AGEX I: the Explosives Regime of Weapons Physics

AGEX I the explosives regime of weapons physics

Timothy R. Neal

he history of explosives re- hitherto not contemplated. The such as triaminotrinitrobenzene search and above-ground ex- achievement of those goals left Los (TATB)—can be dropped from great Tperimentation for nuclear Alamos, at the end of World War II, heights and will shatter but not ex- weapons began with the Manhattan uniquely in possession of the most plode. If exposed to fire in an acci- Project. During the hectic, almost advanced explosive-fabrication tech- dent, TATB will burn, but it is ex- frantic, war days at Los Alamos, it nology on earth and a mission to tremely unlikely to undergo a transi- became clear that, if possible, the fis- make nuclear weapons safer and tion from burning to deflagration or sionable material in the weapon more efficient—a mission that has detonation. Even when exposed to should be plutonium. It was equally continued into the present. high temperature, extreme pressures, apparent that the critical mass of plu- For a long period of time, the or shocks, these materials resist ex- tonium needed to produce a nuclear work on weapons implosions has plosion. Thus, they can be handled explosion would have to be assem- utilized conventional plastic-bonded quite safely with simple precautions. bled in the weapon through a spheri- high explosives, which could be pre- In addition to safety, the stability cal implosion driven by powerful ex- cisely machined. Improvements and reliability of nuclear weapons in plosives (Figure 1). Thus from the were continually made to increase the nation’s stockpile have been on- beginning the development of nuclear the accident resistance of these ma- going concerns. Scientists and engi- weapons was intimately connected terials. The emphasis on safety in neers have continued to study the with and dependent on developing nuclear weapon research led to the compatibility of materials contained fabrication, quality-control, and in- development of insensitive high ex- in weapons during long-term storage spection technology for high explo- plosive (IHE) at Los Alamos. Dur- and to develop new materials for sives (explosives with energies ing the 1970s the Laboratory pio- weapons components. The develop- greater than that of TNT). Initial ex- neered the use of IHE in nuclear ment of new materials has even led periments in the spring and summer weapons designs, which dramatical- to applications in the commercial of 1943 revealed, among other ly decreased the possibility that the sector. For example, a high explo- things, that for the weapon to work explosives would detonate during sive developed in the weapons pro- the design of the explosive charges accidental insults. Most modern gram, nitrotriazolone (NTO), is and the timing of their detonation weapons are designed to incorporate under consideration for use as a gas would have to achieve a precision insensitive explosives. An IHE— producer in automobile air bags.

54 Los Alamos Science Number 21 1993

AGEX I

AGEX I fig1 3/25/93

Chemical Subcritical Compressed reaction propagates by compressing explosive mass supercritical the material ahead of it and reaches

mass 90-percent completion within a few millionths of a second. Such rapid reactions produce strong shock waves. The detonation of a high explosive is typically initiated by a small shock wave that strongly compresses the explosive at a point, causing it to heat up and burn. The exothermic Figure 1. Explosive-driven Implosion chemical reaction happens so rapidly

