Fabrication of Hollow Silica Aerogel Spheres for Direct Drive Inertial Confinement Fusion (ICF) Experiments
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Fabrication of Hollow Silica Aerogel Spheres for Direct Drive Inertial Confinement Fusion (ICF) Experiments Reny R. Paguio, Abbas Nikroo, Jared F. Hund, Christopher A. Frederick, Javier Jaquez, Masa Takagi,1 and Mary Thi2 General Atomics, P.O. Box 85608, San Diego, California 92186-5608 Email: [email protected] 1Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551-9900 2University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093 ABSTRACT Hollow foam spheres are needed for laser fusion experiments on the OMEGA laser facility at the University of Rochester as part of the demonstration of the feasibility of inertial confinement fusion. Previously polymer based foam and aerogel shells have been produced using resorcinolformaldehyde (R/F) and divinylbenzene (DVB). In this paper we discuss the development of silica aerogel (SAG) shells. SAG may have the increased robustness, which is important in processing these laser targets. SAG shells were fabricated by the microencapsulation method using a triple orifice droplet generator. This technique allows for precise control of the shell diameter and wall thickness. Reduction of the aerogel gelation time is crucial to fabrication of intact shells with high yield. In addition, the proper choice of the components of the different phases of the microencapsulation process is essential for fabrication of intact SAG shells with proper sphericity and wall uniformity. The density of shells fabricated is approximately 100 mg/cc and the diameter ranges from 700–2000 μm, with a wall thickness of 50–200 μm. Development of a full density permeation barrier for retention of the fusion fuel will also be discussed. INTRODUCTION Aerogel/Foam shells are used on direct drive inertial confinement fusion experiments on the OMEGA facility at the University of Rochester Laboratory for Laser Energetics (LLE) and will be used in future experiments on the National Ignition Facility (NIF). The shells needed for OMEGA experiments have diameters of ~ 800–900 μm which are scaled down versions of the targets that are required for high gain wetted foam direct drive ignition designs on NIF [1]. The required wall thicknesses and foam densities for these targets are 50–100 μm and 30–150 mg/cc respectively. These foam shells are required to have good wall uniformity because wall non-uniformity adversely effects their implosions. The (low mode) wall uniformity specifications for OMEGA wetted foam targets are usually quoted in terms of non-concentricity (NC) defined by Eq. (1). NC =
Offset of the Inner - Outer Wall 2(Avg. Wall Thickness)
(1)
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The NC requirement for these targets is 5%. They must also hold a fill gas such as D2 or DT. Because the foam material is porous, the foam shell by itself cannot hold a fill gas. For wetted foam shells to hold a fill gas, they require a pinhole free permeation barrier, which is over-coated around the shell. Spherical shells of resorcinol-formaldehyde (RF) aerogels [2,3]
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