AST March, at the behest of theDepartment of Energy, a group

of scientists from around the worldconvened at the University ofCalifornia, Berkeley, to discusspotential scientific applications of theNational Ignition Facility (NIF). TheNIF is a 192-beam, neodymium glasslaser that the Department of Energywill use to obtain information on high-energy-density matter, which isimportant for the generation of energythrough inertial confinement fusion.This information will also be used tomaintain the skills and informationbase necessary to manage the nation’snuclear stockpile, and mostimportantly from a scientificperspective, to pursue basic andapplied research.

The objective of the gathering inBerkeley was to identify those areas

anything from a terrestrial lightningbolt (approximately 104 K) to the coreof a carbon-burning star (109 K).

In short, this versatile and powerfulresearch tool would enable scientists inthese fields to explore previouslyinaccessible regions of the physicalparameter space that could validatecurrent theories and experimentalobservations and provide a foundationfor new knowledge. Following are theareas where the NIF is expected tomake notable contributions to scienceand applied science.

Astrophysics

To obtain information about starsand other astronomical bodies, theastrophysicist produces a sampleplasma in the laboratory and studiesits physical properties. For example,

of research in which the NIF and otherhigh-energy lasers could be used toadvance knowledge in the physicalsciences and to define a tentativeprogram of high-energy laserexperiments. The scientists determinedthat the NIF as well as other high-energy lasers have effectiveapplication in areas relating toastrophysics, hydrodynamics, materialproperties, plasma physics, andradiation physics. Their determinationwas based on the wide range ofexperiments already being performedon high-energy lasers, the diverseinterests of the scientific community,and the extraordinary range of physicalconditions that would be achievablewith the NIF—densities from onemillionth the density of air to ten timesthe density of the solar core andtemperatures that would be relevant to

Science on the NIF

The National Ignition Facility will allow scientists toexplore a previously inaccessible region of physical

phenomena that could validate their current theoriesand experimental observations and provide a

foundation for new knowledge of the physical world.

43

L

to determine a star’s structurethroughout the various stages of itslifetime—that is, its mass, heat,luminosity, and pulsationalinstabilities—the astrophysicist mayrequire information on the radiativeopacity of a plasma that mimics the outer stellar envelope and/orinformation on the equations of state(how density and temperature relate tothe pressure or internal energy) of aplasma that resemble the dwarf starinterior. Furthermore, to get a betteridea of a star’s structure duringvarious stages of evolution, theastrophysicist may be interested inproducing a stellar-like plasma toinvestigate its nuclear reaction rates.The key to success in all of theseexperiments is the ability tosynthesize the very hot plasmas thatcharacterize the stellar environmentduring stages of stellar evolution. The

astrophysical community is interestedin developing this potential withhigh-energy lasers, especially withthe NIF.

Equation of StateUnder many circumstances, the

equation of state of a stellar interioris simple: most of the gas ishydrogen and other light elementsthat have lost a good portion of theirelectrons. Unfortunately, theequation of state of the star’s interioris not as simple when the star is inits later stages of evolution. Densityis quite high, and the materialbecomes strongly coupled; that is,the ions interact strongly and nolonger behave as free particles. Thisbehavior is often accompanied byelectron degeneracy. This leads tothe tendency of the electrons to fillup certain energy states in a way that

forces some of the electrons to bevery energetic, thereby affecting thepressure and internal energy.

The theory of stellar evolution isaffected by uncertainties in theequation of state in a few areas. Forexample, in white dwarfs—the“nuclear ashes,” or compressedcores—of stars that have shed theirhydrogen-containing outer layers andgone through most of their evolution,the pressure from degenerate electronssupports the material against gravity.Near the surface of the material,however, degenerate electrons losetheir dominance. The ions then takeover, setting the specific heat andestablishing the rate at which thewhite dwarf will cool, a process thattakes many millions of years.

Radiative OpacityThe radiative opacity of the

material in stellar interiors plays akey role in determining how starsevolve: what the maximum mass of astable star is, how hot and howluminous the star is while it burns itshydrogen fuel, what pulsationalinstabilities may occur. Previous torecent experiments, astrophysicistswere using a set of opacitycalculations that predicted a verynarrow range of surface temperaturesfor the hydrogen-burning phase ofstellar evolution for all stars—in otherwords, all stars were very hot at thistime despite their differences in mass.These calculations also tied pulsationinstability to stellar luminosity andmass, which resulted in the wrongpulsation periods. The solution thenwas to correct the opacities used inthe calculations.

