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June 1997

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• Tools to Fight Strokes• Better Measurements of Sediment• The Next Generation of Laser Targets

Also in this issue: • Tools to Fight Strokes• Better Measurements of Sediment• The Next Generation of Laser Targets

The NewScience of High Explosives


About the Review

The High Explosives Application Facility(HEAF) at Lawrence Livermore is the center ofsome of the most advanced research in thenation and, indeed, the world.in state-of-the-artenergetic materials. Since its completion in1989, it has been the home of the developmentand characterization of new high explosives, aprocess based less on trial and error in recentyears and more on rigorous scientific principlesand methods. On this month’s cover, PhilipPagoria, an organic chemist at HEAF,synthesizes a new high-energetic compound in aglovebox. For a report on the Laboratory’scapabilities in researching energetic materials ofsignificance to U.S. government and industry,turn to the article beginning on p. 4.

• •

About the Cover June 1997Lawrence

Livermore

National

Laboratory





Lawrence Livermore National Laboratory is operated by the University of California for theDepartment of Energy. At Livermore, we focus science and technology on assuring our nation’s security.We also apply that expertise to solve other important national problems in energy, bioscience, and theenvironment. Science & Technology Review is published ten times a year to communicate, to a broadaudience, the Laboratory’s scientific and technological accomplishments in fulfilling its primary missions.The publication’s goal is to help readers understand these accomplishments and appreciate their value tothe individual citizen, the nation, and the world.

Please address any correspondence (including name and address changes) to S&TR, Mail Stop L-664,Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, California 94551, or telephone(510) 422-8961. Our electronic mail address is [email protected].

Prepared by LLNL under contractNo. W-7405-Eng-48

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SCIENTIFIC EDITORRavi Upadhye

MANAGING EDITORSam Hunter

PUBLICATION EDITORDean Wheatcraft

WRITERSArnie Heller, Dean Wheatcraft, and Gloria Wilt

ART DIRECTORRay Marazzi

DESIGNERSGeorge Kitrinos, Ray Marazzi,and Kitty Tinsley

INTERNET DESIGNERKitty Tinsley

COMPOSITORLouisa Cardoza

PROOFREADERAl Miguel

S&TR is a Director’s Office publication,produced by the Technical InformationDepartment, under the direction of the Office of Policy, Planning, and SpecialStudies.

2 The Laboratory in the News

3 Commentary by Hal GraboskeThe Critical Roles of Energetic Materials Science

Features4 Transforming Explosive Art into Science

Livermore scientists are imposing a more scientific structure on high-explosives research by combining computer simulation codes, state-of-the-art experimental diagnostics, and a culture combining many specialties.

14 On the Offensive against Brain AttackThe Laboratory’s Center for Healthcare Technologies is leading a multidisciplinary team of Livermore researchers in developing the tools needed to provide early, aggressive diagnosis and treatment of stoke.

Research Highlight22 Laser Targets: The Next Phase

25 Patents and Awards

28 Abstracts

S&TR Staff June 1997

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Available fromNational Technical Information ServiceU.S. Department of Commerce5285 Port Royal RoadSpringfield, Virginia 22161

UCRL-52000-97-6Distribution Category UC-700June 1997

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Science & Technology Review June 1997

Commentary by Hal Graboske 2 The Laboratory in the News


Peña gives go-ahead for subcritical testsSecretary of Energy Federico Peña in March gave the go-

ahead for conducting subcritical experiments. Conducted nearly1,000 feet underground, the experiments will test the basicproperties of small quantities of plutonium driven to highpressures using conventional explosives. No nuclear energywill be generated by the experiments. Data on the properties ofplutonium will improve the accuracy of the computer simulationsthat ultimately will provide a level of confidence in weaponsreliability and safety, previously assured by nuclear testing.

“Subcritical experiments are essential to our commitmentsto a world free of nuclear testing, a reliable nuclear deterrent,and are fully consistent with the Comprehensive Test Ban Treaty(CTBT),” Peña said in a statement.

“In addition, these experiments complement other elementsof DOE’s Stockpile Stewardship and Management Program,such as the National Ignition Facility and the AcceleratedStrategic Computing Initiative, additional tools that will helpsupply the confidence in stockpile safety and reliability thePresident has required in order to support the CTBT,” he said.

The first in a series of subcritical experiments is scheduled by Los Alamos for June. The second, by LLNL, will followsometime later.Contact: LLNL Media Relations (510) 422-4599 ([email protected]).

Scientists simulate earthquake ground-motion patternsScientists at Lawrence Livermore and the University of

California at Berkeley have announced results of a study thatmodels the localized severity of ground motion during a majorearthquake along the Hayward fault. The fault runs beneath adensely populated section of the eastern San Francisco Bay Area.

Using one of the world’s most powerful supercomputers atLawrence Livermore, Laboratory computer scientist ShawnLarsen teamed with Berkeley seismologists Mike Antolik andDoug Dreger to simulate expected ground motions for amagnitude 7.1 earthquake along the Hayward fault beginningsouth of Fremont and rupturing 50 miles northwest to SanPablo Bay.

The researchers developed a computer movie based on thisscenario that shows seismic waves propagating throughout the Bay Area and impacting different regions with varying degrees of severity.

“Ultimately it’s our goal that those responsible for earthquakeretrofit and new structure design can take our findings andcombine them with current approaches to make betterengineering decisions,” said Larsen.

The researchers announced their findings at the SeismologicalSociety of America annual meeting, held in April in Honolulu.Their simulations use a sophisticated computer code developedat Livermore and a seismic velocity model developed at Berkeley.Contact: Shawn Larsen (510) 423-9617 ([email protected]).

Defense research yields commercial benefitsDefense research at Lawrence Livermore may help U.S.

companies get a head start in the fiercely competitive internationalcomputer chip market, according to Dena Belzer, author of anew study about the Laboratory’s effect on the economy.

Belzer, a principal consultant with Bay Area Economics inBerkeley, quoted industry giants Intel Corp. and Microsoft assaying that breakthroughs at Lawrence Livermore have beencritical to putting more information onto tiny microchips. Thecompanies said the Lab’s cutting-edge research tools and widepool of scientists from diverse disciplines enabled microchipbreakthroughs such as EUV lithography, a technique for puttinginformation on chips more precisely with strokes that are abouta thousandth of the width of a human hair.

Belzer’s report said that the planned National Ignition Facilitylaser could push the state of the art in several technology areasover the next 10 to 15 years but cautioned that “economicbenefits can only be realized if the national labs continue to havestrong interactive relationships with private industry.”Contact: LLNL Media Relations (510) 422-4599 ([email protected]).

Lawrence Livermore’s high profile in arms controlIn an address to Lawrence Livermore employees in April,

Ambassador Thomas Graham, Jr., U.S. negotiator and specialrepresentative of the President at the U.S. Arms Control andDisarmament Agency, applauded the Laboratory’s contributionsand its high-profile role in achieving a global treaty to endnuclear testing.

According to Graham, Lawrence Livermore was the only oneof the nation’s three nuclear weapons laboratories representeddirectly on the U.S. negotiating team for the test ban.

The Laboratory’s representative, physicist William Dunlop,“was one of the top 10 people on our negotiating delegation,”Graham said. “He and the Lab . . . made a very importantcontribution to developing negotiating positions and ultimatelyto the negotiations of the [Comprehensive Test Ban Treaty].

“More and more people in recent years have looked to the[national] labs for support in arms control and nonproliferationbecause they have a lot to offer. And a lot of it is world-class andunique,” he said. He went on to name specialized sensors andother technology for monitoring treaty compliance as examples.Contact: LLNL Media Relations (510) 422-4599 ([email protected]).

AWRENCE Livermore’s involvement in energeticmaterials began at its inception in 1952, when the

Laboratory instituted a research and development program inhigh explosives for nuclear warheads under design. Today,Livermore’s high-explosives capabilities, with extensivefacilities at the main site, Site 300, and recently, at the NevadaTest Site, are the equal of any institution’s in the world.

What makes Livermore’s energetic materials workinternationally distinct is its breadth and depth. At one extreme,as described in the article beginning on p. 4, chemists andphysicists are working to increase the safety and performanceof energetic materials at the molecular level. At the otherextreme, Livermore experts are supporting the safedismantlement of nuclear weapons and the disposition oftheir energetic materials.

Stockpile StewardshipThe Department of Energy’s Stockpile Stewardship and

Management Program is the centerpiece of Livermore’snational security mission. Underlying all of our stockpilestewardship activities is a dedication to assuring that nuclearweapons will continue to be as safe and reliable as they aretoday. The relationship of energetic materials to the well-being of the nation’s nuclear stockpile is particularly strong.Activities range from assisting DOE plants in fulfilling theirmanufacturing mission to understanding the aging of highexplosives in the stockpile and predicting their useful lifetime.

Current energetic materials research and development insupport of stockpile stewardship focuses on several areas. Inthe important area of enhanced surveillance, we aredeveloping new, minimally invasive methods of detecting theproducts of high-explosives decomposition as well aspursuing theoretical and experimental work to predict andcharacterize potential decomposition pathways. Ourcontinuing emphasis on safety, performance, and reliabilitydrives an intense effort to improve our ability to model thebehavior of energetic materials under both normal andabnormal conditions. This research necessitates theacquisition of additional equation-of-state data and bettermodeling of the very complex coupling of thermodynamic,chemical-kinetic, and hydrodynamic behavior in a burninghigh explosive.

