An Expanding Role for AGEX: Above-Ground Experiments For

An Expanding Role for AGEX Above-Ground Experiments for Nuclear Weapons Physics

Philip D. Goldstone

Procyon, the high-explosive pulsed-power generator at Los Alamos

or the last fifty years U.S. ex- Imagine, twenty years from now, a bilities of others), if we are without pertise in nuclear-weapons de- military commander or a member of any fully integrated nuclear tests and Fsign and engineering was a nat- a stockpile-surveillance team notic- without ongoing development pro- ural by-product of an active developing that one of the weapons being grams. It will be necessary, but not ment and testing program. With the stored has changed in appearance. sufficient, for the scientists and engi- end of the cold war, the development They will want to know, “is this still neers involved in this enterprise to of agreements to retire most of the safe, and would it work if needed?” do research; the research itself must U.S. and former Soviet weapon stock- They will call the Laboratory and stress their judgement in weapons piles, and plans to phase out U.S. nu- ask the experts regarding this science and engineering. Further- clear testing by 1996, the way the nu- weapon. Will they be able to rely on more, to prevent losing capability clear weapons program has operated the answer they get? A competent within a generation, these research is changing dramatically. However, technical capability—including the programs must be technically inter- nuclear weapons will remain a pres- honed expertise and judgement of esting to attract and train new scien- ence in the world for the foreseeable weapon designers and engineers—is tists and engineers. future, and the U.S. will retain a the principal element required for In the absence of underground smaller but very real nuclear capabili- the assurance and stewardship of testing, a high premium will be ty. We at the weapons laboratories whatever nuclear stockpile the nation placed on predictive computational will therefore have a vital responsi- retains. At Los Alamos, where over capability for design and engineering bility—that of long-term stewardship 80 percent of the weapons that are of weapons. If our computer-simula- of the stockpile, including continued expected to remain in the stockpile tion capabilities were perfect, then, assurance of reliability and safety. were designed, this responsibility is in principle, actual testing would not Since the weapons remaining in the particularly important. be required for us to be confident of stockpile will age, eventually we will Without nuclear testing we will the performance of an aging, modi- have to provide competent assess- simply not be able to maintain to- fied (for improved safety, for exam- ments of the need to modify or re- day's level of competent judgement. ple), or even redesigned device. place aging weapons or components. The challenge we now face is to con- Computational-simulation capabili- This role will continue indefinitely tinue to exercise our weapons R&D ties are simply insufficient for this into the future and beyond the career expertise so that we retain—as much challenge, despite many years of de- lifetime of many of today's weapon as possible—the ability to assess our velopment and the ongoing revolu- scientists. own systems (and the nuclear capa- tion in high-performance computing.

