Materials Science and Technology: Condensed Matter and Thermal Physics Simulation of Direct Drive Target Injection into ‘Hot’ Chambers James K. Hoffer & Timothy R. Gosnell Condensed Matter and Thermal Physics Group, Material Sciences Division Los Alamos National Laboratory Presented at the Laser IFE Workshop U. S. Naval Research Laboratory February 6, 2001

Materials Science and Technology: Condensed Matter and Thermal Physics Overall Objective…………. Response of target materials to injection stresses FY 01 Deliverables………..1. Start experiments to measure DT yield strength and modulus. 2. Start experiments to measure effect of rapid thermal load on layered target PI Experience………………. Extensive experience with DT and DT layering (POC: A. Nobile) Proposed Amount………….. $ 400 k Relevance of Deliverables [X] NIF……………………Research in materials in NIF targets [ ] Laser RR Facility…. [ ] Other DP/NNSA…… [X] Energy………………Needed for injection into chamber Related OFES activities……Design equipment for thermal stress ($ 50 k) Target Injection-2: Target Materials Response - LANL

Materials Science and Technology: Condensed Matter and Thermal Physics There are two major building blocks to produce a ‘simulation’: ‘Simulating’ a DT filled target: –there are two routes: spherical target filled & layered with solid DT (not really a surrogate at all, but an actual target!) cylindrical surrogates that permit direct heating both the above are routinely done at LANL Simulating a hot chamber: –again there are two main routes: heating of a spherical or cylindrical shroud to high temperatures –(difficult to do quickly and the temperature will be ramping) illumination of a cold spherical shroud or ‘integrating sphere’ to duplicate black-body radiation from a hot shroud –easy to do quickly but difficult to match the IR spectrum –choice of window materials critical to IR spectrum on target

Materials Science and Technology: Condensed Matter and Thermal Physics By using a spherical beta-layered target, rapid heating of a metallic shroud can simulate ballistic injection of a prototype beta-layered direct drive target:

Materials Science and Technology: Condensed Matter and Thermal Physics Both shroud heating and IR illumination concepts have practical limitations: The highest power that can be practically applied to a shroud in a conventional cryostat is only a few kW –thermometry is difficult if the mass is low –high heating rates imply high thermal ramping rates and the shroud temperature will significantly overshoot the desired value, leading to target runaway and loss of primary containment (an important ES&H concern!) With IR illumination, a true black-body spectrum will be difficult to achieve –the source spectrum may not be black-body –window materials will cut off the far IR

Materials Science and Technology: Condensed Matter and Thermal Physics Heat input to a layered D 2 target due to Stefan (black-body) radiation from a hot surface: (as a function of the emissivity e cold of the cold surface) times in red will not survive a 50-ms-long injection process!

Materials Science and Technology: Condensed Matter and Thermal Physics The solid D 2 fuel by itself is transparent to much of the black-body radiation, especially at 300 K: 

Materials Science and Technology: Condensed Matter and Thermal Physics Solid DT is equally transparent to black-body radiation, especially at 300 K:

Materials Science and Technology: Condensed Matter and Thermal Physics If all heating is solely due to IR absorption of DT ice, (i.e., none due to the shell wall) very long survival times are expected (for the NRL IFE target):

Materials Science and Technology: Condensed Matter and Thermal Physics We have just measured an IR spectrum on ‘Kapton’ brand polyimide, using a 13 micron-thick-film:

Materials Science and Technology: Condensed Matter and Thermal Physics Now we may calculate a ‘lifetime’ based on absorption only in a Kapton plastic shell and compare that to the Omega experiment:

Materials Science and Technology: Condensed Matter and Thermal Physics For the NRL IFE target, the times are longer:

Materials Science and Technology: Condensed Matter and Thermal Physics Here we compare DT to Kapton absorption for the NRL IFE target:

Materials Science and Technology: Condensed Matter and Thermal Physics What is the appropriate experiment to investigate target ‘lifetimes’? Question: Does the IR absorption of the DT layer play a significant role? –target lifetimes based on DT absorption are ~ 1000 times longer than observed at 300 K at Omega!! –lifetimes based on polyimide absorption are ‘about right’ –hence, the total IR absorption appears to be dominated by the plastic shell Answer: No! –therefore, heating of the DT layer and consequent behavior can be adequately simulated by adding heat directly to the outside surface –our present apparatus, with an appropriately designed cylindrical or toroidal layering chamber, is well suited for this type of study

Materials Science and Technology: Condensed Matter and Thermal Physics The information necessary for more accurate calculations is the infrared absorption spectra of plastics, but not that of DT. We need to measure the IR absorption spectrum of actual plastics at low temperatures: –IR absorption in plastics is known to be strongly temperature dependent: –we must accurately measure the near-IR spectrum, less accurately the far-IR, and not neglect the visible –we can use simple slabs of material –we may need additional operating funds (far-IR optics are expensive!)

Materials Science and Technology: Condensed Matter and Thermal Physics If we know the effects of heating to near the triple point of DT, then we can adequately calculate ‘lifetimes.’ Longer lifetimes (up to ~10 x) are possible if we allow for a portion of the DT ice to melt. Do we know if the act of melting will affect the shape of a DT layer in free fall? –the most accurate experiment may have to be done on the space shuttle or the ‘vomit comet.’ We cannot simulate melting in zero-g, but we can rapidly heat DT layers and look for stress-induced irregularities in the solid. –With the addition of a high-speed, high-precision camera, our present apparatus will be sufficient for this type of study

Materials Science and Technology: Condensed Matter and Thermal Physics Using direct heating, we need to assume a value for the surface emissivity, but thereafter, both opaque shells and foam filled shells can be investigated. Empty torus side view (windows not shown) Filled with foam, bored out to yield a 75 micron- thick layer at the waist Filled with DT and beta- layered to yield a solid layer 100 microns thick. Using an offset foam bore, the thickness of DT above the foam varies.