L We also have a continuing responsibility to assist DOE’sPantex Plant in meeting the needs of the stockpile; we mustalso meet the requirements of our own hydrotesting programs.For example, we are currently doing research focused onimproving the chemical synthesis processes for highexplosives (e.g., a new route to the synthesis of theinsensitive high explosive TATB), and we maintain a viablesynthesis, processing, and assembly area at Site 300.

Department of Defense and Other ActivitiesLivermore’s role in energetic materials has broadened beyond

its primary nuclear weapon-related mission. Today, there areextensive activities in advanced conventional weapons, rocketand gun propellants, antiterrorist work, demilitarization, andindustrial applications of energetic materials. We are activelyassisting the Department of Defense in a wide range of activitiesin this area, particularly those focused on insensitive highexplosives, environmentally sound demilitarization of surplushigh explosives and propellants, and modeling the performanceand safety of high explosives and propellants. Two currentexamples of Department of Defense projects are thedevelopment of molten salt as an environmentally benignmeans of destroying energetic materials and the explorationof new synthesis routes to reduce the cost of high explosives.

In support of Livermore’s nonproliferation program, weare developing the means to reliably detect and identifyhigh-explosive compounds and formulations, with the addedinterest of potentially determining their origin.

Providing the Scientific FoundationThe excellent science carried out at Livermore enables

the outstanding advances in energetic materials research anddevelopment described above and in the following article.This work ranges from understanding detonation science atthe molecular level to predicting structures for exciting newhigh explosives. This continuing excellence in the scienceand technology of energetic materials has made possible thewide range of Livermore contributions since its inceptionand promises significant breakthroughs in the years ahead.

■ Hal Graboske is Associate Director of Chemistry and Materials Science.

The Critical Roles ofEnergetic Materials Science

5


EW products typically take years of effort to synthesize yet disintegrate in a

few millionths of a second when used. Despitetheir brief lifespan, energetic materials,particularly high explosives, are in demand asnever before by the Department of Energy,Department of Defense, and industry for theirunique properties: shock waves producingpressure up to 500,000 times that of Earth’satmosphere, detonation waves traveling at10 kilometers per second, temperatures soaringto 5,500 kelvin, and power approaching20 billion watts per square centimeter.

Explosives have been around since Chinesegunpowders appeared during the 11th century.However, until the past 15 years, theirdevelopment has been characterized by anapproach based largely on intuition and trialand error. Now high-explosives scientists areimposing more rigorous scientific structure andtechniques upon all aspects of their work.

For centuries, intuition and trial anderror dominated the development of

high explosives. Now, high-explosivesresearchers at Lawrence Livermore

are imposing more rigorous scientificstructure and techniques upon

their work.

For centuries, intuition and trial anderror dominated the development of

high explosives. Now, high-explosivesresearchers at Lawrence Livermore

are imposing more rigorous scientificstructure and techniques upon

their work.

F

For example, Lawrence Livermoreresearchers are combiningbreakthrough computer simulationcodes, state-of-the-art experimentaldiagnostics, and a culture in whichtheoretical, synthesis, and experimentalchemists and physicists work alongsideeach other. At the same time, they areworking more closely with theirpartners in the energetic materialscommunity, from DOE’s Pantex Plantin Texas to the Air Force’s WrightLaboratory at Eglin Air Force Base,Florida, to small explosives companiesin the San Francisco Bay Area.

Advances in energetic materials,which include high explosives,propellants, and pyrotechnics, benefitDOE’s Office of Defense Programs,DoD’s warheads and propulsion efforts(especially the 12-year-old DOE/DoD“Memorandum of Understanding onConventional Munitions”), NASA’sspace exploration programs, the FederalAviation Agency’s explosive detectionefforts, and many industries, includingmining, oil exploration, and automobile.The continuing demand is driving asearch for better theoretical models ofthe behavior of energetic materials andan improved diagnostic capability tomeasure the complex chemical andhydrodynamic processes duringdetonation.

According to Ron Atkins, director ofthe Energetic Materials Center (EMC),

a joint effort of Lawrence Livermoreand Sandia National Laboratories, U.S.industry has scaled back its energeticmaterials research because of safetyand financial considerations. Likewise,the Department of Defense’s ownenergetic materials research facessignificant budget pressures, whileacademia does not have the costlyfacilities to carry out such research. Asa result, says Atkins, “the national labsare becoming the country’s mostimportant repository of energeticmaterials expertise.” Atkins is headinga task force representing severalLivermore directorates in work toensure that the Laboratory will remaina national resource for energeticmaterials expertise over the nextdecade and beyond.

Livermore researchers have studiedand synthesized high explosives fordecades because they are an integralelement of every nuclear weapon.Today, under the EMC umbrella, theirwork encompasses a wide range ofbasic research and programmaticactivities. Lawrence Livermorechemists are synthesizing newcompounds that yield more energy, aresafer to store and handle, and are lessexpensive and more environmentallyfriendly to produce. They also aredesigning new paths to synthesizingexisting energetic molecules that arecheaper and easier on the environment.

Understanding Is Key GoalLivermore scientists are conducting

experiments to better understand thefundamental physics and chemistry ofenergetic materials, particularly withregard to their stability, sensitivity, andperformance. “Despite a century ofwork, scientists still do not understandwhat happens in a detonation wavethoroughly enough to predict all thedetails of how a given explosive willbehave under various conditions,” saysRandy Simpson, head of the EnergeticMaterials Section in the Chemistry andMaterials Science Directorate.

Simpson and his colleagues are alsoinvolved in fundamental surveillanceactivities associated with themaintenance of the nation’s nuclearweapons stockpile. Performance andsafety testing (see Science &Technology Review, December 1996,pp. 12–17) assures that the highexplosives in nuclear warheads willremain dependable despite decades ofstorage. Another aspect of stockpilestewardship work is developing safeand environmentally sound methods fordismantling and disposing of thousandsof kilograms of high explosivesremoved from retired nuclear weapons.Going a step further, Livermorechemists are investigating processesthat would permit the reuse of thesehigh-quality, expensive materials in thecommercial marketplace.


Energetic Materials

Transforming ExplosiveArt into Science

Transforming ExplosiveArt into Science

http://www.llnl.gov/str/12.96.html

7


Energetic Materials

have been few despite steady progress inexplosive power and insensitivity overthe past century. The last energeticmaterial to “hit it big” was HMX (cyclo-tetramethylene-tetranitramine),discovered during World War II as acontaminant in a batch of anotherexplosive material. Since then, Simpsonsays, there have been TATB (triamino-trinitrobenzene, a highly insensitive highexplosive for nuclear weapons) duringthe 1970s and a few specialty materials,but certainly nothing used as widely asTNT (trinitrotoluene) (Table 2).

The reason for the paucity of newenergetic materials is the fact that theymust meet so many differentrequirements such as high energy density,insensitivity to mechanical insults,resistance to chemical decomposition,inexpensive synthesis from readilyavailable reagents, and the ability to beformulated with other materials forfabrication into practical devices.

Despite the difficult requirements,Livermore chemists are optimistic thatthey can improve the safety andperformance of current and futureweapons systems. It is a balancing actbecause the compounds must bepowerful enough to do the job and at the

same time insensitive enough to preventaccidental explosion. For someapplications, the priority is on improvingsafety, especially with nuclear weaponsand with explosives stored on ships.

For other applications, higher powerand energy are of greatest interest.(Energy is the capacity of an explosive todo work, whereas power is the rate ofenergy release, or how rapidly theexplosive can accelerate metal. Energy ismeasured in joules, power in joules persecond.) In this area, several newLivermore explosives have beendeveloped for Air Force weaponsdirected at penetrating “hard targets”such as underground reinforced concretebunkers. In the same performance arena,smaller shaped charges using Livermoreformulations are demonstrating velocitiesup to 10 kilometers per second to penetratethick steel armor plate some 6 to 8 timesthe diameter of the shaped charge.

Developing new energetic materialsis a complicated process in which manycandidate molecules are considered, afew synthesized, even fewer formulated,and only a small handful adopted by themilitary or industry. The laborious processinvolves computer modeling, plenty oflaboratory work, and thorough testing.

Starting at the ChalkboardThe road to a new high explosive

begins the old-fashioned way, whencandidate molecules are drawn on achalkboard by both theoretical andsynthesis chemists. Theoreticalchemists tend to suggest more“flamboyant” molecules than thesynthesis chemists because they haveless experience in the laboratory, quipstheoretical chemist Larry Fried. Once agroup of candidates is agreed upon,Fried and his colleagues take over,screening the molecules with a host ofcomputer codes.

The codes help guide the synthesischemists by predicting the inherentcharacteristics of the cyber-compounds.Fried says the process is similar to thatfound in the pharmaceutical industry.In that business, too, trial and error andhuman hunches used to be predominant,but now sophisticated computers arehelping to point the way to prime-candidate molecules for synthesis.