Explosion of a fission weapon is initiated by the implosive force generated by the det- that the pressure of the reaction prod- onation of a layer of high explosive surrounding the fissile fuel. The detonating high ucts compresses the fuel around it explosive compresses a subcritical mass of fissile material to form a supercritical causing that fuel, in turn, to heat up mass that then rapidly releases nuclear energy through an uncontrolled fission chain and react, and so the detonation pro- reaction. ceeds to spread out from the point of initiation just like a spherical wave. Research on Safety ered to be more tolerable. Now that This compression-driven reaction and Performance of the Soviet threat is retracted and our travels at supersonic velocities and is High Explosives current intent is to dismantle or called a detonation wave. The lead- store needed nuclear arms rather ing edge of the detonation wave is a The end of the Cold War has led than brandish them, the public de- shock front; that is, there is a discon- to increased emphasis on safety. An serves even greater assurances about tinuity in pressure, temperature, and overriding worry is that an accident safety. Accident analyses have density across the front. The pres- might cause the explosive in a nu- therefore been extended to address sures built up in the gaseous reaction clear weapon to release its energy, extremely low-probability accidents. products behind the shock front are thus causing the assembly of a criti- Complex, multiple-accident scenar- typically on the order of a few hun- cal mass and the production of some ios now being considered include dred thousand atmospheres, and the sort of nuclear yield. Even if a nu- the possibility that after a bomber temperatures are typically between clear yield is totally averted through loaded with nuclear weapons catches 2000 and 4000 kelvins. inherent design features, the explo- on fire, another large plane crashes Most accidental insults to a nu- sive-energy release might still dis- into it. Can the new “wooden” in- clear weapon would not produce perse radioactive plutonium across sensitive high explosives withstand shock waves that could initiate the the countryside. Nuclear weapons both the high temperature and the detonation of high explosives. have long been designed to avoid or severe impact that would be in- However, exposure to fire along drastically reduce such threats. For volved in such an accident? with the impact of a crash might ini- example, all weapons in the stock- In order to predict the response of tiate a deflagration, a burn front that pile are inherently “one-point” safe; explosives in various accident scenar- propagates by heat conduction rather that is, the initiation of the explo- ios, research has been under way to than compression and therefore pro- sive at some random point will not further understand the detonation ceeds about a million times more produce a nuclear yield. Weapons process in high explosives. Unlike slowly than a detonation. A defla- have also been tested against the gasoline, which must be mixed with gration in explosives and propellants raging inferno of a jet-fuel burn to the oxygen in the air in order to burn might, however, build up into a full- assure their safe response should, completely and rapidly, high explo- scale detonation. for example, a bomber loaded with sives contain enough oxygen to under- The deflagration-to-detonation nuclear weapons catch on fire. How- go extremely rapid and complete transition is a significant safety con- ever, during the Cold War, as we exothermic (heat-producing) chemical sideration in all industrial, military, stood eyeball to eyeball with the So- reactions. The high explosive is said and nuclear weapon applications of viets, certain low risks were consid- to undergo detonation if the chemical high explosives and propellants. A

1993 Number 21 Los Alamos Science 55 AGEX I

comprehensive study of this problem high explosives are slower and seem sible to use basic models to simu- involving a consortium of university to depend on their location inside late the behavior of explosives even and government laboratory partici- the explosive charge. Thus the mod- in complex geometries. Thus the pants is under way, and the results eling of detonations in IHE has been wave of the future emphasizes care- of the study are being incorporated a far more difficult problem. fully selected benchmark experi- into engineering codes for predictive Through a very strong experimental ments to characterize explosive be- design and safety assessment of nu- program scientists have been able to havior followed by the linking and clear weapons. When the deflagra- confirm theoretical predictions con- extension of those results through tion-to-detonation process is proper- cerning the behavior of insensitive numerical simulations on super- ly understood, we can effect safety high explosives, in particular, that computers. measures to guard against even a reaction rates are strongly accelerat- Los Alamos scientists are extend- low-risk accident. ed by increases in temperature and ing their historic mission in high ex- The most important thrust of cur- pressure. The results of these exper- plosives research to discover at the rent explosives research is to devel- iments on reaction rates have been molecular level what an explosive is op better models of deflagration and used to develop more precise models and how it works. This fundamental detonation through a combination of of the initiation and detonation of research enlists sophisticated spec- experimental and theoretical work. insensitive high explosives and to troscopic experimental techniques to Many advances were achieved in better understand the effects of reac- learn what holds the explosive mole- modeling the detonation of conven- tion rates on the sensitivity of the cules together, how they come apart tional high explosives. The fact that explosive to heat and impact. during initiation and detonation, and chemical reactions in these materials Good models of deflagration and how the released energy builds up can be considered to occur instanta- detonation are essential because the the pressure and temperature of the neously simplifies the modeling of set of possible accidents is too gaseous reaction products so they the detonation wave. In contrast, broad to test each directly. Growing can do useful work (for example, AGEX I fig2 the reaction times of insensitive computing capabilities4/8/93 make it pos- drive the implosion of a metal

Kapton film Barrel Flying kapton

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produces flying kapton kaptonyyyy initiates detonation

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Figure 2. The Slapper Detonator A detonator initiates detonation of a high explosive by creating a short, high-pressure pulse in the explosive. Illustrated here is the operation of a standard detonator called the slapper. An intense pulse of electrical energy causes the metallic bridge to burst. The burst drives the kapton film down a short barrel that cuts the film like a cookie cutter. When the piece of flying kapton hits the explosive, it generates a sufficiently strong pressure pulse to cause the explosive to detonate.