In the last few years, a group ofphysicists at LLNL has been able toreduce the discrepancy by using anew set of opacity calculations.Although these are definitely closerto observation (Figure 1) than the

Science on the NIF E&TR December 1994

44

0.74

4 Mfl5 Mfl

6 Mfl

6 Mfl

5 Mfl

4 Mfl

7 Mfl

0.72

0.70

1.0 3.0 5.0 7.0P0

P1/

P0

Figure 1. The effects of opacity on pulsations of Cepheid stars. The stellar mass (Mfl) ispredicted from the ratio of the first harmonic, P1, to the fundamental, P0. The black lines arethe results based on the new opacity calculations; the red lines are the results based on theold opacity calculations; the dots are the observed ratios. The new calculations, whichpredict a much wider range of surface temperature for the hydrogen-burning phase ofstellar evolution, put observation and theory in agreement. Experiments on the NIF willallow us to verify these effects and confirm our theoretical predictions.

previous calculations, they stillembody many approximations. Thus,to verify opacity at the relevantconditions, astrophysicists will needto conduct direct experiments. Ahigh-energy facility like the NIF willallow them to do this.

Thermonuclear Reaction RatesAlthough astrophysicists have

been studying nuclear reactions fordecades, their experiments haverarely achieved the energies at whichsuch reactions occur in stellarenvironments. With the NIF, theywill be able to conduct experimentsthat achieve such energies.Figure 2 shows the temperature anddensity regimes attainable with theNIF and compares them to theconditions of a star as it progressesthrough each phase of evolutionarynuclear burning. The first regime,which extends up to about 14 keV,shows the temperatures and densitiesthat may be reached in a laser-heatedhohlraum or an imploding capsulewithout nuclear ignition and includesa star’s hydrogen- and helium-burning phases. The second regime,which is between 9 and 60 keV,shows the conditions that might existin a deuterium–tritium capsule afterignition and includes the temperaturesand densities achieved up to a star’scarbon-burning phase.

The nuclear cross sections dependon energy (temperature), and we arecurrently limited by conventionalaccelerator methods’ inability toprobe the relevant energy regimes ofinterest (see Figure 2). The NIF willallow us to measure the nuclearreaction rates at precisely the energiesrelevant to stellar interiors.

In a thermalized NIF capsule,where a temperature of 8 keV couldbe attained, the number of radioactive13N nuclei, which would have a half-life of approximately 10 minutes,

could be counted after the event, orscintillators could be positionedaround the target to detect their pulseof 2-MeV gamma rays during theevent. Because the events would beproduced all at once in this type ofexperiment, the usual low signal-to-noise ratio would be avoided, makingit easy to distinguish the reactionsfrom ambient room background noise.

NIF experiments may further ourunderstanding of nuclear reactionsthat explore the proton–proton chainof hydrogen-burning reactions insolar-type stars and also the carbon-nitrogen-oxygen cycle. As examples,three reactions of interest inastrophysics include the 12C(p,Ì)13Nreaction, the 3He(3He,2p)4Hereaction, and the 3He(4He,Ì)7Bereaction. The first plays an importantrole in every star’s carbon-nitrogen-

oxygen cycle; the latter two form part of the proton–proton chain ofhydrogen-burning reactions in solar-like stars.

Hydrodynamics

Hydrodynamics is the study offluid motion and the fluid’sinteraction with its boundaries. TheNIF will allow us to further ourunderstanding of the hydrodynamicsof inertial confinement fusion andshock wave phenomena in the galaxy.

Because the NIF will be capable ofdepositing a large quantity of energyin a large amount of material over along time and at high densities, it willbe able to generate hydrodynamicflow conditions that are much moreextreme than those generated by wind tunnels, shock tubes, or even

E&TR December 1994 Science on the NIF

45

10–1 10 101 102 103

Temperature, keV

1030

1020

1010

Den

sity

, cm

– 3

NIF laser only

Hydrogen-burning

Carbon-burning

Neon-oxygen-silicon-burning

Helium-burning

BurningNIF capsule

Figure 2. Temperature and density regimes attainable on the NIF overlap with theconditions of a star as it progresses through evolutionary nuclear burning. The first regime,which overlaps with a star’s hydrogen- and helium-burning phases, could be reached in alaser-heated hohlraum or an imploding capsule without nuclear ignition. The secondregime, which overlaps with the conditions achieved up to a star’s carbon-burning phase,might exist in a deuterium–tritium capsule after ignition. Experiments conducted in theseregimes could greatly enhance our knowledge of stellar evolution.

high-energy lasers such as Nova.Thus, scientists will be able toinvestigate a number of flow problemsunder previously unattainableconditions. NIF experiments designedto study these problems—for example,the growth of perturbations at a fluidinterface (unstable flow) andshock–shock boundary interactions(stable flow)—will lead to newunderstandings in fluid dynamics.