Livermore high-end workstationsdo simulations with the speed thatapproaches a supercomputer’s. Thesoftware program GAUSSIAN (usedwidely in the chemical andpharmaceutical industries) is first

6


Energetic Materials

Guiding all of these activities arecomputer codes that mimic energeticmaterials and the very rapid physicaland chemical processes that governtheir detonation (Table 1). The codesreflect longstanding Livermoreexpertise in simulating extremely short-lived events such as nucleardetonations. Continually refined byexperimental data, the codes are pavingthe way for an unprecedentedunderstanding of energetic materials atthe molecular level.

The work is headquartered in theHigh Explosives Applications Facility(HEAF) at Livermore, which representsthe state of the art in high-explosivesresearch with regard to both technicalcapability and safety (Figure 1). Workat HEAF is complemented by activitiessome 15 miles away at Site 300, wherelarge-scale high-explosives processingand testing are carried out.

Searching for New MaterialsSimpson notes that in a world

accustomed to daily announcements ofimportant scientific advances,breakthrough high-energetic materials

Table 1. Codes used in developing energetic materials.

Code Function

ALE3D Hydrodynamic code used in safety analyses such as “cookoff” simulations spanning aremarkably wide time span. (Developed at LLNL.)

CHEETAH Transforms predicted formation energy and density of molecules into performancemeasures such as detonation velocity, pressure, energy, impulse, and impetus.(Developed at LLNL.)

GAUSSIAN Determines the three-dimensional shape of the molecule and the energy binding itsatoms.

MOLPAK Packs molecules together into a low-energy configuration.

TOPAZCHEM, PALM Predict changes in thermal and chemical properties caused by different accident,battlefield, and aging scenarios. (Developed at LLNL.)

Figure 1. (above) Livermore’s High ExplosivesApplications Facility (HEAF), completed in 1989,is playing a major role in developing andcharacterizing high explosives. (right) Speciallydesigned containment vessels are used to safelydetonate high explosives in quantities as largeas 10 kilograms of TNT-equivalent.

Table 2. Molecular structure of important energetic materials.

Material Molecular Structures

TNT (trinitrotoluene)

HMX (cyclo-tetramethylene-tetranitramine),

TATB (triamino-trinitrobenzene)

LX-19(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, which is CL-20, plus a polymer binder)

LLM-105 (2,6-diamino-3,5-dinitropyrazine-1-oxide)

N N

N N

N N NO2NO2

NO2NO2

NO2NO2

O2N

H2N NH2

NO2

NO2

NH2

O2N NO2

NO2

CH3

N

NN

N

NO2

NO2

NO2

NO2

N

N+ NH2

NO2

N2H

O2N

O–

CL-20 in LX-19TATB

TNT

LLM-105 HMX

9


Energetic Materials

HMX-based materials.* LX-19 is basedon CL-20 (developed at the NavalWeapons Center, China Lake,California). Working with the Navy,Livermore experts determined many ofthe characteristics of CL-20 andperformed the first scale-up to kilogramquantities at the Laboratory’s Site 300test area.

A similar effort is aimed atsynthesizing materials with more energythan TNT, the best known high explosivein the world and one that offers lesspower (but better sensitivity) than HMX.For this effort, Livermore has synthesizedLLM-105, an insensitive energeticmaterial with 60% more energy thanTNT. The new material is underevaluation by Ron Lee and his colleaguesin Livermore’s Defense and NuclearTechnologies Directorate.

In the process of developing newcompounds and more efficient pathwaysfor synthesizing existing compounds,the synthesis group has developed aninnovative and cost-effective approachcalled the VNS (vicarious nucleophilicsubstitution) method for producingTATB. The procedure eliminates theneed for chlorinated compounds, whichhave adverse environmental effects.(See the November 1996 Science &Technology Review, pp. 21–23.)Livermore and DOE’s Pantex Plantrecently began a four-year effort to applythe VNS method in order to establish alower-cost industrial supply of TATB.

Once a few grams of a material havebeen synthesized, they are passed on toexperimental chemists for a battery ofsafety tests (Figures 4 and 5). The testsdetermine the material’s sensitivity to

8


Energetic Materials

impossible. It is an iterative process,depending largely on the knowledge,abilities, and intuition of the chemist.Many times, a synthesis scheme cannotbe considered for full-scale productionbecause it ultimately requires too manysteps or reagents that are too costly.”

Much of the synthesis effort isdevoted to developing new energeticmaterials that possess an energy density(the energy that can be released from aspecified volume of material) at least15% greater than that of HMX, the high-energy high explosive against whichcandidate materials have long beenevaluated. HMX replacements areneeded for a host of volume-fixedarmaments such as so-called smart, orprecision-guided, munitions.

Many have been developed atLivermore. One formulation, LX-19, isthe highest power material in the worldbut somewhat more sensitive than

Figure 2. Livermore’s computer code CHEETAH, running on high-end workstations, transformsthe predicted crystal energy and density of molecules into explosive performance measures.Here the explosive molecule HMX is transformed to its detonation products.

Accelerated Strategic ComputingInitiative (ASCI) at the Laboratory arebeing used to assist with modeling thehydrodynamics of candidate explosives,and plans call for ASCI’s use in creatingadvanced predictive models of thechemical reactions that occur whencandidate molecules explode.

Assuming the software programsvalidate the chemists’ premise that thecandidate molecule offers significantpotential, the material is ready to besynthesized.

Synthesis Can Be ToughWhile it takes about one week to

screen a candidate molecule by computer,its actual synthesis in the laboratorycan require a year or even longer ofpainstaking effort.

“It takes a lot of trial and error to getthe synthesis reactions to go,” saysorganic chemist Phil Pagoria (Figure 3).“The chemist must constantly evaluatewhether the project is progressing orwhether the molecule, as planned, is

employed to determine the three-dimensional shape of the molecule andthe energy binding its atoms. Themolecules are then “packed together”into a low-energy configuration forgreatest stability by another widelyused program called MOLPAK.

Finally, CHEETAH transforms themolecules’ predicted thermodynamicenergy and density into explosiveperformance measures such as detonationvelocity, pressure, and energy (Figure 2).CHEETAH, developed by Fried andhis colleagues, is a thermochemistrycode derived from more than 40 yearsof experiments on high explosives atLivermore. With libraries of hundreds ofreactants and 6,000 products in its code,the program is now used throughoutthe world and has become DOD’spreferred code for designing newexplosives and, to a lesser extent,propellants and pyrotechnics (see Science& Technology Review, June 1996, pp. 6–13). The capabilities of themassively parallel computers in DOE’s

Figure 3. Organic chemistPhil Pagoria synthesizesa new high-energeticcompound inside aglovebox to guard againstunwanted moisture.

Figure 4. ScientificAssociate Chet Leemeasures the burn rateof a high explosive underhigh pressure, astandard safety test.

Figure 5. Chemist RosalindSwansiger remotely controlsa performance test of apromising high explosive.

* Experimental molecules are designated byan LLM number for Lawrence Livermoremolecule. Experimental formulations aredesignated by an RX number for researchexplosive. Once the material is inproduction, it acquires an LX designation forLivermore explosive. DoD experimentalmunitions receive an XM number.

CHEETAH

HMX

Water

Carbon dioxide

Nitrogen

http://www.llnl.gov/str/06.96.htmlhttp://www.llnl.gov/str/11.96.html

stability, and mechanical and physicalproperties assist designers in evaluating aformulation and determining appropriateuse in specific devices. Chemicalreactivity tests, for example, identifyincompatibilities between devicecomponents and a formulation. Becausea major objective in formulation isincorporation of the formulatedexplosive into a device, any possibleincompatibility between devicecomponents and the formulation must becorrected early.

Atkins notes that obtaining accuratedata from experiments at the extremetemperature, pressure, and time regimesof high explosives presents enormouschallenges. Many of the tests use

diagnostic tools originally developedfor underground nuclear weapons testsat the Nevada Test Site. Others weredeveloped more recently. One such toolis the multibeam Fabry–Perotvelocimeter, designed by Livermorescientists (July 1996 Science &Technology Review, pp. 12–19). Thisdevice provides high-resolution,continuous velocity data about thebehavior of materials traveling up to3,000 meters per second. With themultibeam system now producing moremeaningful data about the power ofexplosives—the rate at which they arecapable of releasing energy—modelingcodes become increasingly accurate.The device also allows more efficient

use of budgeted funds because oneexperiment provides many sets ofvelocity data, thus taking the place offive separate experiments.

Computer simulations have alsostrengthened formulation activity andtesting. CHEETAH is once again calledinto play, this time to suggest how thevarious formulation ingredients will affectperformance. In addition, TOPAZCHEM-2D/3D, PALM, and more recently, theALE3D code (see box, below) augmentsafety testing by predicting changes inthermal and chemical properties causedby different accident, battlefield, andaging scenarios.

Encouraging results from experimentsand computer simulations lead to still

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Energetic Materials10


Energetic Materials

as 2% and as much as 40% of the volume,can serve several purposes: it can makethe explosive easier to fabricate intouseful shapes, aid in desensitization toshock, or modify the high explosive’sperformance characteristics.