52 Los Alamos Science Number 21 1993 An Expanding Role for AGEX

Therefore, part of the strategy for Second, and possibly more important, fidence in the U.S. stockpile. Explo- meeting the dilemma posed by a experiments provide ongoing valida- sives characterization as well as hy- cessation of nuclear testing is to tion and exercise of weapons capabil- drodynamic testing is discussed in develop higher-performance com- ity and judgement. A proper mix of “AGEX I—The Explosives Regime puting capabilities and to exploit experiments will even actively in- of Weapons Physics.” those capabilities by running more volve the fabrication, engineering, The second physical regime that accurate and more predictive codes. quality assurance, and fielding skills must be addressed by experiments is Although the weapons-design vital to weapons capability. Third, the “high-energy-density” (or AGEX codes may be viewed as an “archive” only a technically stimulating re- II) regime, which is typically where design physics is contained search program that combines experi- achieved only after the initial pro- for future designers to use, contin- ment with theory is capable of at- duction of nuclear energy. It in- ued comparison of code predictions tracting and retaining quality scien- volves temperatures from tens to against experiment—reality—is re- tists and engineers in the weapons thousands of electron-volts or eV (1 quired to provide validation of the effort. Finally, in its own right, ex- eV is equivalent to 11,600 kelvins) codes and the designer's judgement. perimental capability—the ability to and pressures greater than 10 The physics, materials behavior, and measure and interpret data, whether megabars (1 megabar equals 1 mil- engineering associated with nuclear that data derives from an under- lion atmospheres). Technical issues weapons is extremely complex. In a ground weapons test, an above- in the high-energy-density regime matter of moments, weapon compo- ground experiment, or from other include radiation flow and the inter- nents are brought from their normal sources such as nonproliferation ac- action of radiation with matter; radi- physical state to the most extreme tivities—is an essential component of ation hydrodynamics and hydrody- conditions found in the solar system. U.S. nuclear weapons stewardship. namics at extreme pressures; nuclear It is the act of continually testing The following two articles dis- cross sections and neutron interac- both the codes and the designers cuss the two physical regimes that tions; and the behavior of dense against experiment that develops and must be addressed in experiments plasmas. maintains expertise and that prevents related to weapons design. The “ex- Whereas the explosive regime can theory and reality from diverging. As plosives regime” (or AGEX I) in- be more or less directly accessed at long as a simulation code contains cludes the physics and chemistry of full scale, achieving the high-energy- implicit approximations and assump- explosives as well as the behavior of density regime over anything ap- tions, its value is intimately tied to matter subjected to the pressures, proaching the full spatial and time the judgement of those who use it and shocks, and temperatures that may scales of a weapon would imply an interpret its output. A divergence of be achieved with typical high-explo- extraordinary amount of energy, com- theory and reality can (and often sive configurations. For example, parable to that of a nuclear explo- does, in many human endeavors) re- the behavior of heavy-metal assem- sion. Although this is clearly neither sult in a false sense of confidence in blies compressed by high explosives achievable nor desirable in an above- computation or design judgement that is an important element of weapon ground experiment, various pieces of is potentially disastrous. This is the design and engineering. the physics can be accessed and stud- principal underlying reason for regu- Above-ground experiments direct- ied by using an appropriate set of larly performing nuclear tests, even if ly applicable to many (though not specialized facilities. The efforts to we could anticipate never having to all) important aspects of the high-ex- develop and use such facilities for modify a stockpiled weapon again. plosives regime of weapon design are the study of the high-energy-density Therefore, in the absence of under- possible without a nuclear explosion. regime are discussed in “AGEX II— ground testing, appropriate “above- The principal example is that of hy- The High-Energy-Density Regime of ground experiments” (AGEX) must drodynamic testing, including flash Weapons Physics.” be vigorously pursued. radiography of high-explosives-dri- The role of experiments is, first, to ven assemblies. The DARHT (Dual- provide the physics data and the un- Axis Radiographic Hydro Test) facil- derlying physical models to improve ity is the single most important new the predictive nature of the codes. AGEX capability for the future con-

1993 Number 21 Los Alamos Science 53

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

Metallic Explosive yy

bridge yyyyyy

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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 understanding 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 characterization, hydrodynamic modeling, and technology development are a special 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 densities. 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

AGEX II the high-energy-density regime of weapons physics

Stephen M. Younger

The Pegasus II capacitor bank for pulsed-power experiments uclear explosives achieve access to these unique conditions eral hundred kilobars and reach tem- higher temperatures and has been sharply reduced, and by the peratures of several eV. These con- Npressures than any other ob- end of 1996 it will disappear entire- ditions are reproducible in the labo- ject in our solar system. Pressures ly. There is an urgent need to devel- ratory and a great deal of data are in excess of ten million atmo- op laboratory techniques that will available to describe material re- spheres, temperatures over 1000 allow us to simulate the conditions sponse and hydrodynamic processes electron volts (1 eV corresponds to found in a nuclear explosive both to at these pressures. (See “AGEX I— 11,600 kelvins) and very high densi- provide more accurate information The Explosives Regime of Weapons ties typify a nuclear explosion. on the physics of matter at high en- Physics.”) When the fissionable Under these conditions even the ergy density and to provide a vehicle material in the weapon reaches a heaviest atoms are almost complete- for continued development of the critical mass, however, a chain reac- ly ionized, and neutron radiation is special skills required to maintain an tion occurs, which causes the rapid so intense that higher-order nuclear understanding of nuclear weapons. generation of energy. This chain re- processes (such as multple capture) action occurs on a time scale short become common. Our knowledge of compared to the ratio of the size of such extreme energy-density condi- The Physics of Nuclear the device to the sound speed, so the tions has been gained through a Explosives material does not have a chance to combination of theoretical calcula- expand during the energy-generation tions and experiments performed on Nuclear weapons are very com- phase. Since the energy cannot go actual nuclear explosions. With the plex devices. During the high-ex- into kinetic energy, it goes into ther- reduction in the number of under- plosive phase of the weapon, materi- mal energy, raising the temperature ground nuclear tests, however, our als are subjected to pressures of sev- of the material to extraordinary val-

