backlit capsule were fielded before the DT layered shot. After backscatter subtraction, laser drive degradation is needed to match observed X-ray drives. VISAR measurements determined drive degradation for the picket, trough, and second pulse. Time dependence of the total Dante flux reflects degradation of the of the third laser pulse. The same drive degradation that matches Dante data for three beryllium shots matches Dante and bangtimes for plastic shots N130501 and N130812. In the picket of both Be and CH hohlraums, calculations over-estimate the x-ray flux > 1.8 keV by ∼100X, while calculating the total flux correctly. In beryllium calculations these X-rays cause an early expansion of the beryllium/fuel interface at ∼3 km/s. VISAR measurements gave only ∼0.3 km/s. The X-ray drive on the Be DT capsule was further degraded by an unplanned decrease of 9% in the total picket flux. This small change caused the fuel adiabat to rise from 1.8 to 2.3. The first NIF beryllium DT implosion achieved 29% of calculated yield, compared to CH capsules with 68% and 21%.

X-ray Thomson scattering measurements of temperature and density from multi-shocked CH capsules

Proof-of-principle measurements of the electron densities, temperatures, and ionization states of spherically compressed multi-shocked CH (polystyrene) capsules have been achieved using spectrally resolved x-ray Thomson scattering. A total energy of 13.5 kJ incident on target is used to compress a 70 μm thick CH shell above solid-mass density using three coalescing shocks. Separately, a laser-produced zinc He-α x-ray source at 9 keV delayed 200 ps-800 ps after maximum compression is used to probe the plasma in the non-collective scattering regime. The data show that x-ray Thomson scattering enables a complete description of the time-dependent hydrodynamic evolution of shock-compressed CH capsules, with a maximum measured density of ρ > 6 g cm−3. In addition, the results demonstrate that accurate measurements of x-ray scattering from bound-free transitions in the CH plasma demonstrate strong evidence that continuum lowering is the primary ionization mechanism of carbon L-shell electrons.

Oxygen Profile Determination in NIF GDP Capsules Using Contact Radiography

CH capsules, produced with glow discharge plasma coating, pick up oxygen continuously and irreversibly during storage. The added X-ray opacity from this oxygen affects the shock velocity during target implosion, requiring compensation in the shock timing. We developed a radiography technique that nondestructively characterizes the oxygen profile and have used it to track the evolution of the oxygen profiles in various types of storage conditions. Modified storage protocols have reduced the amount of pickup, and our database enables estimation of the oxygen profile at shot time such that the impact on target implosion performance can be minimized.

Advancements in Capsule Surface Defect Characterization

CH capsules, produced through glow discharge plasma coating, routinely suffer from surface defects including domes with gradually sloping sides and dust particles with sharp edges. Surface defects seed instabilities during implosion experiments on the National Ignition Facility and lead to radial jets, which increase mixing at the center of the implosion hindering the shell compression. Avoiding such defects requires characterizing the entire shell surface. In addition, the global position of the defects must be recorded in order to coordinate shot results with the initial surface perturbations. Further work was done to enable side-by-side comparison with optically acquired images to aid in capsule surface inspection throughout the capsule production process.

Defect-induced mix experiment for NIF

The Defect Induced Mix Experiment (DIME-II) will measure the implosion and mix characteristics of CH capsules filled with 5 atmospheres of DT by incorporating mid-Z dopant layers of Ge and Ga. This polar direct drive (PDD) experiment also will demonstrate the filling of a CH capsule at target chamber center using a fill tube. Diagnostics for these experiments include areal x-ray backlighting to obtain early time images of the implosion trajectory and a multiple-monochromatic imager (MMI) to collect spectrally-resolved images of the capsule dopant line emission near bangtime. The inclusion of two (or more) thin dopant layers at separate depths within the capsule shell facilitates spatial correlation of mix between the layers and the hot gas core on a single shot. The dopant layers are typically 2 μm thick and contain dopant concentrations of 1.5%. Three dimensional Hydra simulations have been performed to assess the effects of PDD asymmetry on capsule performance.