Formulations chemist Mark Hoffmanacknowledges the role of artistry inarriving at a sound formulation but notesthat Livermore people can tap 45 years’worth of experience with high explosives.Much of the artistry is spent juggling thetradeoffs among sensitivity, performance,and cost. As a formulation increasesinsensitivity to explosion (for safetyconsiderations, for example), performancetypically suffers. Hoffman notes: “Itdoes no good to have a weapon on board

impact, heat, friction, electrostaticdischarge, and shock. Most candidatematerials fail at this point. Those thatpass are sent on to other chemists forincorporation in a mixture of ingredientscalled a formulation. Simpsonacknowledges that the process is “stilllargely an art” but adds that it is becoming“more precisely scientific all the time.”

A World of Tradeoffs Formulating high explosives for

unique applications may require a medleyof ingredients, including energeticcrystalline powders, energetic liquids,inorganic oxides, metals, and binderssuch as thermoplastics, thermosets, andgels. The binder, which takes up as little

a tank that does not possess enoughpower to destroy or incapacitate anopposing tank. But it’s inappropriate tocarry a weapon that’s so sensitive thatit explodes in response to a few bumpsin the road.”

Formulators work closely with otherchemists, who can quickly obtain safetyand performance measurements usingdifferent quantities of a formulation.With as little as 1 to 2 grams, chemistscan only perform critical safety tests.With 50-gram quantities, they canevaluate how well the ingredients of aformulation come together to form thenew explosive. As formulations arescaled up to kilogram quantities,important tests of performance, thermal

The very destructive power of high explosives places a premiumon all aspects of their safety, including manufacture, transportation,storage, and handling. Likewise, much of Lawrence Livermore’shigh-explosives work involves determining the sensitivity ofexisting high explosives and rocket propellants to fire, accident, andterrorist attack.

Safety has also come under the purview of computer codes. “Wewould like to do predictions of safety at the start of thedevelopment process, much as we determine other characteristics ofcandidate molecules,” says theoretical chemist Larry Fried, who isexploring using the widespread computer code GAUSSIAN todetermine how much energy it takes to break a molecular bond asan indicator of sensitivity to accidental detonation. He is alsoexploring the conversion of intermolecular phonons (quanta ofvibration energy) to intramolecular vibrational states as part of acomputational model that could eliminate inherently unstablemolecules from consideration before they are synthesized.

Fellow theoretical chemist Al Nichols has been working withcomputational scientists from the Defense and NuclearTechnologies Directorate to transform ALE3D, a three-dimensionalhydrodynamic, explosive-safety code developed at Livermore (seefigure on p. 11). With the ALE3D team, Nichols has added thermaland chemical capabilities to the code so it can answer safetyquestions about high explosives, in particular a stringent militarythermal safety test called “cookoff.” Thanks to ALE3D, Livermoreis the first research center to simulate cookoff by depicting aremarkably wide time span. The code models deformations in aheated explosive device from the time they begin at the rate of

millimeters per day to the instant of explosion when deformationrates increase to kilometers per second.

Safety efforts include working with the Air Force on its missilepropellants. One study, a part of the Titan IV program, is looking atthe safety ramifications of solid propellant falling from an errantrocket launch, as happened earlier this year when an Air ForceDelta rocket blew up at Cape Canaveral, Florida, raining propellantdown on the ground below. Another study concerns the propellantsof the Air Force Minuteman III missile.

In performing the safety studies, says experimental chemicalengineer Jon Maienschein, Livermore chemists are doing businessdifferently by modeling every experiment before it is conducted. Inthat respect, says Energetic Materials Section leader RandySimpson, Livermore scientists do a smaller number of experimentsthan are done at other sites, but they thoroughly instrument eachone and precede major experiments with computer simulations.

Maienschein notes that Livermore personnel are working moreclosely with colleagues and sponsors in DoD. “Both they and werecognize that we can do more by teaming up with each other.” Theprocess, he says, encourages creative thinking about, for example, anew generation of transducer-based systems that continuouslymonitor important safety data such as temperature in high explosives.

Energetic Materials Center Director Ron Atkins notes that in aworld of diminished federal outlays, collaboration is clearly theway to achieve important advances with the greatest cost-efficiency. “We’re working hard to build bridges to the armedservices, DOE centers like the Pantex Plant in Texas, and othernational labs,” he says.

Spotlight on Safety

The ALE3D computer code is capable ofsimulating a “cookoff” safety test bymodeling the rate of deformations in aslowly heated high explosive over awide time span. (a) A model of the testat setup. The high explosive is encasedin steel and aluminum and boltedbetween two metal end caps. Heaterssurround the metal container and heatthe 7.6-centimeter-tall device at the rateof 3.3°C per hour. (b), (c), and (d) aresnapshots of the simulation of thematerial’s deformation as a function of(respectively) temperature, pressure,and chemical change after 50 hours ofheating. ALE3D simulations such as thistell energetic-materials scientists ingreat detail and in slow motion how,when, and with what violence new high-explosive compounds deform whenburned. In (b), (c), and (d), the velocityof deformation is 80 meters per second.

(a) (b)

(c) (d)

Metalend caps

Metalcontainer

Heaters

Explosive

http://www.llnl.gov/str/07.96.html

metals for construction. Now buildershave a host of different materials fromwhich to choose. “We’re leaving the IronAge of energetic materials becausemilitary planners are no longer limited toTNT and HMX,” he says. “We’re seeingspecific new materials for specific militaryapplications.”

The driving force is the ascendancy ofsmart munitions. Because these weaponsroutinely hit their targets, smallimprovements in the lethality of thewarheads can significantly increase theireffectiveness. What’s more, fewer andsmaller munitions mean that moreexpensive energetic materials may be used.

As part of this new effort, Livermorechemists are working with the Navy toadapt LX-19 and similar CL-20formulations to the military’s XM-80program. Multiple small submunitions,each containing about 10 grams ofexplosives, will be grouped in shells andshot out of Navy guns. Capable oftraveling long distances, the shells, whichhave a propulsion system guided byglobal positioning satellites, willaccurately destroy enemy fortifications.

Simpson is confident that computercodes will continue to become moresophisticated so that a code such asALE3D will be used as a design tool tomodel safety elements of energetic devicesas diverse as rockets or automobile airbags. It is a safe bet that with other aspectsof high explosives, as well, Livermoreresearchers will play a large part in thenew age of high explosives.

—Arnie Heller

Key Words: ALE3D, CHEETAH, Fabry–Perot velocimeter, GAUSSIAN, highexplosives, High Explosives ApplicationsFacility (HEAF), HMX (cyclo-tetramethylene-tetranitramine), MOLPAK, PALM, stockpilestewardship, TATB (triamino-trinitrobenzene),TNT (trinitrotoluene), TOPAZCHEM.

13


Energetic Materials

larger-scale formulations of 400 gramsor greater done at Site 300. When thematerial properties are optimized, theformulation process is developed forscale-up to production quantities forfinal technology transfer.

Livermore chemists are alsoworking to improve efficiencies in theproduction world. They are exploringthe use of injection molding equipment

much like that used to make plastic toyparts. Such machines could be ideal formaking shaped charges, which typicallycontain a number of complex folds that aredifficult to fashion using standardproduction machinery (Figure 6).

Leaving the Iron AgeSimpson describes the Iron Age as a

time when builders were limited to a few

12


Energetic Materials

For further information contact Randall Simpson (510) 423-0379([email protected]).

The research, development, and testing of new energetic materials done at theLaboratory’s High Explosives Applications Facility is, like all science done atLivermore, a multidisciplinary team effort. In this instance, the team membersoperate under the auspices of the Energetic Materials Center (EMC), sponsoredjointly by Lawrence Livermore and Sandia national laboratories. Chief contributorsto the new science of high explosives being done at Livermore are (left to right):ALBERT NICHOLS, a theoretical chemist currently working to model safetyaspects of high explosives used in nuclear and defense applications; RANDALLSIMPSON, an experimental chemist who develops new energetic materials andcharacterizes their initiation and detonation properties; RONALD ATKINS, directorof the EMC and coordinator of the team’s work; RONALD LEE, a physicist whodevelops new explosive initiation systems; JON MAIENSCHEIN, an experimentalchemical engineer involved in computer simulations of the safety of energeticmaterials before their testing; MARK HOFFMAN, a formulations chemistresponsible for formulating high explosives for unique applications within strictsafety, performance, and compatibility guidelines; LAWRENCE FRIED, atheoretical chemist who screens candidate high-explosives molecules usingadvanced computer codes; and PHILIP PAGORIA, an organic chemist, who isexpert in synthesizing new high-energetic compounds.

About the Scientists

(d)(c)

(a) (b)

Figure 6. At Site 300 facilities, injection-moldable explosives are developed as part of an effort to enhance production methods.(a) Mark Hoffman formulates a moldable high explosive. (b) Hoffman and Kirk Pederson pour the explosive to a transfer funnel,from which it is poured into a deaerator–loader. (c) Frank Garcia operates the deaerator–loader to remove air from the explosivebefore loading it into the explosive device. (d) Mike Kumpf displays the finished precision explosive device.

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Stroke Initiative

smaller. Recognizing that LawrenceLivermore has capabilities inmicrofabrication and other technologiesthat could be used to reduce the size ofmedical devices, the Center forHealthcare Technologies established aprogram to create a new standard ofstroke care.