1993 Number 21 Los Alamos Science 63

AGEX II

ues and thus raising the pressure to fortunately, low-density samples lack velocities. Such experiments are many millions of atmospheres. some of the unique aspects of dense made more complex by the presence Laboratory studies of the proper- plasma. The relevant figure of merit of the hydrodynamic tamper or other ties of high-energy-density matter for dense matter is the coupling para- artifacts of the plasma-containment face two major challenges: First, one meter, Γ, the ratio of the average mechanism. must reproduce the very high densi- electrostatic energy between neigh- No single above-ground experi- ties and temperatures typical of a nu- boring ions to the average thermal ki- mental facility can simultaneously clear explosion. Second, one must be netic energy. For low Γ thermal reproduce all of the relevant condi- able to probe the conditions in the processes dominate and the plasma tions found in a nuclear explosion. sample, usually via an x-ray burst (to behaves as an ensemble of individual At Los Alamos we have assembled a probe atomic properties) or a pressure particles. For high Γ the electrostatic broad array of high-energy-density pulse (to probe material equations of force dominates, the plasma becomes facilities, including pulsed-power state). The energy required to heat a “stiff,” and it can even condense into machines, lasers, and the LAMPF sample is roughly given by (3/2)nkT, a solid phase. The goal of high-ener- accelerator, that allow us to access a where n is the density of particles gy-density physics is to produce a broad range of high-energy-density (nuclei plus ionized electrons), k is sample dense enough to resemble a conditions for the study of physics Boltzmann’s constant, and T is the strongly coupled plasma yet hot relevant to nuclear explosives. temperature. A simple calculation enough for the level of ionization to shows that normal-density uranium at be representative of the material in a 1 keV has an energy density of about nuclear explosive. This requires both Athena: Pulsed Power for AGEX II table 500 megajoules per cubic centimeter3/27/93. raw energy, to heat a sample of sig- High-Energy-Density Even for a sample 1 millimeter across nificant size, and power, to rapidly Physics the net energy required is 500 kilo- heat the sample before it expands to joules, a substantial amount for labo- low density. In Greek mythology Athena was the goddess of wisdom who carried Examples of High-Energy-Density Physics the thunderbolts of Zeus. At Los Alamos Athena is the program that The High Pulsed Nuclear Jupiter Lasers uses pulsed-power technology to ex- Sun explosives power explosions plore high-energy-density physics in Temperature (eV) 103 1 ~1 >100 100 >103 support of the nuclear weapons program. The advantage of pulsed 9 6 5 8 7 7 Pressure (atm) 10 10 10 10 10 >10 power for high-energy-density Density (g/cm3) 10 1 1 100 10 >10 physics is that many megajoules of energy can be stored in very compact devices and then rapidly deliv- ratory experiments. Also, in contrast Diagnosing a high-energy-density ered to an experiment. The Athena to fissioning metals, which generate plasma is also challenging. No ma- program uses two methods to gener- heat internally, laboratory samples terial probe can withstand the condi- ate intense electrical pulses: a large must be heated by an outside energy tions of a hot dense plasma, so re- capacitor bank called Pegasus II and source. The heating takes several mote measurements are essential. X a high-explosive pulsed-power gen- nanoseconds, long enough for the rays, either those emitted by the erator called Procyon. sample to begin to disassemble. The plasma itself or those absorbed when resulting density and temperature gra- an intense probe signal is passed Capacitor-bank pulsed power. The dients complicate the interpretation of through the plasma, can reveal much Pegasus II capacitor bank consists of the experiment. Reducing the density about the atomic properties of the 144 capacitors wired in parallel and allows one somewhat greater flexibil- material. Strong shock waves can arranged around a central target ity, since hydrodynamic tampers can be launched into the sample to de- chamber. Over the course of several be used to keep the material from ex- termine its equation of state via the minutes, a high-voltage power sup- panding during the experiment. Un- measurement of shock and particle ply charges the capacitors. Pegasus