56 Los Alamos Science Number 21 1993 AGEX I

sphere). Such studies improve the lightning or other accident sources. gle experiment. In spite of the com- ability to predict how a new explo- In contrast, the new optical power monplace descriptor, “hydrodynam- sive will behave and may also lead sources cannot be triggered by exter- ics,” nothing is taken for granted. to an improved first-principles ap- nal sources in any accident scenario. Every aspect of the broad subject of proach for prescribing explosives The disposal of high explosives detonations, the interaction of with specific desired characteristics. and propellants that are removed gaseous explosive products with Collaboration of theorists and organ- from weapons systems and the envi- inert materials, and the possible ef- ic chemists at Los Alamos has re- ronmental redemption of waste from fects of material strength on the re- cently led to the discovery of a new explosive and propellant manufac- sulting flows is examined extensive- class of insensitive high explosives turing are technologies of prime cur- ly. In a common type of experiment, that, unlike previous explosives, are rent interest. Research and develop- a metal plate is placed in contact very rich in nitrogen and contain ment on safe, environmentally ac- with the high explosive, and the much less carbon and oxygen. The ceptable methods for explosive and high explosive is detonated with the first of these to be synthesized, propellant disposal are under way at goal of determining how effective it LAX-112, is less sensitive than TNT Los Alamos and involve interdisci- is at pushing on the metal plate. and produces a more powerful deto- plinary collaborations among many The pressure exerted by the detonat- nation than TATB. Work is continu- parts of the Laboratory. Methods ing explosive is typically about a ing to find an explosive within the ranging from base hydrolysis to bio- million times greater than atmos- new class that is even more insensi- logical degradation and supercritical pheric pressure—much higher than tive but retains the high performance water oxidation are under investiga- the yielding strength of any ordinary of LAX-112. tion. In the latter, the explosive is metal—and causes the metal to The design and engineering de- broken down into innocuous gases move rapidly, covering distances of velopment of systems to initiate the that can then be released. a few millimeters in a millionth of a detonation of high explosives com- second. Early diagnostics consisted ponents is also a part of explosives of electronic gauges and high-speed work. These initiation systems use Above-Ground optical motion picture cameras that electrical capacitor discharge units Hydrodynamic Experiments took a few pictures at the rate of a to explode a bridgewire and thereby with High Explosives million pictures per second. create a high-pressure pulse in a In addition to the experiments small region of the explosive. Re- The demonstrations in 1943 that with metal plates, experiments were cent advances in initiation systems an explosive-driven implosion of a also carried out on weapons assem- include improved safety features. metal sphere or cylinder was possi- blies containing surrogates for the For example, the requirements for ble opened up to study the behavior fissile material. Such experiments stored electrical energy are much of matter under the extreme pres- allowed measurements to be made lower, and traditional exploding sures, shocks, and temperatures gen- on the early stages of implosion. bridgewires have been replaced by erated by high explosives. This spe- The results were then used to cali- flying slappers (Figure 2). Empha- cialized science is termed hydrody- brate computer simulations of sis on improved safety has also led namic testing because solids and weapons implosions that included to the development of safer explo- metals seem to flow like liquids the behavior of the fissile material. sives for initiation systems. when driven by the detonation of In the 1960s, a major new diag- The next generation of initiation high explosives. nostic was added to the repertoire— systems will be based on even safer Firing sites are the laboratories flash radiography. The technique in- detonators. Laser-driven slappers or for hydrodynamic testing. Because volves the use of a high-energy elec- direct optical deposition will create each experiment self-destructs dur- tron beam to produce extremely the shock waves that initiate detona- ing a test, the entire experiment short-duration bursts of x rays. Dur- tion. In more traditional systems must be rebuilt before it can be re- ing a hydrodynamic test a single x- metal wires were coupled to the deto- peated. Scientists therefore cast ray burst passes through the rapidly nator; hence, those wires could feed about continually for ways to obtain moving test object and is recorded electrical pulses to the detonator from more experimental data from a sin- on film. The resulting x-ray image

