Plastic Ablator Research Articles

Compression and heating of spherical fusion targets by indirect (x-ray) drive on the Iskra-5 facility

Experiments have been carried out using the Iskra-5 facility in order to study the behavior of x-ray targets in response to a highly symmetric x-ray field. Results are presented of experiments using targets in the form of a spherical copper hohlraum coated with gold on the inside, with six laser entrance holes and a glass microtarget filled with DT gas located at the center. In some experiments the central capsule was coated with a plastic ablator layer of varying thickness. An analysis of the experimental results showed that on the whole, they are satisfactorily described by spherically symmetric gasdynamic calculations.

A comparison of three-dimensional multimode hydrodynamic instability growth on various National Ignition Facility capsule designs with HYDRA simulations

Three similar cryogenic ignition capsule designs for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)] are analyzed to determine surface roughness specifications required to mitigate the growth of hydrodynamic instabilities. These capsule utilize brominated plastic, polyimid and copper-doped beryllium ablator materials respectively. Direct three-dimensional numerical simulations with the HYDRA radiation hydrodynamic code [M. M. Marinak et al., Phys. Plasmas 3, 2070 (1996)] examine the growth of multimode perturbations seeded by roughness on the outer ablator and inner ice surfaces. The simulations, which showed weakly nonlinear behavior for optimized surfaces, were carried through ignition and burn. A three-dimensional multimode perturbation achieves somewhat larger amplitudes in the nonlinear regime than a corresponding two-dimensional simulation of the same rms amplitude. The beryllium and polyimid capsules exhibit enhanced tolerance of roughness on both the ice and ablator surfaces.

Effects of variable x-ray preheat shielding in indirectly driven implosions

The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x-ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4× increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x-ray image core size with increasing Ge content is confirmed.

Ignition target design and robustness studies for the National Ignition Facility

Recent results are presented from two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasmas Phys. Controlled Thermonucl. Fusion 2, 51 (1975)] calculations of the indirectly driven hohlraum and ignition capsules proposed for the National Ignition Facility (NIF). The calculations concentrate on two capsule designs, the baseline design that has a bromine-doped plastic ablator, and the beryllium design that has a copper-doped beryllium ablator. Both capsules have a cryogenic fuel layer. Primary emphasis in these calculations is placed upon robustness studies detailing various sensitivities. Because of computer modeling limitations these studies fall into two categories: those performed with integrated modeling where the capsule, hohlraum, and laser rays all are modeled simultaneously with the laser power levels as the only energy input; and those performed in a capsule-only mode where an externally imposed radiative flux is applied to the exterior of the capsule, and only the capsule performance is modeled. Integrated modeling calculations address sensitivities to, e.g., the laser pointing; among other things, capsule-only calculations address yield degradation due to the growth of hydrodynamic instabilities seeded by initial surface roughnesses on the capsules. Limitations of the calculational models and directions for future research are discussed. The results of the robustness studies performed to date enhance the authors’ confidence that the NIF can achieve ignition and produce 10–15 MJ of capsule yield with one or more capsule designs.

Cryogenic deuterium target experiments with the GEKKO XII, green laser system

A series of experiments were conducted using cryogenic deuterium targets to study fundamental physics and implosion dynamics with the GEKKO XII glass laser system [IEEE J. Quantum Electron. QE-17, 1639 (1981)]. Preheat sources were found to be due to a shock wave and hot electrons. A new method to measure the fuel ρR using proton spectra was employed. Measured in detail were the implosion dynamics of cryogenic deuterium foam with a plastic ablator and a CH shell with a controlled pressure of deuterium gas targets. Under current experimental conditions sources of nonuniformity were discussed in terms of Rayleigh–Taylor instability.

Implosion of D2 temperature-controlled cryogenic foam targets with plastic ablators.

