Physics, P-DO
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Beryllium Ablator Microstructure and Stability Experiments
J.A. Cobble, T.E. Tierney, D.C. Swift (P-24), N.M. Hoffman, D.L. Tubbs (X-1), A. Nobile, R.D. Day (MST-7) IntroductionControlled fusion in the laboratory remains the unfulfilled grand challenge of the nuclear age. Fusion ignition of an inertially confined fuel capsule is a demanding task.1 As in building a fire, the first task is assembling the fuel mass; however, for inertial-confinement fusion (ICF), this is extremely more difficult than collecting firewood. The fusion fuel must be compressed to a density of ~ 400 g/cm2, which is over 30 times the density of solid lead and must be maintained at this density while its temperature is raised to over 200,000,000 K. Fuel assembly begins with as high a density fuel as possible—in this case, a thin shell of cryogenic deuterium-tritium (DT) ice at ~ 18 K. The surface of the shell, both inside and outside, must be smooth to ~ 1 µm,2 and the radiation drive, which compresses the fuel, must be uniform to better than 1%.1 These specifications are necessary to prevent hydrodynamic instabilities, which, while the irradiated capsule wall is compressing the fuel within, can rip the wall apart, release the fuel, and mix wall material with the fuel that remains in the core at the central hot spot. Only the highest quality compression will result in the density and the temperature of fuel necessary for ignition. The first ablator material to be considered for the capsule wall was plastic. However, the technical hurdles to be overcome to field a plastic shell containing cryogenic DT ice are immense and the cost is prohibitive (~ $150 million). An alternate to plastic, beryllium has higher density, which leads to shorter laser drive for ignition; lower opacity, which leads to a higher implosion velocity; higher tensile strength, which allows a DT-filled capsule to be handled at room temperature; and higher thermal conductivity, which also relaxes cryogenic requirements.2 However, crystalline beryllium, a naturally anisotropic material, supports different sound speeds depending on the direction of propagation. As long as a beryllium wall remains in the solid state during compression for fuel assembly, velocity shear at the beryllium-DT interface could lead to unfavorable hydrodynamic effects, primarily the classical Rayleigh-Taylor (RT) instability.3 Physics Division and Applied Physics Division are collaborating in experiments to determine the magnitude of this potential problem. Meanwhile, Material Science and Technology Division is working to reduce the size of beryllium grains so that the number of grains in the wall with random orientation smoothes the effect of non-uniform velocity fields. Eventually during the radiation drive, the shock heating of the beryllium will cause melt. Then, the question becomes whether the material retains sufficient memory of RT behavior to spoil the compression. For ignition at the National Ignition Facility (NIF), the early-time radiation pressure on the beryllium capsule will be 1–2 Mbar, exactly in the region for beryllium melt. Thus, experimental verification of beryllium behavior under various drive conditions is important to the success of future ignition experiments at NIF. We are therefore conducting beryllium ablator microstructure and stability experiments at the University of Rochester’s Omega laser. Our main objective is to measure RT-instability growth rates in beryllium, first in machined sinusoidal perturbations and then from individual grains of various preselected sizes with “face-on” x-ray radiography. RT-instability growth is to be stimulated by laser-driven radiation in a gold vacuum hohlraum for as many growth times as needed to achieve detectability. We adopted the preliminary goals of
Experimental Rational and ResultsOmega experiments typically last from 0.5–3 ns, which is insufficient time to detect microstructure-induced RT-instability growth in the beryllium. However, the longer the laser pulse, the less energy that can be extracted because of conversion efficiency problems in generating the 351 nm laser light at Omega. Our aim was to stagger two separate laser pulse shapes to integrate the longer drive within our targets.4 Then, after applying the appropriate laser drive to the hohlraum, we had to determine whether the measured radiation temperature, Trad, and pressure, which drive the instability growth in the beryllium, matched theoretical predictions. Given the pulse length for heating the target, gold blowing in from the wall has sufficient time to stagnate on the hohlraum axis, blocking the later-arriving x-ray backlighter beam and hiding details of the RT-instability growth in beryllium, which has a much lower opacity than the gold. However, theoretical considerations suggest that radiation heating of the beryllium sample in the hohlraum could form a hot beryllium plasma bubble, putting pressure on the gold plasma and inhibiting its influx. Therefore, our final task was to qualify the ability of the vacuum hohlraum to maintain high visibility of the beryllium package within for face-on radiography.