1993 Number 21 Los Alamos Science 57 AGEX I

the premier high-energy radiographic capability in the world. Because flash radiography does not perturb the experiment in any way, it yields an accurate measure of whether the explosive performance matches theoretical and engineering predictions. At first flash radiography stood alone as an isolated diagnostic. But because of the high cost of such experiments, electronic and optical diagnostic capabilities were soon added to the PHERMEX firing site. Thus began our current approach to hydrotesting: diagnose each experiment as thoroughly as possible to get the most return for the invest- ment and to maximize the under- standing of total system behavior. This philosophy continues into the future with the construction of the new DARHT (dual-axis radiographic hydrodynamic test) Facili- Figure 3. PHERMEX ty. Dual axis means that the facility PHERMEX and its associated image-analysis tools have been continually upgraded has two x-ray machines that produce and maintained as the premier high-energy radiographic facility in the world. A radio- x-ray bursts from two directions frequency linear accelerator directs a pulse of 30-MeV electrons to a tungsten target (Figure 4). At present the images where the energy of the electrons is converted into bremsstrahlung radiation. This are captured on x-ray film or spe- burst of x rays is used to make radiographic images of hydrodynamic tests involving cialized storage media residing in a high explosives. The photograph shows the thick cylindrical reinforced concrete recoverable cassette that wards off bunker that houses and protects PHERMEX from the blasts generated during a hy- blast and shrapnel damage. Only a drotest. Woven-steel blast mats covering one end of the bunker are adjacent to the tiny fraction of each x-ray burst ac- explosive firing point. Electrical signals generated by the hydrotest begin their journey tually penetrates the hydrotest ob- to the recording equipment underground in the structure shown in the lower portion of ject to record an image on the detec- the photograph. tor, so extensive image-analysis techniques are needed to quantify of the test object effectively country’s first such facility and was, the resulting pictures. If the two “freezes” the motion of explosive- in certain respects, ahead of its time. bursts are generated at different driven weapon components. Such It contains a large radio-frequency times, the resulting images allow de- radiographs are analyzed, in great linear accelerator that produces a termination of velocities of the ma- detail, to determine whether the be- beam of relativistic electrons with en- terial in the interior of the test ob- havior of the weapon components ergies of 30 MeV. The beam is di- ject. As an alternative, the two pic- agrees with theoretical predictions. rected at a tungsten target where the tures can be taken at the same time The machine called PHERMEX energy of the electrons is converted but from different positions to give a (pulsed high-energy radiographic ma- into bremsstrahlung radiation, most of “stereoscopic” view that yields a chine emitting x rays) was built main- it in the x-ray range. Through contin- type of three-dimensional image. ly for such weapons-system hydrody- ual redesign and upgrade programs Finally, there is the option of orient- namic testing—or hydrotesting, as we PHERMEX and its associated image- ing one x-ray machine to one area of call it (Figure 3). PHERMEX was the analysis capabilities have remained a hydrotest to obtain the best possi-