Critical to defining this standard wasthe “Workshop on New Technologyfor the Treatment of Stroke,” which theCenter sponsored in March 1995. Theworkshop was attended by internationallyrecognized stroke clinicians andresearchers, cardiologists with experiencein medical devices used to treat heartattack, and scientists and engineers fromLawrence Livermore and Los Alamosnational laboratories. Instead of thetypical conference agenda of successstories, clinicians described significantareas of unmet need for diagnosing andtreating stroke victims, particularly thewant of medical devices that mightsatisfy those needs.

Out of the workshop grew a vision ofthe future of stroke care and a frameworkfor the priorities of a multidisciplinaryteam of Laboratory researchers who,with the help of Laboratory DirectedResearch and Development funding,are developing much-needed tools todiagnose and treat stroke. The Lawrence

Livermore stroke initiative team’svision of the future of stroke care issummarized in Figure 1. It focuses onthe greatest unmet clinical needs—restoring blood flow, preventinghemorrhage, improving treatmentdecisions with sensors, and identifyingthe at-risk population with newscreening technologies. (For a primeron the kinds, causes, and treatment ofstroke, see the box on p. 19.)

The Livermore team consists ofspecialists in biomedical engineering,biology and bioscience, laser medicineand surgery, micro-engineering,microsensors, and computer simulation.It also has key collaborators fromacademic medical centers and privatecompanies. These partnerships are thebasis for rapidly moving the medicaldevice concepts from the researchlaboratory through development,clinical trials, regulatory approval, andmanufacture so that the resulting newtools can have a timely impact on thelives of the thousands of people whohave strokes each year.

Since the workshop, the stroke-initiative team’s research has developedseveral proof-of-principle prototypes.The work falls into four categories:microsensors for brain and clotcharacterization, optical therapies for

breaking up clots in the blood vessels ofthe brain, laser–tissue interactionmodeling, and microtools for treatinganeurysms (a leading cause ofhemorrhagic stroke).

Sensors to Diagnose ClotsThe Laboratory’s stroke initiative has

made substantial progress in developingmicrosensors that improve understandingof the biochemistry of stroke as well asoffer the potential to improve strokediagnosis, monitoring, and treatment.The sensors have the potential to identifythe types of clots that cause stroke, tomonitor patients during therapies thatdissolve clots or protect brain cells withdrugs, and to determine the health ofbrain or blood vessel tissue at a strokesite prior to treatment.

Development and use of thesesensors, like much of the team’s work,are predicated on the availability ofmicrocatheters. These tiny, hollowtubes, which are available from anumber of manufacturers, can containoptical fibers to which microsensors andother diagnostic, treatment, andmonitoring tools being developed at theLaboratory are attached (Figure 2).Inserted in the femoral artery, themicrocatheters are guided by microwiresthrough the circulatory system to the clot

14


N the fall of 1994, a group in Lawrence Livermore National

Laboratory’s Center for HealthcareTechnologies began asking a pointedquestion whose answer was toprofoundly affect the focus of a majorpart of the Center’s research: Given thatboth heart attack and stroke result fromdisruption of blood flow, why arecardiovascular conditions treated withaggressive medical intervention whilecerebrovascular conditions usuallyreceive passive intervention withemphasis on rehabilitation? Why isstroke not treated as “brain attack”? Theanswer, they found, was not that there issomething fundamentally different aboutthe two potentially deadly maladies.Instead, what they found was that doctorsfrequently did not have the proper toolsto treat stroke as quickly andaggressively as they treat heart attack.

Heart attacks and strokes usuallyresult from decreased blood flowinterrupting the supply of oxygen andnutrients to tissue. Most frequently, theflow is decreased because of ablockage, but flow can also bedisrupted by malformations or ruptureof the vessels. An important differencebetween a heart attack and a stroke isthat the size of the blood vesselsinvolved in a stroke are significantly

Under the leadership of the Laboratory’s Center forHealthcare Technologies, a multidisciplinary teamis developing a variety of much-needed tools toprovide stroke victims with early, aggressivediagnosis and treatment.

I

On the Offensiveagainst BrainAttack

On the Offensiveagainst BrainAttack

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Stroke Initiative

to dissolve blood clots in the vessels ofthe brain. D dimer is a substance withantigenic properties (i.e., capable ofstimulating an immune response) andis produced as a result of a complexbiochemical process when clot-dissolvingdrugs are injected via a microcatheterinto blood clots.

Livermore’s D-dimer biosensor canact as a diagnostic tool by indicatingwhether the blockage is caused by plaqueor by a clot. Clot-dissolving drugs willnot dissolve plaque; therefore, if anelevated concentration of D dimer is notdetected at the site of blockage, then theblockage may be caused by somethingother than a clot, and alternative therapyis needed. In addition, because treatmentusing clot-dissolving drugs is highlyvariable, the D-dimer biosensor couldhelp eliminate the guesswork related tothe dosage and infusion rates. It couldhelp physicians develop a diagnosis andtreatment plan faster and reduce the riskof hemorrhage resulting from treatmentto dissolve clots.

Medical PhotonicsMembers of the stroke-initiative team

are developing a catheter-based systemthat uses laser energy to break up clots.The system will deliver low-energy laserpulses through a fiber-optic microcatheter(Figure 3). The laser energy will bedirected at a cerebral clot, and byconversion of optical light to acousticstress waves, it will break up the clot andrestore blood flow in cerebral arteries.The concept is simple. The challenge isto determine the proper pulse strengthneeded to break up the clot withoutharming viable vessel tissue.

Research has focused on the opticaland mechanical material strength andfailure properties of the clots and tissuefound in the cerebral blood vessels.2Using a tunable optic parametric oscillator(OPO) laser system at Livermore’sMedical Photonics Laboratory (Figure 4),the medical-lasers team has conductedin vitro experiments to send laser pulses

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Stroke Initiative

site in the brain where the tools attachedto them can do their work to combatbrain attack.

Lawrence Livermore researchers have,for example, demonstrated in vitro fiber-optic and electrochemical sensors formeasuring pH at stroke sites.1 Thesesensors can establish brain tissue viabilityby direct measurement of pH in braintissue or through indirect measurementin blood near the stroke site. Livermorescientists have developed miniatureintracranial (direct brain tissue)electrochemical and fiber-optic pHsensors, which neurosurgeoncollaborators at the State University ofNew York at Buffalo have used for invivo animal testing.

The measurement of blood pH is oneof a number of chemical “markers” thathave been identified to assess the healthof blood-vessel tissue at the site of astroke, thereby providing guidance instroke therapy. When tissue dies, lacticacid builds up and blood pH decreases.So if blood pH is below normal (7.4) ator near the stroke site, then brain celldeath has occurred, and the use ofneuroprotectant drugs to minimize braindamage is unwarranted. If, on the otherhand, pH is close to normal, cell deathhas not occurred, and neuroprotectantdrugs become a therapeutic option.

In the Laboratory’s fiber-optic pHsensor, a pH-sensitive dye,seminaphthorhodamine-1 carboxylate(SNARF-1C), is mixed with transparentsilica sol-gel and dip-coated onto anoptical fiber tip. In laboratory tests, the tipis placed in blood, and the dye is excitedby a tungsten–halogen light source or alow energy density laser. The emissionspectra of the dye is pH sensitive. Thesetests showed that the sensor had goodsensitivity in the pH range of 6.8 to 8.0,indicating possible use for in vivo sensingof blood pH in the neighborhood of astroke site.

Livermore scientists are alsodeveloping a D-dimer biosensor tomonitor stroke patients during therapies

(a) Current response to stroke

Stroke symptomsWeakness or numbness in extremeties on one side; sudden blurred or lost vision in one eye; sudden, severeheadache; etc. (See p. 19.)

Ambulance to hospital

Hospital diagnosis (hours to days after symptoms)• Administration of neuroprotectants• Brain imaging and scanning• Determination of stroke type (ischemia vs hemorrhage)• Neurological examination of effects

Hospital treatment• Surgery to remove plaque or relieve hemorrhage pressure• Drugs to prevent clots andto promote infusion of blood into affected brain area in ischemic stroke• Coagulants to reduce hemorrhage effects

Hospital/outpatient rehabilitation

Recovery/chronic care

(b) Livermore stroke initiative's vision of brain attack response

In-ambulance care• Determine stroke type• Early administration of neuroprotectants• Early administration of thrombolytics, as appropriate

Hospital diagnosis (less than a few hours after symptoms)• Microsensor-assisted determination of stroke type and treatment options• Brain imaging and scanning• Neurological examination of effects

Hospital treatment• Therapies based on stroke type• Ischemia— laser clot busting— nerve-growth stimulants— anticoagulants— neuroprotectants— thrombolytics• Hemorrhage— sensor-assisted therapy— microtools to treat aneurysms— coagulants— pressure-release surgery

Figure 1. (a) Current response to stroke is more passive than active and lacks urgency largelybecause of the want of tools to diagnose and treat stroke early. The more time that passes after astroke, the less the chance of even partial recovery from its effects. (b) The overriding goal of theLaboratory’s stroke initiative is to improve those chances through early intervention. The Centerfor Healthcare Technologies’ vision for the future of stroke care concentrates on providing tools forearly, aggressive medical intervention to improve a stroke victim’s chances for full recovery ormore productive rehabilitation.