64 Los Alamos Science Number 21 1993

AGEX II

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ddddddddddddddddddddddddddddddddddddddddddddvcddddddddddddddddddvc cQddddddddddddddddddddddddddddddd

dddddddddddddddddddddddddddddddcddecdddddddAddddddddddddddddddIc cRdddddddddddddddddddddddddddddd the two current-carrying electrodes and other implosion parameters. dddddddddddddddddddddddddddddddcddecdddddddAddddddddddddddddddIc cRdddddddddddddddddddddddddddddd

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ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd of the capacitor bank. A current I In the second class of experi- ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd

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ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd flowing in the cylinder produces a ments, a sample is placed inside the ddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd magnetic field imploding cylinder to study its material properties under extreme pres- Figure 1. Implosion of a Thin B = µI/2πr sures and temperatures. An implod- Foil Cylinder at Pegasus II ing cylinder can compress a sample A side view of the implosion of a 2500- (where µ = 4π × 107). The interac- to a pressure of several megabars. angstrom-thick, 5-centimeter-radius, tion of this magnetic field with the At this extreme pressure the atomic aluminum cylinder on the Pegasus ca- drive current results in an inward structure of the sample becomes dis- pacitor bank is shown at various times, pressure, torted. The forced overlap of atomic with an arbitrary zero time. Note the orbitals causes changes in the trans- onset of short-wavelength perturbations P = B2/2µ, port rates of heat, charge, and pho- at early times, followed by the growth of tons. Serious research on those longer-wavelength perturbations, and that causes the rapid implosion of transport phenomena in hot dense eventually the complete breakup of the the cylinder. When the Pegasus II matter is just beginning to be pur- aluminum foil. capacitor bank at full charge is dis- sued. Since the implosions are near- charged through a gold-foil cylinder ly adiabatic and create high-pressure its axis, the kinetic energy of collapse with outer radius of 2 centimeters conditions that last on the order of a is converted to thermal energy, and and wall thickness of 0.1 millimeter, microsecond, Pegasus II is an ideal the matter in the foil becomes a hot the cylinder implodes in about 6 mi- test bed in which to study the struc- plasma. The matter then rapidly cools croseconds. The peak pressure is ture and dynamics of matter under by emitting an intense burst of x rays. 1.1 megabar and the peak implosion extreme conditions. Kinetic energy is proportional to the velocity is about 1.3 centimeters per In the third class of experiments, square of the velocity, so implosions microsecond. very fast implosions are used to gen- at higher velocities produce more x Pegasus II can be used for three erate intense x-ray bursts. These rays. Since the magnetic pressure main classes of experiments. Sim- bursts are needed for a wide range of during the implosion is independent ply discharging the bank through a experiments on radiation transport and of the mass of the foil, the implosion cylindrical target provides a test bed the interaction of x rays with matter. velocity can be increased by reducing for studying implosion hydrodynam- When the cylindrical foil implodes to the thickness of the foil. Cylindrical