Imposed Perturbations To study the growth of an imposed

perturbation under continuousacceleration, we shock planar foils of fluorosilicone by x-ray ablation.The foil trajectory is recorded by aradiographic streak camera so that we can check the bulk movement of the sample. The image’s contrast inoptical depth is then measured as a function of time. From thismeasurement, we deduce the evolutionof the imposed perturbation.

Figure 3 shows two images of a foilfrom a perturbation experimentconducted on the Nova laser. Theimage on the left shows that foil hasbecome increasingly bright; this is the result of thinning caused by the“bubble and spike” shapecharacteristic of the nonlinear regime.The image on the right, takenapproximately 2.6 µs after the start of the drive for a duration of 100 ps,shows no transverse distortion of the foil.

We obtain quantitative data bytaking intensity traces at different timestransverse to the grooved structure, asshown in Figure 4. Note that thecurves, which represent different timesin the growth of the perturbation, areoffset for ease of comparison. At earlytimes, the growth is small and stillsinusoidal, indicating that theinstability is in the linear regime. Late

Science on the NIF E&TR December 1994

46

(a)

Pos

ition

Position Position

(b)

Tim

e

In, e

xpos

ure

50-µm increments

Burn-through

Spike

1.5

1.1

0.7

0.3

–0.1

t, ns

1.9

2.7

2.3

Non

-line

arLi

near

Bubble

Figure 3. One- and two-dimensional views of the backlight absorbed by a foil with initialperturbation. The one-dimensional view (a) shows that the end of the bright spot occurswhen the backlight ceases to radiate effectively. The face-on, two-dimensional view (b)shows little if any distortion of the foil.

Figure 4. Traces foran accelerated foilwith an imposedperturbation. Thecurves are offsetvertically to allowsimple comparison.The dashed linesindicate the backlightintensity, whichvaries across the foilas a function of time.

in time, the growth is larger anddistinctly nonsinusoidal, exhibiting the characteristic bubble-and-spikeshape. The rapid flattening of themodulations in the top two curvesresults from the burn-through thatoccurs when the bubbles break out ofthe back side of the foil. At this point,the spikes are still being ablated away;however, they can no longer bereplenished by matter flowing downfrom the bubbles.

These experiments extend from thesingle-mode example described hereto multiple-mode experiments and toburied interfaces with imposed modestructures. The limiting case of arandom set of perturbations at aburied interface requires a somewhatdifferent technique.

Impact CrateringMany phenomena in impact

cratering occur on temporal andspatial scales that are very large whencompared to those of the impactingobject; as a result, we can model the

impact as a point source of highenergy and momentum density. This modeling is usually done bydepositing focused laser energy insmall spheres of high-Z (high-atomic-number) material or by generatingprodigious shocks and post-shockpressures with flyer foils.

A study of concentrated impactson surfaces shows that scaling lawsapply to craters formed by impact andsurface energy deposition. A proof-of-principle experiment designed toexplore the effects of impact crateringon simulated soil (Figure 5) indicatesthat further research in this areawould be of great interest. The abilityof the laser to deposit large amountsof energy in a spot volume withoutresidual gases—the by-product of thesame experiment performed with highexplosive—indicates its utility as asimulation source. On the NIF, theamount of energy deposited and thein situ diagnostic potential wouldmake investigation of hydrodynamicresponse possible in real time.

Material Properties

For the last several decades,scientists have been studying thephysical properties of materials—e.g., their equations of state,opacity, and radiative transport—byconducting experiments with gasguns, high explosives, and high-pressure mechanical devices.Although these experiments haveprovided a wealth of precise data ona wide range of materials, they arelimited because they do not provideinformation on material behavior atthe extreme pressures andtemperatures of scientific interest,i.e., pressures from 1 to 100 terapascals and temperatures up to a few hundred electron volts.Although a few laser-driven andshock-wave experiments have beencarried out in this range of interest,the resulting data are quiteimprecise and do not validate anyof the theoretical models of materialbehavior.