Figure 2. Optical fibers about the diameter of human hair can be enclosed ina hollow microcatheter and steered by microwires through the circulatorysystem to stroke sites in the brain. Microsensors and microtools are attachedto these fibers to provide rapid, improved stroke diagnosis and treatment.

Figure 3. The technique used to break cerebralclots consists of converting laser energy toacoustic stress waves. The challengeis to break up the clot withoutdamaging the bloodvessel.

Infusioncatheter

Laser energy

Fiberoptic

Clot

Acousticstresswaves

Microcatheter

Optical fiber

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Stroke Initiative


the tip, and the initial stress wave, whichcan generate high peak pressure within400 nanoseconds (billionths of a second),quickly propagates away from the tip. Inthe moderate case, energy is deposited inan extended zone beyond the fiber tip(Figure 5b). The initial stress evolves fromthe heated region within 500 micrometers(millionths of a meter) of the end of thetip, and the largest stress gradients,which develop within 20 nanoseconds,are directed radially and are situated inthe immediate vicinity of the tip.

into a blood phantom (water colored withred food coloring). They have identifiedtwo distinct regimes of dynamicresponse, one due to strong laser lightabsorption, the other to moderateabsorption.

In both cases, the confined stressesimparted to the liquid are substantialand determine most of the importantdynamics. In the strong absorption case,energy is deposited in a thin zone nearthe fiber tip (Figure 5a). A thermallygenerated vapor bubble develops around

Eventually (within 100 nanoseconds), acloud of tiny bubbles develops inresponse to the stress caused by laserheating. In both cases, this expansionand collapse of bubbles exert pressureand shear forces on a clot, which leadultimately to its breakup.

The Laboratory recently entered intoa Cooperative Research andDevelopment Agreement (CRADA)with EndoVasix Inc. of Belmont,California, which will eventuallymarket the laser “clot-busting”

Stroke (or “brain attack”) results from vascular diseaseaffecting the arteries supplying blood to the brain and occurs whenone of these vessels bursts or is clogged. Part of the brain isdeprived of the oxygen and nutrients it needs to function, the nervecells die within minutes, and the part of the body controlled bythese cells cannot function. Sometimes the devastating effects ofstroke are permanent because the dead brain cells are not replaced.

Stroke is the leading cause of permanent disability in the U.S. and the third leading cause of death. Each year, 550,000Americans have strokes. One-third of them die. Many of thesurvivors, who currently total over 3 million, have decreasedvocational function (71%); of these 16% remain institutionalized,and 31% need assisted care. The personal cost is incalculable; theannual cost for treatment, post-stroke care, rehabilitation, and lostincome to victims (but not their family caregivers) is $30 billion.

Types of StrokeThere are two main types of strokes, ischemic and

hemorrhagic. Clots—cerebral thrombuses or cerebral embolisms—cause ischemic strokes. Cerebral hemorrhage or subarachnoidhemorrhage causes hemorrhagic strokes. Ischemic strokes are themost common, hemorrhagic strokes the most deadly.

Cerebral thrombosis occurs when a blood clot (a thrombus)forms in an artery in or leading to the brain, blocking the bloodflow. It is the most common cause of ischemic stroke. Cerebralembolism occurs when a wandering clot (an embolus) or someother particle occurs in a blood vessel away from the brain,usually the heart. The clot is carried by the bloodstream until itlodges in an artery leading to or in the brain.

A cerebral hemorrhage occurs when an artery in the brainbursts, flooding the surrounding tissue with blood. Bleeding froman artery in the brain can be caused by a head injury or a burstaneurysm, a blood-filled pouch that balloons out from a weak spotin the artery wall. A subarachnoid hemorrhage occurs when ablood vessel on the surface of the brain ruptures and bleeds into thespace between the brain and the skull (but not into the brain itself).

Hemorrhagic strokes cause loss of brain function both fromloss of blood supply and from pressure of accumulated blood onsurrounding brain tissue. The amount of bleeding determines theseverity. If hemorrhagic stroke victims survive (which they do in50% of the cases), their prognosis is better than that of ischemicstroke victims. With ischemic stroke, part of the brain dies anddoes not regenerate. With hemorrhagic stroke, pressure from theblood compresses part of the brain, but the pressure diminishesgradually and the brain may return to its former state.

About 10% of all strokes are preceded by “little strokes”called transient ischemic attacks (TIAs). They are more useful forpredicting if, rather than when, a stroke will happen. They occurwhen a blood clot temporarily clogs an artery and part of thebrain does not get the blood it needs. The symptoms, which arethe same as stroke symptoms, occur rapidly and last a relativelyshort time, usually between 1 and 5 minutes. TIAs can last up to,but not more than, 24 hours. Unlike stroke, when a TIA is over,people return to normal, because the nerve cells were notdeprived of oxygen long enough to die.

Diagnosis and TreatmentDiagnosing that a stroke has occurred and its type and

severity takes time—time that stroke victims may not have.Diagnostic tools are tests that image the brain, such ascomputerized axial tomographic (CAT) scans, magneticresonance imaging (MRI) scanning, and radionuclideangiography or nuclear brain scan. Tests that show the electricalactivity of the brain are also used. The two basic tests of thistype, an electro-encephalogram (EEG) and the evoked responsetest, measure how the brain handles different sensory stimulisuch as flashes of light, bursts of sound, or electrical stimulationof nerves in an arm or leg.

Tests that show blood flow to and in the brain are also usedfor diagnosis. One of these is the Doppler ultrasound test, whichcan detect blockages in the carotid artery. Another is carotidphono-angiography, wherein a stethoscope or sensitivemicrophone is put on the neck over the carotid artery to detectabnormal sounds (bruits) that may indicate a partially blockedartery. Yet another is digital subtraction angiography, in whichdye is injected into a vein in the arm and an x-ray machinequickly takes a series of pictures of the head and neck. Fromthese x rays, doctors can determine the location of anyblockages, how severe they are, and what can be done about them.

Surgery to remove plaque from artery walls, drugs thatprevent clots from forming or getting bigger, acute hospital care,and rehabilitation are all accepted ways to treat stroke.Sometimes treating a stroke means treating the heart, becausevarious forms of heart disease can contribute to the risk ofstroke, particularly those caused by clots that form in a damagedheart and travel to the brain. But compared to the diagnosis andtreatment tools that have been developed for heart attack, thosefor brain attack seem extremely limited and have not advancedgreatly in recent years.

* Heart and Stroke Facts (The American Heart Association, Dallas, Texas, 1994), pp. 21–27. This booklet is available from the American HeartAssociation’s National Center, 7272 Greenville Avenue, Dallas, Texas 75231-4596 (telephone: 1-800-242-8721).

Charge-coupleddetector

Beamsplitter

Beamsplitter

Probe laser beam

Mirror

Mirror

Quartztube containingblood phantom

Quartz microfiber

1 to 5 millijoulesof laser energy

Compensationtube

Figure 4. Livermore scientists use atunable optic parametric oscillator (OPO)laser to create a series of laser back-lighted images (see Figure 5 below) of thepressure distribution of laser energy withina blood-like fluid.

18 Stroke Initiative

Figure 5. In laboratory experiments todevelop a laser system to break up clots,Livermore researchers have identified tworegimes of dynamic response. (a) In thestrong absorption case, heat from the laserenergy generates a vapor bubble about thefiber tip. (b) In the moderate case, initialstress evolves from the heated region verynear the fiber tip. Within 100 nanoseconds, acloud of tiny bubbles develops in response tothe stress caused by laser heating. Theexpansion and collapse of these bubblesexert pressure and shear forces on the clot,ultimately leading to its breakup. Fiber-optic tip

Peak pressureabout 250 bar after

20 nanoseconds

Peak pressureabout 400 bar after440 nanoseconds

(a) Strong absorption case (b) Moderate absorption case

Brain Attack Facts*

New Vision of Stroke CureThe work of the stroke initiative at

Lawrence Livermore hopes to remedythe paucity of tools for diagnosing andtreating strokes. Its vision of stroke careincludes medical devices for screeningpeople without symptoms for strokerisk. It places special emphasis on thedevelopment of tools to provide earlierrather than later diagnosis of stroke typeand assessment of brain cell damage sothat appropriate treatment can beinitiated rapidly. It has guidedLivermore researchers in thedevelopment of technology to break upstroke-causing clots with laser energy aswell as microsensors and microtools toassist in the diagnosis and treatment ofvarious kinds of brain attack. And itlooks forward to providing the meansfor more instances of full recovery,fewer stroke-related disabilities, andless need for chronic care.

—Dean Wheatcraft

Key Words: brain attack, laser “clot-busting,” laser–tissue interaction modeling,LATIS code, medical photonics,microsensors, neuroprotectant drugs, shape-memory microgripper, stroke.

References1. Sheila A. Grant and Robert S. Glass,

Sol-Gel-Based Fiber-Optic Sensor forBlood pH Measurement, LawrenceLivermore National Laboratory,Livermore, California, UCRL-JC-125297 (January 1997). (Submitted toAnalytica Chemica Acta.)

2. P. Celliers et al., “Dynamics of Laser-Induced Transients Produced byNanosecond Duration Pulses”Proceedings of the Society of Photo-Optical Instrumentation Engineers,2671A (1996).