1993 Number 21 Los Alamos Science 65 AGEX II

there is a relatively low-cost alternative, namely, the amplification of electric power pulses with high explosives. At Los Alamos we have developed a series of high-explosive pulsed-power generators that have produced currents as high as 150 million amperes in compact, relatively low-cost units. The present device, called Procyon (Figure 2), can deliver more than 1 megajoule of energy into an implosion. Figure 3 illustrates the operation of a high-explosive pulsed-power generator. A small capacitor bank sends a current pulse through a coil wound loosely about a copper cylin- Figure 2. The Procyon High-Explosives Pulsed-Power System der that is filled with high explo- The white cylinder at left is a Mark IX high-explosive generator. To the right is an sives. This current creates a mag- explosive opening switch, and at the extreme right are the plasma flow switch and netic field in the gap between the the implosion target. The clear tubes extending from the target contain diagnostic coil and the copper cylinder. As the equipment. magnetic field reaches its peak value, the high explosive in the cop- foils as thin as 2500 angstroms (less to reduce the rise time of the bank per cylinder is detonated. The than the wavelength of visible light) from several microseconds to several cylinder expands, and as it closes have been imploded on Pegasus II to hundred nanoseconds, we can make the gap between itself and the coil, velocities in excess of 10 centimeters energy deposition in the cylindrical it squeezes the magnetic field into a per microsecond. However, the onset foil more efficient. Second, by in- smaller and smaller volume and of hydrodynamic instabilities limits creasing the radius of the foils, we can thereby increases the magnetic-field the usefulness of very thin foils. The extend the implosion time. We hope energy. At maximum field compres- very rapid acceleration endured by that an optimal combination of the sion the switch shown in the figure these foils enhances the growth of two techniques will maximize the is opened, allowing the field energy Rayleigh-Taylor instabilities. The coupling of the energy in the capacitor to be extracted in the form of a foils can become so unstable that they bank to the kinetic energy of the foil greatly amplified current pulse that break up before reaching the axis, in while minimizing the growth of dele- flows through the target. which case various pieces of the foil terious hydrodynamic instabilities. Even though high-explosive arrive at different times. This prolon- pulsed-power generators can pro- gation of the arrival of the imploding High-explosive pulsed power. Pe- duce tens or even hundreds of mega- shell results in a lower effective ener- gasus II produces pressures of joules of electrical energy in a sin- gy density in the plasma and hence a megabars and energy densities of gle pulse at affordable prices, the softer x-ray spectrum. Also, if the hundreds of kilojoules per cubic pulse is several microseconds long, implosion velocity is so high that the centimeter in a volume of about a so the power eventually delivered to implosion is over before the capacitor cubic centimeter. The production of a target is at most a few tens of ter- bank has the opportunity to deposit all significantly higher pressures in a awatts (1 terawatt = 1012 watts). At of its energy in the target, then the re- similar-sized volume would require present it is difficult to compress sulting x-ray burst will be less in- storing tens to over a hundred mega- this energy into a much shorter, tense. Two techniques are available joules of electrical energy in a very higher-power pulse, which would be to overcome these limitations. First, large capacitor bank. This can be an useful for the production of intense by using special switching techniques expensive proposition. Fortunately x-ray bursts or ultrahigh pressures,

66 Los Alamos Science Number 21 1993 AGEX IIfig2 4/7/93 AGEX II

Figure 2. Operation of a High-Explosive Pulsed-Power System

Metal armature As the capacitor bank discharges Field coil through the field coil, the current in the Target coil generates a magnetic field between Capacitor the coil and the metal armature. The High Opening bank switch opening switch is in the closed position, explosive preventing current from flowing to the target.

The high explosive is detonated at one end and expands the armature. The magnetic field is squeezed between the expanding armature and the field coil and greatly increases in magnitude. In this way high-explosive energy is converted into magnetic-field energy. The field energy, in turn, amplifies the current.

At peak compression of the magnetic field the switch is opened and a greatly amplified current pulse flows through the target.