The implosion hydrodynamics of temperature-controlled cryogenic deuterium foam targets with plastic ablators is investigated. The experimental data recorded with x-ray imaging systems are compared to a one-dimensional (1D) simulation. The ablation front trajectories derived from x-ray streak-camera data agree with 1D simulation. However, an x-ray core emission is observed at an earlier time, and for a longer duration than that predicted. For the three liquid-fuel shots, the convergence ratios are, respectively, 0.4, 0.9, and 0.7 times that predicted by the 1D simulation.

Development of foam shell with plastic ablator for cryogenic laser fusion target

A low density, low Z foam shell overcoated with a plastic ablation layer was developed for use as a cryogenic laser fusion target in which main liquid or solid fuel is sustained by the uniformly thick foam layer. The high quality ablator with thickness of 1–10 μm, uniformity of ≳95%, and surface smoothness of <0.2 μm was coated on the foam shell by the interfacial polycondensation method using hydroxyethylcellulose or poly(p-vinyl phenol) (PVP) and isophthaloyl chloride. The tensile strength of the crosslinked PVP at low temperature and its permeability to deuterium at room temperature are comparable with those for polystyrene. In this reaction, we found that the existence of surfactant in the emulsion was essential to form an ablation layer which adhered to the foam layer.

Cryostat to provide a solid deuterium layer in a plastic shell for the Gekko XII glass laser system

A system to provide a liquid or solid deuterium shell target with a plastic ablator for laser implosion experiments was developed. The system is capable of filling a plastic capsule with deuterium gas of 11 MPa at room temperature at the firing position in the target chamber. Then, the target is cooled down to a cryogenic temperature to form a uniform liquid or solid fuel layer inside without exposing it to the atmosphere. Details of the system, tensile strength of polystyrene shells at low temperature, and the residual vapor pressure in the central void of the target at the laser irradiation are described.

Thermal transport studies of 351-nm laser-produced plasmas using extreme ultraviolet spectroscopy

Spectra from an extreme ultraviolet (XUV) grazing incidence spectrograph have been used in the study of thermal transport in laser plasmas. These measurements with XUV lines allowed a diagnosis of much lower temperatures in the heating front than had been previously measured. The OMEGA (24-beam, 351-nm) laser system at the University of Rochester was used to produce plasmas from glass microballoons coated with Al or Ti substrates and overcoated with a plastic ablator. Thermal transport was investigated by measuring the intensity of x-ray and XUV lines as a function of the thickness of the plastic overcoat. These measurements were then compared to the 1D hydrodynamic code lilac. Agreement between the hydrodynamic code and experiment could not be obtained with a reasonable value for the flux limiter ( f≤0.65). Consistent results were reached if nonuniform irradiation and flux inhibition is assumed. The measured burnthrough depths of around 9 μm are similar to previous measurements made with x-ray lines at fluxes below 1015 W/cm2. No evidence of preheat in the heat front was observed.

Laser implosion of microballoons: study of the transition from exploding-pusher to ablative regime

Microballoons filled with an equimolar deuterium-tritium mixture and coated with a plastic ablator of variable thickness are imploded by the eight-beam Octal laser (λ = 1.06 μm; 0.6 TW). An X-ray diagnostic with space-time resolution is used to analyse the implosion of the targets designed to the highest ∫ ρdr. The experimental results are compared with numerical simulations performed by a one-dimensional Lagrangian code. A DT density of about 2 g·cm−3 is obtained; the transition from an exploding-pusher regime to a more ablative one is analysed on the basis of the evolution of the preheat, the hydrodynamic efficiency and the density and temperature performance.

CO2 laser-driven high-density implosion experiments

Glass microballoons, coated with various thicknesses of plastic ablator and filled with deuterium-tritium gas mixtures, were irradiated with the eight-beam Helios CO2 laser. Neutron yield, fuel temperature, compressed fuel density, and implosion time were measured as functions of ablator thickness. Hydrodynamic efficiency and preheating of these targets were estimated. Transition from fuel conditions characteristic of earlier exploding pusher experiments to those characteristic of a more nearly isentropic mode of implosion is illustrated. Fuel densities above 2 g/cm3 were achieved.