3. R. A. London et al., Laser–TissueInteraction Modeling with LATIS,Lawrence Livermore NationalLaboratory, Livermore, California,

UCRL-JC-125974 (February 1997). (To be published in Applied Optics.)

4. M. Strauss et al., “ComputationalModeling of Laser Thrombolysis forStroke Treatment,” Proceedings of theSociety of Photo-Optical InstrumentationEngineers, 2671 (1996).

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Stroke Initiative

into account a raft of variables—size andcomposition of the clot, strength ofblood-vessel tissue, and buildup andtransport of heat during laser clotbusting—this code can numericallysimulate the hydrodynamics of the laser-created energy needed to break up clotsand predict the amount of energy neededto do so without damaging other tissue.

The modeling team has madesignificant progress in simulating thelaser clot-busting process with a focuson short-pulse (1- to 10-nanosecond)interactions of laser light with water andblood clots.4 These advances in LATISare being used to improve the laser clot-busting technology discussed earlier.They will be refined and expanded tobetter determine the parameters of thehydrodynamics at the heart of a safe,effective laser clot-busting system.

technology. EndoVasix is investing inthe development of a prototype systemfor clinical demonstrations beginningwith animal stroke models. Some of thepreliminary animal tests have alreadytaken place.

Laser–Tissue InteractionCentral to the design of clot-busting

tools is the refinement and use ofcomputer codes for modelinglaser–tissue interaction. Based on thelaser–matter interaction codes developedfor inertial confinement fusion atLivermore, the Laboratory’s LATIS(laser–tissue) code provides a basis forpredicting how short-pulse, low-energymedical lasers affect tissue.3 It thuspromote the rational design of clot-busting devices by modeling laser–tissueinteraction during the process. By taking

The medical applications have alsohad positive “spin-back” to the coreprograms at the Laboratory. Forinstance, because the medicalapplications required simulation ofhow laser beams interact with highlyscattering materials, a bug in one of theMonte Carlo x-ray transport subroutinesused in national security applicationswas discovered and corrected.

MicrotoolsMiniaturization expertise from

Lawrence Livermore’s MicrotechnologyCenter, Precision Engineering Group,and Plastic Shop has produced a varietyof silicon, metal, and plastic microsensorsand actuators for the stroke initiative.One of these with the potential to preventstrokes caused by hemorrhage ratherthan clots or other blockages is the “shapememory” microgripper (Figure 6).These tiny devices (less than a cubicmillimeter) have a variety of applications,but the initial one is for treatinganeurysms. Very fine metal thread isplaced into the microgripper, which isconnected to a guidewire cable andmaneuvered to the site of the aneurysm.Closed, it slips into the aneurysmthrough the narrow neck connecting theaneurysm with the vessel wall. Onceinside, the microgripper’s heater isactivated by power sent through theguidewire tether, and the gripper opens,releasing the metal thread into theaneurysm. The gripper then cools,“remembers” its closed shape, and canbe withdrawn through the neck, leavingbehind the metal thread. The threadembolizes (acts as a clot in) theaneurysm, reducing blood flow andpressure in the aneurysm. Without thepressure, the aneurysm eventually fillswith scar tissue and is significantlyless likely to rupture and cause ahemorrhagic stroke.

Figure 6. Microtools such as this silicon microgripper with “shape memory” will be used to treatthe cerebral aneurysms that lead to hemorrhagic stroke. The microgripper is less than 1 cubicmillimeter, that is, about the size of the head of a straight pin. (Approximate size: )

The stroke-initiative team at the Laboratory is a multidisciplinary group of scientistsand engineers from several directorates—Biology and Biotechnology, Engineering, LaserPrograms, Physics and Space Technology, Defense and Nuclear Technologies, andChemistry and Materials Science. The team’s members have combined their expertisein biomedical engineering, biology and bioscience, laser medicine and surgery,micro-engineering, microsensors, and computer simulation to create a variety of toolsto respond quickly and urgently to brain attack and thereby improve the chances of astroke victim’s survival and recovery. They are collaborating with academic medicalcenters and private companies to move these proof-of-principle prototypes as quicklyas possible from the research stage to development, clinical trials, regulatory approval,and manufacture so that they can benefit as soon as possible the lives of people whohave strokes. Pictured left to right are: ABRAHAM LEE, ROBERT GLASS,WILLIAM BENETT, LUIZ DA SILVA, PATRICK FITCH, RICHARD LONDON,SHEILA GRANT, and STEVEN VISURI. (Not shown are PETER CELLIERS,PETER KRULEVITCH, and DENNIS MATTHEWS.)

For further information contact J. Patrick Fitch at the LawrenceLivermore Center for HealthcareTechnologies (510) 422-3276([email protected]).

About the Team

T will take a community of workers to bring the goals of theNational Ignition Facility (NIF) to fruition. While national

attention has been focused on the funding and construction ofthe 192-beam laser facility, scientists at Lawrence Livermoreare working on myriad problems whose solutions arenecessary to NIF’s success. A group of materials scientists,for example, is developing techniques to produce round,hollow shells about 2 millimeters in diameter—smaller thanBB-gun pellets. This work seems incongruous in a projectdominated by a football-stadium-size facility. But when filledwith deuterium or deuterium–tritium fuel, these shells becomethe targets for NIF’s inertial confinement fusion (ICF)experiments. The goal of these experiments is to create fusionignition—intense temperatures and pressures like those at thecenters of stars for a small fraction of a second.

Steve Letts and Evelyn Fearon of the Laser ProgramsDirectorate’s Target Area Technology Program are among thematerials scientists continuing Lawrence Livermore’s morethan 20 years of research and development on laser targets.Their focus now is on targets for NIF experiments. With40 times more energy and 10 times more power than Nova(currently the world’s largest operating laser), NIF willrequire targets about 2 millimeters in diameter, 4 times largerthan those used previously, which are about half a millimeterin diameter.

The increased shell size must be achieved in tandem withmaking the shell very smooth and symmetrical. During anICF experiment, extremely high laser energies are absorbedby the fuel capsule, causing the capsule wall to blow off withsuch tremendous force that the fuel inside is compressed tovery high density. This compression, which must be asuniform as possible, is necessary for ignition. Any capsulesurface or shape irregularities constitute perturbations thatwill grow in amplitude during implosion, because ofhydrodynamic (Rayleigh–Taylor) instabilities (see Energy &Technology Review, April 1995, pp. 1–9). The perturbationscause the inner wall of the capsule to mix with the fuel,cooling it and thereby degrading efficiency.

The Progression of ICF TargetsLetts and Fearon’s technique for making shells uses an

entirely new approach. Previously, plastic shells wereproduced when droplets of polystyrene solution were droppeddown a heated drop tower, where evaporation first caused askin to form on the droplets and then further vaporization ofthe solvent inside the skin caused the droplets to expand intohollow shells.

Because the drop-tower technique produced shells of alimited size range, researchers tried micro-encapsulationtechniques to increase shell sizes. They encapsulated dropletsof water in a polymer solution suspended in an aqueousphase; the solvent containing the polymer would slowlydissipate into the aqueous phase, leaving behind a polymershell. However, the resulting shells were uneven in thicknessand had bubbles in their walls. Steve Letts explains that thesetechniques frequently “wouldn’t produce round shells most ofthe time, so those that were round would have to be carefullypicked out—not an easy task with such tiny things.”

He came up with a new idea. While measuring mass loss inpolymers when they were heated, he identified one polymermaterial that evolved into a gas when heated to about 300°C,disappearing cleanly without any trace or residue. He figuredout a way to take advantage of the material’s uniquecombination of characteristics.

That material was poly(alpha-methylstyrene), or PAMS. InLetts’s new fabrication method, an amount of PAMS isshaped into a smooth sphere, or mandrel, which is overcoatedwith a thermally stable plasma polymer to a desired thickness.The overcoated mandrel is heated to about 300°C, at whichtemperature the PAMS depolymerizes (decomposes) into agas, diffuses through the plasma polymer overcoat (which isthermally stable up to 400°C), and leaves behind a hollowplasma polymer shell (see the figure on p. 23).

Letts postulated that this method would be feasible forproducing fuel capsules of the size needed for NIF if a suitablePAMS mandrel could be formed. In addition, because the shellis built outward from the PAMS mandrel, it might be feasible

I

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Laser Targets

PAMS (96,000 versus 11,000 for beads) can be used to makethem, because, unlike the beads, they do not need hot-watersoftening, which requires the lower molecular weight material.Because they are ultimately depolymerized, some wallunevenness and internal bubbles are tolerable, as long as theshells are spherical and their outer surface finishes are smooth.Compared with bead mandrels, the hollow mandrels haveshown far less distortion during overcoating and pyrolysis.

An Even Coat of Plasma PolymerPlasma polymer is well suited to be fuel–shell material. It

is transparent, which allows fusion experimenters to diagnosethe contained fuel layer. It can be used to coat the mandrelsbecause it can withstand PAMS pyrolysis temperatures andis permeable to the gaseous, depolymerizing PAMS.

In the coating technique used by the Target Area group,the mandrels are agitated in a bouncing pan in the plasmacoating chamber or, in a variation, rolled in a tilted, slowlyrotating pan until they are evenly coated (see the figure onp. 22). The crucial variables determining even coating arejust the right amount of agitation and the correct (not-too-high) temperature.