E&TR December 1994 Science on the NIF

47

Gold hollowsphere

Soil sample

14°Laser

(a) (b)

Figure 5. To explore the hydrodynamic response of a simulated soil to impact cratering, we deposited 4 kJ of laser energy into the 1.5-mmradius cavity of a 16 x 16 x 16-cm aluminum-plate cube filled with grout (a). The energy density provided by 4 kJ of laser energy in 1 ns wasenough to vaporize a hollow gold target at the bottom of the 6-cm deep cavity and the surrounding grout. Less than 200 J of laser radiationescaped the target, as indicated by the top surface of the cube (b). Although this surface is crazed and slightly bowed, the cavity and theentry cone are clearly visible. There is also a profusion of radial cracks and a faint but definite indication of tangential (spherical) cracks.This diagnostic is an example of what we can learn from scaled experiments.

With the NIF, scientists will beable to investigate material behaviorin this range and obtain the primarydata needed to test their theoreticalmodels. The experimental methodsused to obtain these data includecolliding foil experiments (for

equation-of-state data) and high-resolution x-ray measurements (forradiative opacity data).

Indirectly Driven Colliding FoilExperiments

To reach a regime of very highpressure without sacrificing laser spotsize and one-dimensionality, weemploy a variation of the well-knownflyer-plate technique. In thistechnique, a flyer—in this case a foil—stores kinetic energy from the driverduring an acceleration and rapidlydelivers it as thermal energy when itcollides with another foil. The flyerfoil also acts as a preheat shield sothat the target remains on a loweradiabat, that is, for a given pressurethe temperature is kept lower than itwould be if exposed to the driver.

In this type of experiment (Figure 6), the laser beams are focusedinto a millimeter-scale cylindrical goldhohlraum; the radiation that escapesfrom a hole in the hohlraum becomesthe x-ray drive. The hohlraum x raysablate a 50-µm layer of polystyrenewith a 3-mm-thick gold flyer foil. This foil then accelerates through avoid and, near the end of the laserpulse, collides with a stationary, two-step (two-thickness) gold target foil.The shock on the rear side of the targetfoil is then imaged with an opticalstreak camera.

Figure 6b is a typical streakcamera image of shock breakout on atwo-step target foil. The time intervalbetween the two breakout times (onefor each thickness) measures theshock speed in the target. An intervalof 57 ps between breakout on the twosteps corresponds to an average shockvelocity of 70 km/s. According to ourequation-of-state tables, this shockspeed corresponds to a density of 90 g/cm3 and a pressure of 0.74 Gbarin the gold target, which is by far thehighest inferred pressure obtained ina laboratory.

In this experiment, any spatialimbalance in the drive or anyunpredicted edge effects (forexample, those from interactionsbetween the flyer foil and sleevecontaining the target assembly) couldcause the flyer foil to tilt or curveand drive a nonplanar shock into thetarget. However, any nonplanaritywould be readily observed becauseof the relatively large diameter of thefoils; furthermore, any edge-inducednonuniformities would be minimizedbecause the step in the target is at thecenter of the foil. (See pp. 28–29 fora discussion of this experimentrelative to weapons physicsequation-of-state experiments.)

If the target foil is preheated byhigh-energy x rays from thehohlraum before the flyer foil hits it,the measurement is compromised.To test this possibility, we altered the x-ray drive in one experiment so that the overall intensity would beidentical to that in other experimentsbut the intensity of high-energyx rays (those ≥2.5 keV) would bereduced by more than a factor offive. The result indicated that themeasurement was not affected bypreheat.

X-Ray Opacity MeasurementsTo understand the plasma state

and radiative transport, we need toobtain high-quality measurements of the radiative opacity of materials.To do this, we must simultaneouslymeasure the x-ray transmission,temperature, and density of amaterial sample in a singleexperiment. These measurementshave been done successfully onNova using point-projectionspectroscopy (see Figure 7); webelieve that this technique will beeven more successful in similarexperiments on NIF because we will be able to access larger ranges ofmaterial densities and temperatures.

Science on the NIF E&TR December 1994

48

Shock breakout

Tim

e

Streakcamera

2-µm/6-µmgold

target foil

(b)

(a)

3-µm goldflyer foil

Drivebeams

1-mm-long¥ 700-µmgold sleeve

50-µm void

50-µmpolystyreneablator

X rays

Figure 6. (a) Schematic of a radiation-driven shock experiment. The x raysescape from a hole in the cylindrical goldhohlraum and ablate a 50-µm layer ofpolystyrene with a 3-µm gold flyer foil.The flyer foil accelerates through a 50-µmvoid, collides with a two-step gold targetfoil, and launches a compression wave inthe target foil. (b) A typical streak cameraimage of a shock breakout showing thetime interval between the two sides of the step.

