The NIF Ignition Program - PPPL

John LindlLawrence Livermore National Laboratory

December 13, 2004

The NIF Ignition Program

Presentation toFusion Power Associates Meeting

NIF-0202-0XXXXppt15/GHM/tr

Outline

• Ignition Introduction

• Ignition program progress on hohlraums and capsules for ignitionexperiments.

— Hohlraums— Capsules

• Plan for experiments leading to first ignition target shots in 2010

The scale of ignition experiments isdetermined by the limits to compression

200

100

10

1

0.2

Compression (X liquid density)102 103 104 105

50050

5

0.5

Capsuleenergy (KJ)

NIF

Relaxed pressure and

stability requirements

Ther

mon

ucle

ar e

nerg

yCa

psul

e ab

sorb

ed e

nerg

y

Capsule energy gain plotted versus compression •Pressure is limited toP(Max)~100 Mbar by LaserPlasma Interaction (LPI)effects

•Given the pressure limits,hydrodynamic instabilitieslimit implosion velocities to

!

Vimp

< 4"107cm/sec

and this limits themaximum compression•Symmetry and pulseshaping must beaccurately controlled toapproach the maximumcompression

Introduction

There are two principal approaches tocompression in Inertial Confinement Fusion

Inertial Confinement Fusionuses direct or indirect driveto couple driver energy tothe fuel capsule

Spherical ablation withpulse shaping results in arocket-like implosion ofnear Fermi-degenerate fuel

Spherical collapse of theshell produces a central hotspot surrounded by cold,dense main fuel

Direct Drive Hot spot(10 keV)

Cold, densemain fuel(200-1000 g/cm3)

Indirect Drive Low-zAblator forEfficientabsorption

CryogenicFuel for EfficientcompressionDT

gas

Introduction

The impact of most effects that degrade an implosioncan be specified as a hot spot perturbation amplitude

• Long Wavelength Perturbations — Hohlraum asymmetry — Pointing errors — Power Imbalance — Capsule misplacement in

chamber

• Short Wavelength Perturbations — DT ice roughness — Ablator roughness — Ablator microstructure

The hot spot penetration isthe fraction of the hot spot perturbedby the various sources of error

Introduction

IntroductionThe “ignition island” size provides an integralmeasure of the robustness of the NIF ignition designs

IgnitionIsland forGoldhohlraumwith Becapsule

Minimum Driverequirements areset by hot spotpenetrationfraction

MaxDrive temperaturesare set by LPI effects

0.00.15

0.25

Laser Energy (MJ)

325 300

275

250

Lase

r Pow

er (T

W)

We expect ~15% hotspot penetrationfraction based ondetailed 2D and 3Dcalculations at thespecified targetfabrication and NIFperformance levels


Outline





Typical Pulse Shape

Ablator(CH or Be)

DT Ice

DT gas fill

!

(0.3mg/cm3@18°K)

!

(0.5mg/cm3@19°K)Hohlraum Wall: – Au or High-z mixture (cocktail) – Full density or foam

Capsule

Laser Beamsin 2 rings

Laser EntranceHole with window

Shineshield

NIF Indirect Drive target schematic

400 g/cc density isosurface

The Hydra code is used for 3D calculations onthe NNSA ASC computers

Energetics

(White circleat 40 microns)

Ignition - hohlraum only perturbations

Hohlraum Axis (z)

80µm

600 g/cc surface

Ignition - capsuleand hohlraumperturbations

Stagnationshock

Yields calculated in 3D are near 1Dyields with both Gold and cocktail wallhohlraums and plastic or Be capsles

Energetics

10-25%fo thelaserenergy tocapsule

The Indirect drive ignition point designcontinues to evolve to optimize coupling efficiency

1.451.301.100.530.330.2416.5%

1.451.301.100.620.280.2013.5%

Laser light(MJ) Absorbedxrayswall losshole losscapsuleefficiency (%)

1.451.301.100.650.300.1510.5%

Au with CHCapsule

Au with BeCapsule

Cocktails withBe Capsule


Outline





Omega experiments show larger albedo for cocktailwalls made of ((U Nb0.14)0.6,Au0.2,Dy0.2)