although a variety of options are ers two simultaneous pulses, each ertial-fusion program has made im- being evaluated. To reach signifi- 100 picoseconds long and carrying pressive progress in diagnostics de- cantly higher powers, one must em- 100 joules of energy. The laser con- velopment, so that it is now possible ploy other technologies more suited sists of a very low-energy oscillator to obtain x-ray images of experi- to short-pulse generation. Chief that forms the laser pulse, a series of ments with spatial resolution of less among those are high-power lasers. rod amplifiers that increase the ener- than 5 microns and temporal resolu- gy in the pulse to about 1 joule, and tion of less than 100 picoseconds. a set of disk amplifiers that provide Trident was designed to be an Lasers: Higher Energy the final amplification. The pulses easy-to-use tool for high-energy- Densities for Shorter Times are directed into a target chamber density physics. It can deliver laser outfitted with a wide array of diag- pulses with a wide variety of lengths Lasers can produce very short, nostics, including x-ray and optical and shapes for different experi- high-power pulses and direct them spectrometers, framing cameras, and ments. Trident also has a small into small volumes to create very streak cameras. third laser beam, which is used to high energy densities. But the cur- A Trident 100-picosecond laser create a short x-ray pulse next to the rent high-power lasers are very ex- pulse focused into a volume a few target. X radiographs of evolving pensive energy sources and maintain hundred microns in diameter yields experiments can be obtained from high energy densities in a target for an energy density of over 1 mega- the x-ray pulses and are particularly only a nanosecond or so and over joule per cubic centimeter. Although useful for the study of high-pressure volumes only of the order of a cubic the energy density is higher than that hydrodynamics. Trident pulses, millimeter. In spite of these limita- produced in experiments using Pega- when applied to appropriate targets, tions, high-power lasers have proven sus II, our 4-megajoule capacitor can produce shock-wave pressures to be very versatile in the study of bank, the temporal and spatial scales of several megabars and x-ray pulses high-energy-density physics. of the experiments are much smaller of moderate temperatures. Trident is a neodymium-doped and very sophisticated diagnostics Still higher temperatures over some- glass laser at Los Alamos that deliv- are required to acquire data. The in- what larger volumes can be obtained

1993 Number 21 Los Alamos Science 67 AGEX II

on the Nova laser at Lawrence Liver- toseconds, so that even though it is nuclear explosion. Nevertheless, more National Laboratory. Nova, the moving with the speed of light, a Bright Source II can heat thin solid largest glass laser in the world, pro- pulse is only about 900 microns foils to keV temperatures, creating a duces pulses of up to 40 kilojoules in long. The focused pulses have in- high-density and very hot plasma. one nanosecond. We have fielded a tensities of more than 5×1018 watts Hence this laser can be used to number of experiments on Nova relat- per square centimeter, well above probe the structure and dynamics of ed to radiation hydrodynamics and x- pulse intensities produced by Tri- matter at conditions that approach ray-driven implosions. dent or even Nova. The impact of a those found in a nuclear explosion. How far can one go in increasing laser pulse on the surface of a target The hot plasma cools both by expan- energy density by shortening the sample creates pressures of more sion and by the emission of x rays. pulse length of the laser and reduc- than 1 gigabar, but only for about Figure 4 shows a typical x-ray spec- ing the size of the focused optical one picosecond, after which the trum from an aluminum sample illu- spot? Another laser at Los Alamos, sample expands under thermal pres- minated by a Bright Source II pulse. Bright Source II, is providing the sure. (It is interesting to note that Up to 1 percent of the incident laser answer. Bright Source II pushes the the radiation pressure—the pressure energy is converted to line radiation limits of energy density by directing due to the momentum of the light it- around 2 keV. The line radiation is a relatively small amount of energy self—is 1 gigabar, which is compa- useful for studying the interaction of (only a quarter of a joule at present, rable to the induced thermal pres- x rays with matter. although a 10-joule machine is on sure in the target.) During such a The extremely short pulses avail- the horizon) into an incredibly short short pulse the atoms in the target able from the Bright Source II laser pulse that can be focused down to do not have a chance to equilibrate also provide an effective means to only a few microns. Bright SourceAGEX IIfig3and may not approximate fully the study very rapid processes, such as II pulses last less than 300 fem- 4/7/93equilibrium conditions found in a transient chemical reactions. In typical chemical detonations several 0 transient molecular species such as Bright Source II Al XIII 2-1 OH radicals persist only for a short time but are important in determining the overall energy balance in the 8 detonation products. An experiment is currently underway at Bright Source II to measure the OH radical shot) × in a forced detonation—the first 6 ster such measurement of its kind for an × explosive process. Al XIII 3-1 mJ/(keV 4 Nuclear Physics at LAMPF Al XIII 4-1 Moving up again in the energy scale, we encounter nuclear energy 2 densities—where the relevant energy 1.7 1.8 1.9 2.0 2.1 2.2 parameters are not kiloelectronvolts Energy (keV) as in plasmas but megaelectronvolts. The formation of a critical mass Figure 4. X-Ray Spectrum Induced by a Pulse from Bright Source II during the detonation of a nuclear The x-ray spectrum results from the impact of a 0.25-joule, 300-picosecond pulse explosive and the attendant chain re- from the Bright Source II laser on an aluminum foil. The intense lines serve both action result in an intense neutron as a diagnostic of the conditions in the dense radiating plasma and as valuable burst. Neutrons interact with nuclei probes for use in other experiments. through a complex set of scattering