Electric Discharge Ablation of Plastics in Vacuum

Ablation of plastics by electric discharge was studied in developing the solid propellant pulsed plasma thruster, which ideally suits for the control of geostationary satellites. The distinctive feature of this micro-thruster is that plastics propellant is ablated by pulsive discharge. Teflon was found to be completely decomposed by electric discharge in high vacuum. The experiments were carried out in systems evacuated with an oil-diffusion pump or with a turbo-molecular pump. The amount of 2 to 70 µg was ablated by every discharge of 2 to 10 µF capacitor charged up to 0.5∼4 kV. The ablation was strongly affected by the position of the trigger plug, the discharge voltage, the inductance of circuit, etc. Other plastics such as polyethylene, 6-nylon, polyvinylchloride and polymethylmethacrylate were also tested.

The Deformation of a Glacier Below an Ice Fall

AbstractMeasurements have been made on Austerdalsbreen, Norway, to test the theory that wave ogives at the foot of an ice fall are formed by pressure. The pattern of deformation over the first three waves from the Odinsbre ice fall was studied by measuring the absolute and relative motions of 35 stakes during August 1956. There are large variations of longitudinal compression through the wave pattern, but no simple correlation exists between the compression and the crests; nor are the crests consistently rising with respect to the average surface. We conclude that the waves are not forming by pressure in this area. A detailed quantitative explanation of the observed pattern of deformation is found by improving the existing theory of glacier flow. The deformations taking place are explained as essentially independent of the presence of the waves; they are primarily due (a) to the compression that must occur in the ice as its slope decreases and it becomes thicker, and (b) to the bending and unbending to which the ice is subjected as it passes over a bed of changing curvature. Additional contributions to the deformation arise from the widening of the glacier channel, which is important at the immediate foot of the ice fall, and from the annual ablation.The theory explains quantitatively the rotation and bending of the tunnel excavated in 1955; it was also used to make a prediction, which was successfully verified in a further experiment on the glacier in July and August 1957.A pressure mechanism of wave formation can be reconciled with the observations if it is supposed that the compression in the lower two-thirds of the ice fall, which was measured photogrammetrically in August 1956, varies significantly with the seasons. However, the positions and amplitudes of the observed waves are fully accounted for by the combined plastic deformation and ablation mechanism recently described in another paper. There is therefore no longer any need for the pressure hypothesis as a primary cause of the waves.

The Deformation of a Glacier Below an Ice Fall

Abstract Measurements have been made on Austerdalsbreen, Norway, to test the theory that wave ogives at the foot of an ice fall are formed by pressure. The pattern of deformation over the first three waves from the Odinsbre ice fall was studied by measuring the absolute and relative motions of 35 stakes during August 1956. There are large variations of longitudinal compression through the wave pattern, but no simple correlation exists between the compression and the crests; nor are the crests consistently rising with respect to the average surface. We conclude that the waves are not forming by pressure in this area. A detailed quantitative explanation of the observed pattern of deformation is found by improving the existing theory of glacier flow. The deformations taking place are explained as essentially independent of the presence of the waves; they are primarily due (a) to the compression that must occur in the ice as its slope decreases and it becomes thicker, and (b) to the bending and unbending to which the ice is subjected as it passes over a bed of changing curvature. Additional contributions to the deformation arise from the widening of the glacier channel, which is important at the immediate foot of the ice fall, and from the annual ablation. The theory explains quantitatively the rotation and bending of the tunnel excavated in 1955; it was also used to make a prediction, which was successfully verified in a further experiment on the glacier in July and August 1957. A pressure mechanism of wave formation can be reconciled with the observations if it is supposed that the compression in the lower two-thirds of the ice fall, which was measured photogrammetrically in August 1956, varies significantly with the seasons. However, the positions and amplitudes of the observed waves are fully accounted for by the combined plastic deformation and ablation mechanism recently described in another paper. There is therefore no longer any need for the pressure hypothesis as a primary cause of the waves.