When we use point-projectionspectroscopy on Nova, we use eightlaser beams to heat the sample. Then, apoint source of x rays is produced bytightly focusing one of the remaininglaser beams onto a small backlighttarget of high-Z material. X rays fromthe backlight pass through the sampleonto an x-ray diffraction crystal and arethen recorded on x-ray film. Other x rays from the same point bypass thesample but are still diffracted from thecrystal onto the film record. The ratio ofattenuated to unattenuated x rays provides the x-ray transmissionspectrum of the sample. Propercollimation allows a highly quantitativeanalysis of the spectrum. Backgroundfrom film chemicals, sample emission,and crystal x-ray fluorescence can all beseparately determined from the x-rayfilm record.

The sample itself must be uniformin temperature and density.Uniformity of temperature is achievedby heating the sample in a hohlraumthat does not allow laser light toreflect or impinge on it directly; thus,the sample is heated only by x rays.The hohlraum, by providing x-raydrive that volumetrically heats the

sample that is tamped, also maintainsthe relatively high density of thesample and ensures that it is in localthermodynamic equilibrium.

The sample is tamped by plastic sothat as it expands, its density remainsconstant. The thickness of the tamperis determined by calculations, and thedensity of the sample is determinedby imaging. Usually, a second point-projection spectrometer images theexpansion of the sample. The firstpoint-projection spectrometer is usedto measure the sample’s absorption.The relative intensities of thetransitions from the different ionspecies give the ion balance in thesample, which, when coupled to thedensity measurement, gives thetemperature of the sample.

The two point-projectionmeasurements allow density to bemeasured to an accuracy of ±10% andthe temperature to an accuracy ofabout 5%. With these accuracies it is possible to make a quantitativecomparison between the experimentalresults and the theoretical calculationsof the opacity.

In one experiment on Nova, wemeasured the opacity of niobium in

an aluminum–niobium sample. Thesample contained 14% aluminum by weight for the temperaturemeasurement. Figure 8 shows thetransmission of the aluminum and the transmission of the niobium. The dotted lines overlaying theexperimental data are the calculations.In general, there is excellentagreement. This experiment is amilestone. It shows that we can obtainopacity measurements accurateenough to serve as an in situtemperature diagnostic for the sample.The accuracy of the sample’stemperature, measured to be 48 eV (±2 eV), represents a very importantadvance in measuring temperatures of high-energy-density matter. (See pp. 27–28 for a discussion of thisexperiment in relation to weaponsphysics opacity experiments.)

Plasma Physics

In the broadest sense, plasmaphysics is the scientific investigationof the predominant state of matter inour universe, plasma. The study ofplasma physics has been stimulatedover the past four decades by its close

E&TR December 1994 Science on the NIF

49

Film

x

Bragg crystal

Occludedarea:emissionand fog

Tamped opacity sample with two different thicknesses

Hohlraum heated by 8 laser beams

Backlightlaser

Point sourceof x rays

Spectrum Absorptionspectrum

y

y, ¬

x

Figure 7.Schematic of point-projectionspectroscopy for opacitymeasurements. Thelaser-producedbacklight x rays passthrough the targetand are imaged. A Bragg crystaldisperses thespectrum so that a spatially andspectrally resolvedimage is obtained.Temporal resolutionis provided bybacklight duration.

connection with the goal of creatingfusion as an energy source and withthe exploration of variousastrophysical plasmas.

The advanced experimentalcapabilities of the NIF will allow usto produce and characterize large,hot, uniform plasmas. With largeuniform plasmas, we will be able tomeasure electron and iontemperature, charge state, electrondensity, and flow velocity. In short,we will be able to perform a widerange of quantitative experiments ona medium that is a very goodapproximation of a real test bed forplasma physics. Many experimentswill be extensions of the fusionenergy experiments that have beenperformed on smaller, less powerfulhigh-energy lasers. Many others,however, will go beyond therequirements of fusion energy toexplore a range of basic topics inplasma physics, a few of which arediscussed here.

FilamentationWhen a small hot spot, or speckle,

in the laser-intensity profileundergoes self-focusing,filamentation occurs: that is, electrons(and eventually ions) are expelledfrom the filament, causing laser lightto focus more tightly. This process, in

turn, creates an unstable feedbackloop; the more tightly focused thelaser light is, the higher its intensityand the lower its electron density.Eventually, this instability issaturated by diffraction effects,thermal absorption, or parametricinstabilities.