68 Los Alamos Science Number 21 1993 AGEX II

and capture processes, some leading provide absolute measurements of to produce focused intensities over to the production of additional neu- the neutron flux from the nuclear de- 1020 watts per square centimeter to trons and/or the initiation of fission vice. We are currently evaluating permit the study of multiphoton x- and others to the production of sta- new techniques for using proton and ray interactions. This intensity is ble isotopes. To model the dynam- neutron sources to image dynamic high enough to rip apart the vacuum ics of fission in weapons, we must phenomena in opaque samples. in the electrostatic field near a nu- have accurate descriptions of all of cleus to create electron-positron the dominant neutronics processes. pairs, literally creating matter from The knowledge of weaker processes The Future energy. (such as those involving transient nuclear states) can provide valuable The next several years promise to diagnostics on the progress of the be among the most interesting and nuclear burn and contribute to radio- productive ever for high-energy- chemical analyses of nuclear explo- density physics. We have assembled sions. The Laboratory has conduct- an array of facilities to investigate a ed an extensive series of experi- wide range of physics issues of im- ments on nuclear physics important portance to the nuclear weapons pro- for weapons at LAMPF and other gram. The structure and transport nuclear facilities. properties of hot dense matter will LAMPF is the most powerful ac- be systematically studied in a celerator in the world. Although regime where single-atom theories some machines accelerate charged break down and many-body effects particles to higher energies, none is are important. The interaction of capable of delivering as many parti- strong shock waves and x-ray pulses cles per unit time to the target as with matter will continue to be stud- LAMPF. This capability is impor- ied with the aim of providing quan- tant when one wants to study weak titative data for use in our computer processes, including the study of models of nuclear explosions. Ex- Stephen M. Younger is the Program Director higher-order nuclear cross sections. perimental data on hydrodynamics for Inertial Confinement Fusion and High Energy Density Physics at the Laboratory. He received a In addition to the accelerator itself, and hydrodynamic instabilities will Ph.D. in theoretical physics from the University the LAMPF facility includes several allow us to validate increasingly so- of Maryland in 1978. From 1974 to 1982 he per- target areas. The areas of particular phisticated algorithms in new com- formed theoretical studies of atomic processes at the National Bureau of Standards in Washington, concern for the weapons program puter codes, particularly those that D.C., concentrating on electron scattering from are the Weapons Neutron Research will need to be developed to exploit atoms and ions. From 1982 to 1989 he was a (WNR) facility and the Manuel the promise of massively parallel staff member and group leader at Lawrence Liv- ermore National Laboratory, where he special- Lujan, Jr. Neutron Scattering Center computers. ized in advanced thermonuclear weapons design, (LANSCE). Each of our capabilities can be x-ray lasers, electromagnetic weapons, and other LAMPF has been extensively used extended to higher energies for even defense programs. In 1989 he came to Los Alamos and in 1991 he was named to his present by the weapons program. Funda- more interesting applications. The position. In addition to working on defense is- mental aspects of fission have been next advance in Laboratory capaci- sues, Younger has continued to contribute to fun- studied by examining the relative tor banks is Atlas, a 25-megajoule damental atomic and plasma physics. He is a Fellow of the American Physical Society and has timing of fission and neutron emis- machine that will permit us to study served on numerous government panels and sion in fissioning nuclei. Angular high energy densities over tens of committees. distributions of neutrons, gamma cubic centimeters. The Procyon rays, and fission decay products have high-explosive pulsed-power gener- been measured to determine the en- ator will be followed by a more ad- ergy and momentum balance in fis- vanced system that will deliver in sion. Detectors used in nuclear tests excess of 200 million amperes. have been calibrated on LAMPF to Bright Source III is being designed

1993 Number 21 Los Alamos Science 69