58 Los Alamos Science Number 21 1993 AGEX I

ble resolution and orienting the propagated through microminiature cause nuclear testing will no longer other machine to a completely dif- cables that bend around obstructions. be available as the final arbiter, our ferent area for similar reasons. The crushing of these cables as the computational models and codes The biggest advance in measure- detonation proceeds yields mi- must be tightly tied to those phe- ment techniques in the last decade crowave interferometric measure- nomena that can be measured. In has been the development of quanti- ments of the positions and velocities addition, those measurements must tative radiography. Radiographs are of shock and compression waves. A be made more universal to elucidate no longer just pictures of items selection of these techniques is regu- not just the behavior of surfaces but going hither and yon with distance larly applied to each hydrotest to that of interiors as well. Even sur- scales superimposed for measure- measure the position, velocity, and face measurements must attain a ments. Radiography is now able to condition of material surfaces as well new degree of sophistication that determine the density of compressed as the propagation and pattern of will yield information about temper- materials, the location of material wave-like disturbances. ature and material breakup. interfaces to submillimeter preci- The thrust for the future in hy- The explosives AGEX activities sion, and the computer-assisted to- drotesting is increased precision and must rise to a new role in the nu- mographic (CAT) reconstruction of all-encompassing diagnostics. Be- clear defense activities of the Labo- interior sections of a distorted object. The latter process is analogous to the CAT scans used in the medical field (Figure 5). Progress has also been made in other types of diagnostics. Electron- ic measurements have now attained temporal resolution of a billionth of a second, and hundreds of them may be made during a single hydrotest. Ul- trafast color motion-picture cameras are now joined by electronic cameras that are over ten times faster. Lasers are being used as interferometers to precisely measure the velocity of surfaces (see “Line-imaging Laser Inter- ferometer for Measuring Veloc- itues”). The laser light can be trans- mitted and returned to detectors through fiber optics, a method that allows measurements to be made in hard-to-reach places. Laser interferometers have traditionally been used Figure 4. Plan of the Proposed DARHT Facility to measure the velocities of only a A major new initiative in the explosives AGEX program is the design and construction single point on a surface. With the of a dual-axis radiographic hydrodynamics test facility. This high-intensity flash x-ray help of image analysis techniques, test site will contain two high-energy linear induction accelerators at right angles to measurements can now be made each other. The x-ray bursts will be ten times more effective than those available at along an entire line of a test object. PHERMEX, enabling flash radiography of dense objects. The two distinct x-ray Measurement along a line within an bursts will be used to generate radiographs of a single hydrotest at different times and axially symmetric object translates with orthogonal views. The extension to the rounded end of one of the machine build- into a continuous high-precision ve- ings will contain extensive capabilities for optical diagnostics. The electronic diagnos- locity map of an entire surface. In tic equipment for DARHT, like that for PHERMEX, will be located underground near another technique microwaves are the firing site.

1993 Number 21 Los Alamos Science 59 AGEX I/Line-imaging Laser Interferometers

Line-imaging Laser Interferometers

for Measuring Velocities Willard F. Hemsing

ydrodynamic tests create hos- fringes oscillate between bright the vertical polarization component Htile conditions in which high and dark as the test object acceler- are shifted and their oscillations pressures can easily compress ates. The VISAR measures veloci- lag behind those of the horizontal solids and accelerate materials to ty by accurately determining the component. Specifically, the inten- velocities of several kilometers per number of whole and partial oscil- sities of corresponding points in second. Among the advanced diag- lations that occur as the test object the horizontal and vertical compo- nostics for hydrodynamic tests at accelerates. Its useful product is a nents depend on the sine and co- the Laboratory is our line-imaging continuous velocity history for all sine, respectively, of the velocity VISAR (Velocity Interferometer the points that are visible in the at each point on the target. Polar- System for Any Reflector). The image. izing beam splitters separate the VISAR measures the velocities of (a) Our line-imaging VISAR horizontal polarization component points along an illuminated line on uses a cylindrical lens to focus from the vertical component where a fast-moving test object. The in- laser light onto a line on the test light exits from each side of the in- strument exploits the fact that object. Conventional optics image terferometer. This separation pro- when laser light is reflected from a the illuminated line through a spe- duces two pairs of images of the moving surface, the wavelength of cial wide-angle Michelson interfer- interference intensities along the the light is Doppler-shifted in pro- ometer, where a retardation plate illuminated line. The two images portion to the velocity of the point delays the vertical polarization for each polarization are simply that reflects it. The VISAR em- component of one beam by a quar- negatives of each other. ploys optical interference to gener- ter of a wavelength. As a result, Fiber-optic bundles transmit the AGEX I fig6 ate bright and dark bands of light when the beams are recombined4/8/93 to four images to the photocathode of called interference fringes. The produce interference, the fringes of an electronic streak camera. The

C4350 dye pump laser

(a) Line illumination across moving test object

Argon laser Optical Cylindrical filter lens C504 dye amplifier capillary cell

VISAR interferometer Spherical lens Beam splitter

1/8-wave plate Polarizing splitter

Four images Light of illuminated recombines Polarizing line arrive here splitter at camera Electronic streak camera Fiber optic bundles

60 Los Alamos Science Number 21 1993 AGEX I/Line-imaging Laser Interferometers Position along line of metal sine image plate Position along cosine image line of metal plate