The growth of filamentarystructures can be determined by thewidth and length of speckles in theincident laser beam. Because longspeckles are more likely to self-focusthan short speckles, filamentation canbe described by a growth rate alongthe length of a speckle, or 8f2l, wheref is the f-number of the beam (i.e.,beam focal length divided by effectivemaximum beam diameter) and l is the wavelength of the laser light. Ifthe speckle lengths are smaller thanthe scale length of the plasma, the f-number and the wavelength of theincident beam can be very powerfullevers for modifying filamentation.By using large uniform plasmas onthe NIF, we will be able to producesufficiently large filaments to studythis process over a wide range ofwavelengths and f-numbers. We willalso be able to explore this processover a broad range of experimentalparameters by varying the color and f-number of the filamentation beamand by varying the plasma conditions

(e.g., temperature, density, andaverage ion charge).

The primary diagnostic forfilamentation would be the stimulatedRaman scattering signal, which isindicative of the low density in thefilaments. That could be coupled withhigh spatial-resolution imaging, high-resolution optical probing, and astudy of the angular distribution ofscattered light.

Thomson scattering could be usedfor these investigations to makehighly localized measurements ofplasma temperature and density. Itcould also be used to measure thecoherent motion of electrons involvedin ion acoustic and electron plasmawaves. This measurement wouldprovide a temporally and spatiallyresolved measure of the coherentfluctuation amplitude in a specificdirection, as determined by thedetector angle, the scattering volume,and the scattering light source.Measurement of the backgroundfluctuation levels would provideinformation about the initial level ofthe coherent fluctuations, theiramplification, and their saturation. Italso could provide useful informationabout the coupling betweenstimulated Raman and Brillouinscattering if it were done at the samelocation and at the same time as the

Science on the NIF E&TR December 1994

50

Abs

orbt

ion 0.8

0.6

0.4

0.2

1520 1540 1560Energy, eV

1580

Experimental dataPrediction

1600 2100 2200 2400 2500 2600 2700 280023000

1.0

Figure 8. Absorption of an aluminum–niobium sample. The experimental data are in the solid black line and the opacity prediction is thedashed line. The spectrum of the aluminum-potassium alpha lines, which were verified to yield an accurate temperature, were measured onthe same experiment as the niobium spectrum.

coherent motion measurements.Developing an x-ray Thomsonscattering measurement to studycoherent plasma motion in highdensity plasmas is another excitingpossibility.

Formation of Large, UniformPlasmas

Presently, we can producerelatively high-temperature (3000-eV), millimeter-scale plasmasusing the diagnostic complement andexperiments shown in Figure 9.These large, uniform plasmas areused to study phenomena as diverseas plasma–laser interactions andnuclear reaction rates. Theexperimental geometry should bedirectly scalable to the NIF, withnine-tenths of the laser being used toform the plasma and one-tenth beingused to create interactions. Theability to produce these large uniformplasmas on the NIF will allow us tostudy fundamental aspects of ourexperiments, such as hohlraumenvironments and sidescatter, thathave been virtually impossible tointerpret quantitatively.

Short-Pulse, High-PowerExperiments

There is widespread agreementthat the NIF should include a beamline for short-pulse, high-powerexperiments. This capability isespecially important for studyingsuch basic topics as relativistic, ultra-high-intensity regimes of laser–matterinteraction; high-gradient acceleratorschemes; and fast ignition (Figure 10). It is also more amenableto detailed simulation and tosystematic exploration of linear andnonlinear behavior of plasmas.

The high-gradient acceleratorschemes employ plasmas that supportmuch higher energy fields than thoseassociated with conventionalaccelerator schemes. As a result, thedevice will be much more compact

E&TR December 1994 Science on the NIF

51

Gatedspectrometer

(SXRFC)

Laser Laser

Streaked high-resolution

spectrometer(HIX)

Streakedspectrometer

(SSC)

Gated imaging(GXI-3)

Gatedimaging(WAX)

Interactionbeam

Streaked imaging(SSC)

Gatedimaging(GXI-2)

(a) High compression implosion

(b) Channeling laser beam (c) Ignitor laser beam

Pusher

Compressedmaterial

Figure 9. Schematic of the experiments and diagnostic complement (red arrows) used to forma large, hot, uniform plasma. A gas bag (light green) is filled through two tubes in the hoop (darkgreen). The pressure is stabilized by a pressure transducer that causes the fully ionized speciesto yield electron densities of approximately 10–1 cm3. The roughly spherical plasma, which has avolume of 0.066 cm3 and a radius of 2.5 mm, is heated to a temperature of about 3000 eV byheating lasers (blue arrows). A separate interaction beam (light blue arrow) drives theinstabilities in a controlled way. This geometry should be directly scalable to the NIF.