Advances in target design and fabrication for experiments on NIF

The ability to build target platforms for National Ignition Facility (NIF) is a key feature in LANL's (Los Alamos National Laboratory) Target Fabrication Program. We recently built and manufactured the first LANL targets to be fielded on NIF in March 2011. Experiments on NIF require precision component manufacturing and accurate knowledge of the materials used in the targets. The characterization of foams and aerogels, the Be ignition capsule, and machining unique components are of main material focus. One important characterization metric the physics' have determined is that the knowledge of density gradients in foams is important. We are making strides in not only locating these density gradients in aerogels and foams as a result of how they are manufactured and machined but also quantifying the density within the foam using 3D confocal micro x-ray fluorescence (XRF) imaging and 3D x-ray computed tomography (CT) imaging. In addition, collaborative efforts between General Atomics (GA) and LANL in the characterization of the NIF Ignition beryllium capsule have shown that the copper in the capsule migrates radially from the capsule center. 1. ELEMENTAL DIFFUSION THROUGH IGNITION CAPSULES Recent collaborations with LANL and General Atomics (GA) have focused on analysis of ignition capsules, specifically elemental migration throughout the ignition capsules. Currently, the leading choice in the point design for the ignition capsules are graded doped Cu-Be capsules with the secondary design being Ge doped CH capsules. Both of these capsule types were analyzed using 3D confocal XRF imaging and 3D x-ray CT imaging for possible elemental migration. 1.1 Analysis of copper migration in graded doped Cu-Be ignition capsules Previous studies at GA using contact radiography indicated migration of the Cu dopant in the capsule shell. In addition, SEM (scanning electron microscopy) images of pre- and post-pyrolysis indicates formation of domes on the surface of the capsules post-pyrolysis. We believe this is due to thermal migration of copper during the pyrolysis step. It is well known and documented in materials science literature that copper readily migrates when subjected to higher thermal conditions. (1) Since the dome features are seen on the capsule surface after pyrolysis, we believe these domes are that of copper migrating to the surface. To determine if the Cu migration occurred radially throughout the capsule, a few ignition capsules were given to LANL to study. Using a combination of 3D x-ray CT imaging and

Reduction of Isolated Defects on Ge Doped CH Capsules to Below Ignition Specifications

The NIF Ge-doped CH capsule should be free of isolated defects on the outer surface. The allowed number and dimensions of large isolated defects over the entire capsule surface is given by the isolated feature specification.To date NIF-thickness (146 μm) capsules are plagued by a few isolated large domes on the outer surfaces that otherwise meet the atomic force microscope (AFM) spheremap modal power spectra specification. The large domes on the capsule surfaces were mostly caused by particulate contamination from the wear of an agitation tapping solenoid inside the coater. By eliminating the solenoid and using an alternate rotation agitation, most thick-walled capsules become free of large isolated defects and meet the AFM spheremap modal power spectra standard.The number and size of the isolated defects on the outer surface were characterized with a high resolution phase-shifting diffractive spherical interferometer and checked against the NIF isolated defect specification. The results show the isolated defects on the rolled capsule are below the isolated defect specification. The growth modeling of the remaining nanometer-height domes on the capsules indicates most of these small domes come from the mandrel surface.The rolled capsules meet the layer thickness, doping levels and wall thickness specifications and have good wall uniformity of ±0.1.0.2 μm.