(b) 0 Time Velocity

camera rapidly sweeps the images store information, our line-imaging initiated at two separate points, across a charge-coupled device that VISAR can capture many times drove a metal plate. Triangles ex- digitizes them into a microcomput- more data than conventional VIS- tending across the left third of the er. Later, we subtract one image of ARs. We have found its ability to images are the edges of interfer- each polarization from its negative simultaneously record large quanti- ence fringes as they responded to to double the signal and cancel op- ties of information relating differ- the acceleration of the plate. A tical noise. Analysis of the images ent points on a test object extreme- change from dark to bright, corre- yields the velocity histories of ly advantageous. This is most use- sponding to an increase in velocity many points in the line as a contin- ful in measurements in which of 200 meters per second, is visible uous function of time. velocity gradients are important, in the cosine image. The VISAR’s sensitivity to ac- and in tests that destroy expensive (c) An isometric plot of velocity, celeration, instead of to velocity hardware, especially when test-to- deduced from the photograph in alone, best accommodates mea- test variations are important. Al- (b), as a function of position along surements of velocities from 100 though our line-imaging VISAR is the illuminated line and time. The meters per second to over 20 kilo- versatile, its use is precluded when “cliffs” at the lower left indicate meters per second. Its recording smoke blocks its optical path or the acceleration of the metal as it time can vary from milliseconds to when the test-object surface loses was driven by the two converging nanoseconds; the length of the line light reflectivity. pressure waves. The ridge extend- it observes can range from 0.3 to (b) The sine and cosine interfer- ing from the center to the upper 30 millimeters across the target ence images from an experiment in right is a region of high velocity surface. Because it records pic- which two converging detonation caused by the pressure enhance- tures with their great capacity to waves, produced by an explosive ment where the waves collided.

1993 Number 21 Los Alamos Science 61 AGEX I

Figure 5. Quantitative Radiography When metals are subjected to the shock pressures and temperatures created by the detonation of high explosives, they seem to flow like liquids. This figure shows images of an explosively formed penetrator made of copper during its high-velocity (2.4 kilometers per second) flight. The penetrator was originally a cone-shaped piece of copper backed by high explosive. The force of the high-explosive detonation shaped the copper into the form shown here.

ratory and of the nation. Our capa- bilitites in explosives characteriza- tion, hydrodynamic modeling, and technology development are a spe- cial resource to the national materials science community, to U.S. in- dustry, and to the conventional defense community. They are a unique and critical resource to the nuclear AGEX I fig5b (a) This radiograph is the average of four different radiographic films of the pene- 3/3093 weapons community. As availability trator in flight. Of interest here is the detailed shape of the inner cavity. The of under-ground nuclear testing lighter areas represent greater material thickness. fades, above-ground hydrotesting will become the keystone for nuclear weapon design, qualification, and 2 External contour safety assessment.

0 Internal contour Radial distance (cm) -2

-12 -10 -8 -6 -4 -2 0 Axial distance (cm) (b) The line drawings of the internal and external contours of the penetrator were estimated by a least-squares fitting of an analytical model to the x-ray film densi- ties. In the forward portion of the penetrator, where axial symmetry is high, the edges of the contours are thought to be accurate to within 0.2 millimeter.

Timothy R. Neal has been Division Leader of Expolosives Technology and Appliations since 1991. He joined the Laboratory in 1967 as a staff member with the Flash Radiograph Group. In 1979 he served as Program Manager for the Con- fined Testing Program, and in 1980, he was Asso- ciate Division Leader for Dynamic Testing. From 1981 until February 1990 he served as Group Leader for Hydrodynamics, where he (c) This cross-sectional view of the penetrator is a computer-assisted tomographic oversaw the consolidation of groups involving (CAT) reconstruction of the interior of the penetrator made from a high-quality ra- flash radiography, image analysis, and hydrody- diograph like the one shown in (a). The gray scale represents material density. namics. He served as Adjunct Associate Profes- sor of Physics at New Mexico State University, The combination of good edge location and density reconstruction results from a instituted the continuing U.S./United Kingdom high-quality original radiograph and excellent image-analysis capabilities. The exchange in weapons hydrodynamics and the knowledge of both edge location and density variation is critical to the interpreta- U.S./France exchange in image analysis, and was instrumental in developing the Dual Axis Radi- tion of hydrodynamic experiments. ographic Hydrodynamics Test (DARHT) construction project.

62 Los Alamos Science Number 21 1993