Figure 10. Fast ignition requires high compression, two laser systems, and system diagnoses.(a) In the first step of this process, a gas-filled sphere is imploded. The core of the compressedgas is at densities of 600 g/cm3. (b) In the next step, a laser with a pulse duration of 100 ps and anintensity of 1018 W/cm2 creates a channel by pushing the critical density surface toward the core.(c) Finally, the heater, or ignitor, beam is turned on. This beam interacts with the density gradientand generates hot electrons at MeV energies. These electrons penetrate into the core of thecompressed gas and cause an instantaneous rise in the local temperature of the core.

and potentially cheaper. A number ofnovel schemes have been proposedand studied at laser powers not quitehigh enough to produce the desiredelectron velocity. If these schemesprove successful, applications totunable sources of x rays are alsoenvisioned.

Radiation Sources

The conversion of laser energy intoshort wavelength radiation is a majorgoal of many high-energy laserexperiments. On the NIF, we will be

able to convert laser energy to a widevariety of x-ray and particle sourcesneeded to address several importantquestions in basic and appliedphysics. For example, we will beable to produce intense broadbandthermal x rays from high-Z targets,coherent amplified x rays (x-raylasers) from high-gain plasmas,intense neutron pulses fromimplosion plasmas, and intensepulses of hard x rays from fastelectrons. Accurate energy spectraand absolute measurements of theconversion of laser energy into all

types of radiation and particle fluxes will play an important role in benchmarking our basicunderstanding of laser–plasmainteractions and atomic physics.

Broadband x rays generated byNIF laser plasmas will be used toproduce and characterize large,uniform plasmas relevant to inertialconfinement fusion and astrophysics.The high temperatures and densitiesproduced during implosion andsubsequent ignition will be anexcellent source of continuum x rays—those extending from thesoft x-ray region to MeV with pulsedurations of less than l00 ps.

Besides producing importantcoherent radiation sources (i.e., x-raylasers), NIF will offer a critical testof our atomic modeling, allowing usto extrapolate existing neon-like andnickel-like collisional x-ray lasers towavelengths of about 20 angstroms(Å = 10–10 m). At these wavelengths,we can use x-ray laser interferometry(the interference created by splittingand then recombining the x-ray laserbeam) to measure electron densitiesin plasmas exceeding solid densities.Also, the short-pulse capability ofthe NIF may enable us to developnew x-ray lasers that emit radiationat wavelengths shorter than 10 Å;such bright, coherent sources wouldbe very useful in characterizing solidmatter for materials science andbiophysics research.

The NIF will be able to generatemore than 1018 neutrons in a single100-ps pulse, making it very usefulfor producing uniform, high-density,low-temperature plasmas. It will beable to generate fast electrons withhundreds of kiloelectron volts inenergy—which is a potential source of high-energy x rays forbacklighting and probing plasmas.

Science on the NIF E&TR December 1994

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Figure 11. Equivalent radiation temperature vs time for three gold hohlraum experimentsperformed on Nova. The experiments used a total energy of 18 kJ. Laser entrance holeswere in the sides of the 500- and 1000-µm-length hohlraums and through the ends of the2800-µm-length hohlraum. The curves for each hohlraum show the reproducibility of thedata. The black line indicates the equivalent radiation temperature with the M band; the redline indicates the equivalent radiation temperature without the M band. The contribution ofthe M band to the radiation temperature was greatest in the small hohlraums because of theclosure of the holes; it was very small in the large hohlraums because they were large andbecause their viewing angle did not accommodate a view of the laser-irradiated spots.

0.30

0.20

0.10

0

0.40

2-ns pulse1000 µm ¥ 1000 µm

1-ns pulse1600 µm ¥ 2800 µm

1-ns pulse500 µm ¥ 500 µm

Without M bandWith M band

0 0.5 1.0 1.5 2.0 2.5 3.0Time, ns

Tem

pera

ture

, keV

These radiation sources aresupplemented by the possibility ofusing radiation enclosures, orhohlraums (such as those shown inFigure 11), to generate radiationenvironments and x-ray drive fluxes.These sources will be able to producefar in excess of the approximately200 eV produced by hohlraumsources available on current high-energy lasers. These sources will beof higher effective temperature andalso will be able to provide uniformx-ray drive over far larger areas thanis possible with today’s sources.Thus, the advantages of using x-rayheating for the study of hot, densematter will be greatly enhanced onthe NIF.

