weapons March/April 2003 I HERT I HE Microstructure I Recycling I DARHT Approval I I Stockpile Stewardship Exhibit I Weapons Science and Engineering at Los Alamos National Laboratory Los Alamos, NM 87545 of Point Raymond J. Juzaitis Deputy Associate Director • View Weapons Physics Informatics, Complexity Science, and National Security Informatics and complexity science play a critical environment, climate, and infrastructure. This abil­ role in much of what we do at Los Alamos. In­ ity requires integrating the Laboratory’s strengths formatics uses a suite of information management in computation and large­scale simulation, experi­ tools to understand, manage, and predict complex mental science, and theory and modeling. phenomena. Complexity science requires an under­ standing of the global nature of complex systems Stockpile Stewardship and crosses the boundaries that have classically Our special responsibility as stewards of the nuclear separated the scientific disciplines. stockpile changed in 1992, when President Bush initi­ ated a nuclear testing moratorium and, later, when Los Alamos has long been involved in studies of President Clinton created the Stockpile Ste­ardship complex systems, from our early work on turbu­ Program to ensure the safety, reliability, and per­ lence in the 1940s and continuing through the formance of the nuclear stockpile in the absence of pioneering work of Metropolis, Stein, and Ulam on non­ Stockpile stewardship motivates us to pursue a predictive linear systems; understanding of extremely complex systems and high-impact our research in problems such as the spread of disease chaos theory; and and the impact of global climate change. the breakthrough work of Mitch­ ell Feigenbaum in the late 1970s that led to the full­system nuclear testing. Stockpile stewardship con­ establishment of our Center for Nonlinear Stud­ stitutes the core mission of our Laboratory and is one ies in 1981. Our capabilities today in informatics of the most difficult technical challenges this nation and complexity science emerge from our core has ever faced. This program requires our weapons strengths—development of transdisciplinary ap­ scientists to formally assess the current health—safety, proaches to solving very large and complex reliability, and performance—of the warheads de­ problems; integration of theory, modeling, sim­ signed and built by Los Alamos and to base the ulation, experimentation, and visualization; our validity of complex 3­D simulations on laboratory ex­ computational capabilities; and our ability to study periments and data from prior nuclear tests. phenomena from the nanoscale to the macroscale. For 50 years, weapons designers relied heavily on These capabilities are of enormous relevance to the nuclear testing. They extrapolated their designs Los Alamos national security mission. They define from a known region of tested performance and who we are and how we approach problem solv­ tried to avoid “cliffs,” or regions of performance ing. We utilize a science­based predictive capability space that were marginal with respect to perfor­ as the fundamental tool in helping us understand mance. Our limited understanding of these cliffs complex phenomena in a number of key areas— was due to nonlinear behavior that could not be weapons, threat reduction, bioscience, energy, Continued on page 18 • 1 High- Explosive Radio Telemetry High­explosive radio telemetry (HERT) is a system use of glass fiber obviates electromagnetic energy for monitoring high­explosive (HE) performance transmission into the HE, thereby preventing in a high­fidelity flight test unit (FTU). The high­ a mechanism for premature detonation. When a fidelity FTU contains a warhead with nonfissile in detonation wave strikes the tip of a fiber sensor, place of fissile materials and with HE components a light pulse is created that travels along the fiber connected to an arming, firing, and fuzing sys­ to the telemetry unit. The sensor tip is gold coated tem—all mounted in a reentry body (RB) identical (blinded) to prevent the detonation­wave precur­ to that of an actual weapon system. The HERT sys­ sor light from prematurely reaching the telemetry. tem consists of two main elements: A thin coat of lutetium oxy­orthosilicate (LSO) (1) a set of sensors imbedded in the HE com­ is applied to the tip of the gold­coated fiber. The ponents of the FTU to monitor the performance of LSO emits a very rapid (few­nanosecond rise time), these components as they explode and very intense light pulse when struck by a shock (2) a telemetry unit that transmits these data wave. Finally, for mechanical protection and further from the FTU to receiver stations on the ground. blinding, the entire tip is coated with a layer of Kry­ lon™ black paint. The constraints on the HERT system are for­ midable. First, the sensors used in this system The current version of the HERT unit measures ap­ must be nonintrusive and safe, i.e., they must proximately 2 × 3 × 5.5 in. and weighs 1.5 lb. The be sufficiently small to not interfere with the HE light impulses from the sensors are first converted components they