Trad was monitored by Dante, a ten-channel, filtered-x-ray-diode array.5 The results were consistent from shot to shot and are illustrated in Figure 2 with the desired radiation drive. Although it appears that Trad is too high initially, the Dante measurement is increasingly less reliable below ~100 eV. For that reason, we also employed VISAR, a velocity interferometry system for any reflector6, to measure the free surface velocity of the beryllium sample, the shock-break-out time, and the radiation pressure. The on-going VISAR analysis shows that during the first 3 ns, the radiation pressure is indeed about 1 Mbar. A gated x-ray framing camera (XRFC) filtered for gold M-band emission at 2.5 keV was used to monitor influx of gold from the hohlraum walls over the course of the radiation drive. Axial images in Figure 3 show the radial progress of the gold. During the third nanosecond of drive (at 10.5 mm on the y axis of Figure 3), the slowest component of the gold has moved ~ 300 µm at a speed of ~ 3 × 107 cm/s. The fastest gold ions already appear to have stagnated on axis by 4 ns into the drive as shown by the black dot in the centers of the images. By 5 ns, the gold emission from the laser entrance hole (LEH), where the crossing laser beams have highest intensity, is heavy and uniform.
Future directionsIn the 20 laser shots for this campaign to date, we have learned that we can join different laser pulses together in a hohlraum to create a smoothly varying 6 ns radiation drive to study RT-instability growth in beryllium samples. Our first effort has apparently resulted in a 1 Mbar drive, as measure by VISAR, which Dante confirms reaches ~ 180 eV at its peak, as required by theoretical demands. However, we have measured the influx of gold in a vacuum hohlraum and find that it is a threat to successful radiography. X-ray backlighting of sinusoidally perturbed beryllium samples has produced evidence of RT-instability growth and of the presence of the beryllium bubble, which only partially inhibits gold plasma from encroaching into the center of the target. In the future, we hope to solve the gold influx problem with gas-filled targets, for instance, with 1 atm of CH4. Issues surrounding the gas-filled targets include the drive penalty for heating the gas—as much as a few hundred J—and possible laser-plasma instabilities. The latter may cause backscatter of 10% or more of the laser light away from the target. A drop in Trad of ~ 10 eV is expected. The overall pulse duration might need to be shortened to boost the laser energy available for the required drive. Hopefully, laser energy will be available to evaluate a pulse shape for a 2 Mbar drive. We are encouraged that an Omega hohlraum environment can be created to validate beryllium as the material of choice for NIF ignition experiments. Experiments at the Trident Laser FacilityRelated laser-ablation experiments have been performed at the Trident Laser Facility at LANL to measure the strength of beryllium on a nanosecond time scale. The elastic and plastic response was monitored with VISAR from the sample surface and by in situ x-ray diffraction from the shocked beryllium. The effective strength was several times greater than that on the previously explored microsecond scales. These experiments also allow us to measure spatial variations in response caused by the microstructure of the material. Together with the Omega results, we are obtaining the detailed understanding of beryllium properties needed to set requirements for indirect-drive ignition experiments on NIF. References
AcknowledgmentThis work has been done under the auspices of the U.S. DOE. We acknowledge the expert assistance of many workers at the University of Rochester Laboratory for Laser Energetics, including resident staff from Lawrence Livermore National Laboratory. For further information, contact James Cobble, 505-667-8290, cobble@lanl.gov. |
Following initial laser profile design, we worked with Omega personnel to create two pulse shapes that when combined produced the desired ~ 6 ns laser drive. The integrated pulse consists of three beams carrying the “foot” of the drive for > 3 ns followed by ten 2.5 ns triangular-shaped beams, which carry the bulk of the energy into the hohlraum. The timing of the foot drive and the triangular drive must be exact to within tens of picoseconds to maintain a smooth transition between the two sets of laser beams, which are symmetrically placed in azimuth around the wall of the hohlraum. Figure 1 indicates the placement of the laser pulses into the hohlraum target with a prediction, arising from a hydrodynamic model, for the position of the beryllium bubble with the application of over 4 kJ of laser energy.
To assess the face-on radiography, we placed a gated axial XRFC opposite the LEH to the right of the beryllium sample in Figure 1 and, using M-band radiation from the LEH, backlit the beryllium, which for the first experiments was machined with a sine wave having a 100 µm period and an amplitude of 2.5 µm. Figure 4 shows images from a single strip line from the XRFC taken at 5 ns into the laser pulse. The RT-instability growth of the sinusoidal variations has grown to the threshold of visibility as seen in modulations of the exposure, and an underexposed region near the hohlraum axis is revealed. Through analysis of parallax from the adjacent pinholes on the strip line, the center of mass of this quasi-opaque region is found to be one-quarter of the way back from the LEH—at exactly the position on axis where gold is not illuminated by laser beams. This observation provides evidence for an extensive beryllium bubble blowing out from the beryllium sinusoid, but which unfortunately does not reach far enough forward to prevent gold from obscuring the view.