Radiative Properties

The importance of radiativeproperties in high-energy-densityplasma derives from three factors:• First, the radiative property can bethe best indicator of the level ofscientific knowledge in a particulararea. For example, when scientistswant to develop new descriptions of atomic structure, they look attransition energies.

• Second, radiative properties serve as primary data for numerous otherstudies. For example, spectral linelists are inadequate for many of thecharge states of heavier elements.Thus, scientists measure andcategorize the energies of highlyionized species for a variety of uses.• Third, radiative properties serve asnoninterfering probes. For example,by looking at the emission orabsorption spectrum of a plasma,scientists can obtain fundamentalinformation about the plasma’sionization balance, rate processes,densities, temperatures, andfluctuation levels. The radiativeproperties are therefore a powerfuldiagnostic of the plasma state.

Experiments on high-energy lasershave done much to enhance ourknowledge of the radiative propertiesof hot, dense matter; thus, we expectthat experiments on the NIF, such asthose employing interferometry andplasma spectroscopy, will advancethat knowledge even further.

Interferometry ExperimentsFor years, optical probing of

high-density or large plasmas hasbeen difficult because of the high

absorption of the probe, the effectsof refraction, and the impossibilityof going beyond critical densities.Recently, we did an experiment to see whether an optical measuringdevice, known as a Mach-Zehnderinterferometer, and a standard 3-cm-long yttrium x-ray laser could beused to probe these plasmas moresuccessfully.

In this experiment (shownschematically in Figure 12), theoutput from a standard 3-cm-longyttrium x-ray laser was collimatedby a multilayer mirror and injectedinto the interferometer. An imagingoptic from the interferometer thenimaged a plane within theinterferometer where a plasma wasproduced. Figure 13 shows therecorded interferogram of theplasma. The fringes, or contrastmodulations, due to the plasma are clearly visible, indicating thefeasibility of this technique.However, plasma blow-off, evident in the central region of the image,completely obscures the laser,indicating that the technique still has its limits. On the NIF we will be able to push the x-ray laserinterferometer to shorter x-ray laser

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Figure 12.Schematic of theexperimental setupfor x-ray laserinterferometry.

X-ray CCDdetector

Laser irradiated target

Collimatingmultilayer

mirror

X-ray laserMach-Zehnderinterferometer

Imagingmultilayer

mirrors

wavelengths, making it an even moreimportant diagnostic tool in the studyand characterization of large-scaleplasmas.

Summary

The extraordinary range ofphysical conditions that will beachievable on the NIF will advanceknowledge in the physical sciences. Itwill give us the ability to synthesizeand analyze the plasmas thatcharacterize the stellar environmentduring its evolution. It will enable usto investigate a number of stable andunstable flow problems underconditions that cannot be obtained byconventional means, such as windtunnels, shock tubes, or other high-energy lasers. It will give us theability to investigate materialbehavior at pressures from 1 to 100 terapascals and temperatures upto a few hundred electron volts sothat we can validate our theoreticalunderstanding of material behavior atextreme conditions. We will be ableto convert NIF laser energy to a widevariety of x-ray and particle sourcesneeded to address importantquestions in basic and applied

physics. Finally, the NIF will enableus to push the x-ray laserinterferometer to shorter x-ray laserwavelengths, making it an even moreimportant diagnostic tool in the studyand characterization of large-scaleplasmas. The NIF will allow us toexplore a previously inaccessibleregion of physical phenomena thatcould validate our current theoriesand experimental observations andprovide a foundation for newknowledge.

Key Words: astrophysics; high-pressure physics;hydrodynamics; National Ignition Facility—high-energy laser experiments; plasma physics; radiationsources; radiative properties.

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1200

600

00 1000

µm2000

µm

Figure 13. Interferogram of a high-density plasma produced by x-ray laserinterferometry. The plasma is made byirradiating the surface of a mylar plasticsample (solid horizontal band) with an x-raybeam. The bright spot above the plasticsample is the self-emission of the plastic.

For furtherinformationcontact Richard W. Lee(510) 422-7209.