Successful PyrolysisDuring pyrolysis, the shells can collapse, burst, deform,

or shrink. While collapse is mainly caused by nonuniformcoating, the other problems result from thermal effects. Toavoid them, the researchers devised a temperature programthat controls the rate of PAMS decomposition. It consists ofraising the pyrolysis temperature by 10°C every minute until200°C is reached, holding it there for 30 minutes to allow low-

22


Research Highlight

Laser Targets The Next Phase to incorporate various layers during the overcoating process,

which would be useful for diagnosing shell performance. Themethod would be successful if good quality mandrels could bemade, an even overcoat could be deposited on the mandrel, andpyrolysis (heat treatment) could be accomplished withoutdistorting or collapsing the resulting shell.

Spherical, Smooth MandrelsEvelyn Fearon coordinated PAMS mandrel production. She

and the other fabricators ground commercial PAMS beads intosmaller sizes, put them through a sieve, and suspended themin a water solution hot enough to soften them, thus takingadvantage of surface tension to pull the bead into a sphere.Bead surfaces were smoothed further by exposing them tosolvent vapor while dropping them down a heated column.During the drop, the bead’s thin surface layer dissolved anddried, leaving a surface roughness of less than a billionth of ameter (as measured by an atomic-force microscope).

Smooth, spherical bead mandrels were fairly easy to make.However, they tended to distort from the heat generatedduring overcoating and become nonsymmetrical or coatunevenly. To overcome the heat effects, Fearon experimentedwith higher molecular weight PAMS and lowered theovercoating temperature, but the adjustments did not whollyovercome the distortion problem.

The Target Area group turned to hollow mandrels made bymicro-encapsulation and supplied by General Atomics of SanDiego, California, another DOE contractor. Hollow mandrelshave two advantages. They contain less PAMS todepolymerize, and thus, less force is exerted on the overcoatduring depolymerization. Second, higher molecular weight

Two-millimeter, poly(alpha-methylstyrene) (PAMS)bead mandrels are rolled in a small, tilted, slowlyrotating pan until evenly coated with plasma polymer.Heat treatment decomposes the PAMS and it diffusesthrough the plasma polymer coating, leaving behindsmooth, spherical, thin-walled hollow shells that are thetargets for laser fusion experiments.

Plasmapolymercoating

Heattreatmentto 300°C

Micro-encapsulatedPAMS substrate made

to appropriate size

Hollow plasma polymershell about 2 millimeters

in diameter

Livermore scientists have developeda technique for producing hollowlaser-target shells by starting with ahollow poly(alpha-methylstyrene)(PAMS) mandrel and thenovercoating it with thermally morestable plasma polymer. The coatedmandrel is heated to 300°C over30 hours or more; the PAMSdecomposes and passes throughcoating, leaving a spherical, hollowplasma polymer target shell.

http://www.llnl.gov/etr/04.95.html

25


temperature volatiles to escape, and then ramping it up by0.2°C every minute up to 300°C, where it is held for 30 hoursor more, depending on the size of the shell. The plasma polymershrinks gradually and uniformly during pyrolysis, and thussphericity is maintained. Experimenters observe and measurethe shrinkage only to predict the size of a completed shell.

An optical microscope is used to measure the wallthickness and diameter of pyrolyzed shells, a scanningelectron microscope is used to determine how smooth andfree of particle defects shell surfaces are, and an atomic-forcemicroscope is used to make detailed measurements of thesphericity and roughness of the shell.

Challenges AheadThe techniques described here have now been adopted

by General Atomics as the preferred method for making0.5-millimeter-diameter capsule targets for Nova ICFexperiments at Livermore and 0.9-millimeter-diametercapsules for ICF experiments at the Omega Laser facility atthe University of Rochester. The success in moving thisresearch proof of principle to actual target production iscertainly encouraging. However, significant challenges stillface Livermore’s laser-target scientists.

Currently the development efforts at Lawrence Livermoreare focused on adapting the technology developed by Lettsand Fearon to the production of 2-millimeter-diameter

capsules for NIF. This effort has two parts. The first, beingled by Ken Hamilton, is to develop micro-encapsulationtechniques to form PAMS microshells with the required outersurface sphericity and surface finish. To meet NIF specifications,these shells must be no more than 1 micrometer, or a millionthof a meter, out of round; that is, the radius to the outer surfacecan vary by no more than 1 micrometer (out of 1,000) as onemoves across the surface. Solving this extremely difficultproblem will require significant improvements in currentmicro-encapsulation technology. Once it is solved, thesecond part will be to maintain the sphericity of the shellthrough the coating and thermal treatment to remove the PAMS.

Members of the Laboratory’s Target Area TechnologyProgram will continue to refine laser target technology. Beyondmaking targets for current ICF experiments, they must focuson developing targets for the real NIF event—ignition. ThePAMS technique is being investigated for that use.

—Gloria Wilt

Key Words: laser target, National Ignition Facility (NIF), inertialconfinement fusion (ICF), fuel capsule, plasma polymer, polymershell, micro-encapsulation, poly(alpha-methylstyrene) (PAMS),hydrodynamic instability.

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For further information contact Steve Letts (510) 422-0937 ([email protected]) orEvelyn Fearon (510) 423-1817 ([email protected]).

Laser TargetsEach month in this space we report on the patents issued to and/orthe awards received by Laboratory employees. Our goal is toshowcase the distinguished scientific and technical achievements ofour employees as well as to indicate the scale and scope of thework done at the Laboratory.

Patents and Awards

Patents

Patent issued to

John S. Toeppen

Rex Booth

Thomas E. McEwan

Kurt H. WeinerThomas W. Sigmon

Daniel W. ShimerArnold C. Lange

Patent title, number, and date of issue

Method for Optical and Mechanically CouplingOptical Fibers

U.S. Patent 5,560,760October 1, 1996

Charge Line Quad Pulser


Precision Digital Pulse Phase Generator


Process for Forming Retrograde Profiles in Silicon


E-Beam High Voltage Switching Power Supply


Summary of disclosure

An inexpensive technique to splice optical fibers that does not causedeformation of the host fibers, does not require repeated thermal cycling ofthe optical fibers, does not cause thermal and photonic degradation of thefibers even at high power applications, does not cause the fibers toprematurely deteriorate with age, and is suitable for use with optical fibershaving a core diameter of as much as 1,000 micrometers or greater. Asolder–glass frit having a melting point lower than the melting point of theoptical fibers is used to splice the two optical fibers together.

A quartet of parallel coupled planar triodes that is removably mounted in aquadrahedron-shaped PCB structure. Releasable brackets and flexiblemeans attached to each triode socket make triode cathode and grid contactwith respective conductive coatings on the PCB and with a detachablecylindrical conductive element enclosing and contacting the triodeanodes.The configuration permits quick and easy replacement of faultytriodes. By such orientation, the quad pulser can convert a relatively low andbroad pulse into a very high and narrow pulse. A maximum impedancemismatch within a quartet planar triode circuit of less than 10% is maintained.

A timing generator comprising a crystal oscillator connected to provide anoutput reference pulse. A resistor–capacitor combination is connected toprovide a variable-delay output pulse from an input connected to the crystaloscillator. A phase monitor is connected to provide duty-cycle representationof the reference and variable-delay output pulse phase. An operationalamplifier drives a control voltage to the resistor–capacitor combinationaccording to currents integrated from the phase monitor and injected intosumming junctions. A digital-to-analog converter injects a control current intothe summing junctions according to an input digital control code.

A process for the formation of retrograde profiles in silicon, either previouslydoped crystalline or polycrystalline silicon, or for introducing dopant intoamorphous silicon so as to produce the retrograde profiles. This processinvolves the formation of higher dopant concentrations in the bulk than at thesurface of the silicon. By this process, n- and p-well regions in CMOS(complementary metal oxide silicon) transistors can be formed by a simple,flexible, and inexpensive manner. This technique has particular application inthe manufacture of silicon integrated circuits where retrograde profiles aredesired for the n- and p-well regions of CMOS transistor technology and forburied collectors in bipolar transistors.

A circuit device for generating a ground-level voltage feedback signal forcontrolling the output voltage of one of a plurality of dc–dc converter moduleshaving their outputs connected in series to form a supply output lead. Eachmodule includes a switching device for producing a pulsating voltage ofcontrolled duty cycle, an inductor mechanism for converting the pulsatingvoltage into a smooth direct current, and an inverter mechanism for producingfrom the direct current an alternating current through the primary of atransformer. The transformer has at least one secondary winding inductivelycoupled to the primary winding for producing an output voltage of the module.

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Patent issued to Patent title, number, and date of issue Summary of disclosurePatent issued to Patent title, number, and date of issue Summary of disclosure

George P. RobersonMichael F. Skeate

Gary W. Johnson

Kurt H. Weiner

Alexander R. MitchellPhilip F. PagoriaRobert D. Schmidt

Howard NathelJohn H. KinneyLinda L. Otis

Image Matrix Processor for FastMultidimensional Computations


Apparatus for Controlling the Scan Width of aScanning Laser Beam


Method for Shallow Junction Formation


Vicari

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