are monitoring, and they must to electrical impulses. The programmable logic ar­ not be capable of initiating premature detonation ray then encodes the arrival times of these impulses in these components. Second, the telemetry must into a hexadecimal (16­symbol) data format. The be small enough to fit within the limited space quadrature modulator modifies both the frequency of the RB and lightweight to not perturb the RB and amplitude (I and Q) of the carrier wave from flight characteristics. The telemetry must also be the frequency source, according to the values of extremely fast to collect data from the sensors and these hexadecimal data, and sends the modulated transmit to the ground receivers before the explod­ signal to the amplifier. Next, this signal goes to ing FTU completely disassembles. the antenna that sends it to ground receivers. This custom­designed, 16­state, quadrature­amplitude The HERT sensors consist of 0.005­in.­diameter modulation scheme allows data to be transmitted quartz fibers that extend from the telemetry to the a factor of four times faster than by using stan­ sensor tips, which are embedded in the HE. The dard binary coding with frequency­modulated 2 • transmission only. The data are Power in transmitted at 100 Mbit/s to ensure capture before the Connectors for FTU disassembles. 32 fiber sensors The HERT system shown here is being tested as part of the Shock/vibration W76 Lifetime Extension Pro­ isolation mount gram. The first flight of this system, which will be primarily an instrumentation test, will oc­ cur on a US Navy Mk4A/D5 TM signals out submarine­launched test flight, (radio frequency) designated FCET 30, planned for November 2003. Æ HERT mod 2B telemetry unit. Erwin Schwegler, 667-2993, [email protected] Quartz fibers (three each) Gold "blinding" coating To telemetry Thin LSO layer Three HERT fiber-optic shock sensors. Transmission antenna Optical interface, program logic, and quadrature modulator “I” Fiber array Xylinx 2 of 64 Quadrature Power (max) Programmable Amplifier Logic Array “Q” Modulator Optical to Electrical Converter 64 Ch. Frequency Source HERT block diagram. • 3 Microstructural Characterization of High-Explosive Materials Microstructural aspects of HE materials are known range of length scales, several techniques must be to influence both shock and nonshock initiation incorporated: polarized light microscopy, scanning events and are thus of great interest from both electron microscopy, and small­angle scattering safety and performance perspectives. Differences techniques. We combine these techniques to create in particle size and the number and size of defects a comprehensive description of the microstructure (cracks, pores, etc.) can dramatically affect HE over length scales ranging from 10­10 to 10­2 m. performance. Microstructural changes may be Here, we present a brief description of these three induced during formulation and processing, may techniques and examples of their application. be generated by aging, or may result from physi­ cal or thermal insult. A quantitative description of Polarized light microscopy (PLM) studies allow us these features is important for advanced computer to probe relatively large features (1 µm to 10 mm) modeling of HE material properties and hence the of HE materials. For PLM analysis, the HE mate­ predictability of weapon performance. rial is mounted in a low­viscosity epoxy, then cured and polished. The samples are examined in reflected A complete description of HE microstructure re­ light and photographed with a high­resolution digi­ quires the ability to probe features with sizes that tal camera. Historically, one of range from millimeters to angstroms. Because no the inherent weaknesses of PLM has been the quali­ single characterization method can cover this entire tative nature of the results. To extract quantitative 4 • details from the PLM images, such as area percentages and crystal size distributions, we are employing commercial image­processing software that uses customizable routines that involve varying levels of color thresholding and geometric con­ straints to differentiate between microstructural components. Scanning electron microscopy (SEM) is used to probe micro­ structural features ranging in size from 10 nm to 1 mm. We have recently acquired a field emission scanning electron microscope (FESEM). With a conventional SEM, nonconductive materials must be coated with a conduc­ tive material to prevent charging and damage under high electron SEM image of typical HMX crystals before pressing. beam voltages. However, the Microstructural Characterization FESEM allows imaging at rel­ these contributions, allowing page 6. By using these various atively low voltages (200 eV to for quantitative assessment of imaging tools, we have quanti­ of High-Explosive Materials 30 keV), and in many cases even microstructural features. Due fied the decrease in the average nonconductive materials can be to the penetrating nature of neu­ size of the HMX crystals with imaged without applying a con­ trons and x­rays, SAS techniques increasing pressure and have ductive coating.