Science Seminars

Production and Measurement of Engineered Surfaces for Use in Inertial Confinement Fusion Research

Inertial Confinement Fusion uses the optical energy from a very high power laser to implode spherical capsules that contains a fuel mixture of deuterium and tritium. The capsules are made of either Beryllium, plastic, or glass and range from 0.1 mm to 2 mm in diameter. As a capsule implodes, thereby compressing the fuel to reach nuclear fusion conditions, it achieves temperatures of millions of degrees Centigrade and very high pressures. In this state, the capsule materials act like fluids and often a low density fluidic material will push on a higher density material which can be a very unstable condition depending upon the smoothness of the interface between the two materials. This unstable condition is called a hydrodynamic instability which results in the mixing of the two materials. If the mixing occurs between the fuel and a non-fuel material, it can stop the fusion reaction just like adding significant amounts of water to gasoline can stop the operation of an automobile.

Hydrodynamic instabilities are not well understood; therefore, Los Alamos National Laboratory has conducted numerous experiments to gain a better understanding of how these instabilities are initiated and grow at typical fusion conditions. The experimental packages for one set of these experiments required surfaces to be generated with specific power spectra and average roughness values. These specific surfaces were produced on the outside of aluminum bands that had inner diameters of 0.5 mm and were 8 μm thick. The shapes of the specific power spectra required ranged from: 1) flat over a specific spatial wavelength band to 2) flat over specific wavelengths with a peak at a specific wavelength, to 3) a peak at a specific frequency. A variety amplitudes, frequencies, and roughness values were required for these experiments.

A fast-tool-servo, mounted to an ultra-precision lathe, was used to generate these surfaces. A single crystal diamond Scanning Tunneling Microscope tip was used as the cutting tool for surfaces with the flat power spectrum and for the one with the flat power spectrum with a specific peak. A conventional single crystal diamond tool was used to generate the sinusoidal surfaces.

After the surfaces had been produced, the thickness of the aluminum band had to be measured. The surface roughness was a significant percentage of the band thickness, so providing a single number for the thickness was difficult. In the surface roughness region, the density transitions from full density in the bulk material to zero density. Since, density is an important parameter for this set of experiments it was decided to measure the effective density profile and use the value where the effective density is 50% of full density as the band thickness. The bearing area ratio statistic, calculated from optical profilometer data, was used to make this assessment.

For a related set of experiments, it was important to determine the density variation at the interface of two foam surfaces. A mathematical model that represented the bearing area ratio of the foam surfaces was created and used to simulate the effective density variation at the interface of aerogel and plastic foam surfaces for various bulk density and surface roughness values.

This talk discusses the methods used in the fabrication and metrology of all of the surfaces mentioned above, and describes the mathematical model used to simulate the effective density variation at the interface of aerogel and plastic foam surfaces.


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