Progress toward fabrication of graded doped beryllium and CH capsules for the National Ignition Facility

Current ignition designs require graded doped beryllium or CH capsules. This paper reports on the progress toward fabricating both beryllium and CH capsules that meet the current design criteria for achieving ignition on the National Ignition Facility (NIF) [S. Hann et al., Phys. Plasmas 12, 056316 (2005)]. NIF scale graded copper doped beryllium capsules have been made by sputter coating, while graded germanium doped CH capsules have been made by plasma polymer deposition. The sputtering process used for fabricating graded beryllium shells was produced with a void fraction of ∼5%. Varying the deposition parameters can lead to several different beryllium microstructures, which have been tuned to reduce the void size and fraction to within specifications. In addition, polishing of beryllium-coated shells reduces the outer surface roughness of shells to ignition specifications. Transmission electron microscopy has been used to characterize void fraction and grain structure of beryllium coatings. The plasma polymer deposition process has produced dense, void-free graded doped CH shells that nearly meet the ignition surface finish requirements. Layer thickness and dopant concentrations have been measured by quantitative contact radiography.

Polar direct drive: Proof-of-principle experiments on OMEGA and prospects for ignition on the National Ignition Facility

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11 2763 (2004)] shows promise for achieving direct-drive ignition while the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41 B39 (1999)] is initially configured for indirect drive. Experiments have been carried out using 40 repointed beams of the 60-beam OMEGA laser system [T. R. Boehly et al., Rev. Sci. Instrum. 66 508 (1995)] to approximate the NIF PDD configuration. Backlit x-ray framing-camera images of D2-filled spherical CH capsules show a characteristic nonuniformity pattern that is in close agreement with predictions. Saturn targets (similar capsules surrounded by a plastic ring) increase the drive on the equator, suggesting that highly symmetric PDD implosions may be possible with appropriate tuning. Two-dimensional (2D) simulations reproduced the approximately threefold reduction in yield found for the non-Saturn PDD capsules. Preliminary simulations for a NIF Saturn design predict a high gain close to the 1D prediction. These results increase the prospects of obtaining direct-drive ignition with the initial NIF configuration.

Hot dense capsule-implosion cores produced by Z-pinch dynamic Hohlraum radiation.

Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.

Improved performance of direct-drive inertial confinement fusion target designs with adiabat shaping using an intensity picket

Hydrodynamic instabilities seeded by laser imprint and surface roughness limit the compression ratio and neutron yield in the direct-drive inertial confinement fusion target designs. New improved-performance designs use adiabat shaping to increase the entropy of only the outer portion of the shell, reducing the instability growth. The inner portion of the shell is kept on a lower entropy to maximize shell compressibility. The adiabat shaping is implemented using a high-intensity picket in front of the main-drive pulse. The picket launches a strong shock that decays as it propagates through the shell. This increases the ablation velocity and reduces the Rayleigh–Taylor growth rates. In addition, as shown earlier [T.J.B. Collins and S. Skupsky, Phys. Plasmas 9, 275 (2002)], the picket reduces the instability seed due to the laser imprint. To test the results of calculations, a series of the picket pulse implosions of CH capsules were performed on the OMEGA laser system [T.R. Boehly, D.L. Brown, R.S. Craxton et al., Opt. Commun. 133, 495 (1997)]. The experiments demonstrated a significant improvement in target yields for the pulses with the picket compared to the pulses without the picket. Results of the theory and experiments with adiabat shaping are being extended to future OMEGA and the National Ignition Facility’s [J.A. Paisner, J.D. Boyes, S.A. Kumpan, W.H. Lowdermilk, and M.S. Sorem, Laser Focus World 30, 75 (1994)] cryogenic target designs.

Convergent Rayleigh-Taylor Experiments on the Nova Laser

Experiments have been done on the Nova laser to investigate the effects of convergence on Rayleigh-Taylor growth. We ablatively accelerated planar CH(Br) foils or hemispherical CH(Ge) capsules with identical initial perturbations. Experimental results showed the decrease of the effective wavelength of the perturbation and the early onset of nonlinearity in the converging case compared to the planar case. Numerical simulations using the 2D Lagrangian code FCI2 are in good agreement with the experimental results. (c) 1999 The American Physical Society.