Agreement with LASNEX calculationsindicate that cocktail materials maybe advantageous compared to Au.Integrated hohlraum experimentswith unsmoothed beams have notdemonstrated higher TRAD butmeasure higher laser backscatter

Energetics

Hohlraum experiments with split back plateverify high x-ray re-emission

Intensity ratio of Cocktail/Au is in agreement with

LASNEX calculations

Cocktail

Re-emission

Inte

nsity

Au

1

0.8

1.2

Radiation Temperature

Cock

tail

/ Au

100 200 300

LASNEX

450 eVPhoton energy

Detector

Laser

Cocktail

Au

• LEH shields will be retested on Omega in NIF-like multicone geometry• Similar advantages possible for NIF ignition hohlraums - dependson beam propagation with added plasma source

1 ns square pulse - identical pointing

LEH shields provided a 20% increase incapsule radiation flux at Nova and an extrasymmetry tuning degree of freedom

Energetics

Enhancements in hohlraum coupling efficiency expectedby 2010 will substantially increase the ignition island

Energetics

Coupling efficiency = ηabsηHηabs = 1 - backscattered fractionηH = hohlraum coupling efficiency

The goal ofHohlraumEnergetics isto maximizehohlraumcouplingefficiency

IgnitionIsland

(e)

η = 0.105

CH with Au

At 3ω, drive measurements and Lasnex simulationsagree closely over >two orders of magnitude in T4

R

Nova and Omega Experiments

Energetics

The first four NIF beams are operational andhave been used for a number of experiments

View from inside the targetchamber

Quad 31b beamtubes

NIF Project

An international team has successfully activatedhohlraum diagnostics at NIF and the first hohlraumexperiments are in close agreement with calculations

Thinwall AuHohlraum4-16 kJ, 1- 9 ns,1015-1016 W/cm2

with beamsmoothing NIF

Q31B

0

100

200

300

400

0 2 4 6 8 10

Tr (eV)

t (ns)

1.2 mm

2.4 mm

1.6 mm

0.6 mm hohlraum

Hohlraum Tr vs timeRaw data

Peak Hohlraum Tr vsLaser Power

0

100

200

300

400

0 2 4 6 8 10

DataPreshot simulation

Tr (eV)

Laser Power (TW)

Hohlraum Tr(Dante)

18 channel soft x-ray detector (0.1-

10 keV)

Energetics


Outline





Several developments have led us to adoptthe current Beryllium capsule point design

• With a given ignition hohlraum drive , beryllium absorbs ~1/3 moreenergy than plastic

• Beryllium has better short wavelength stability - an experimentalprogram will be required to establish the acceptable level of Bemicrostructure

• Many previous difficulties in Be fabrication have been solved (filling,diagnosing layer)

• Compatibility with fill tubes allows a staged cryo system with aninitial less complex (and less costly) warm transport capability(instead of a cold transport system)

• Although fill tubes are a design and target fabrication complexity,current simulations and fabrication progress lead us to conclude wecan make them small enough for success - an experimental programis needed to test the calculations

Target Fabrication

3D calculations show the importance of controllingcapsule surface roughness

• Capsule simulations have asymmetry and fabrication perturbations

• 3D asymmetry inferredfrom integratedhohlraum simulation

• Nominal “at spec”perturbations on ice andablator in low, intermediate,and high modes

• Gave 21 MJ (90% of 1Dcalculation)

140 ps beforeignition time

Plastic/DTinterface

Hohlraumaxis

60 g/cc density isosurface

Ignition time

Stagnationshock

400 g/cc density isosurface (different scale)

Implosion Dynamics

0.35%

0.7%

848928

1105 µm

934940

9951011

0.0%

Cu (%)

300 eV design:

Be

DT

0.0%0.35%

200Initial ablator roughness (rms, nm)

0

5

10

15

20

25

Yield(MJ)

OptimizedUniformDopant CHDesign

Surface achieved withCH (“NIF standard”)

400 600 8000

Graded-dopantBe(Cu)

Original CHDesign

GradedDopantCH Design

The graded doped Be capsule cantolerate 60x the NIF standard

Be Capsule designs using graded dopants for pre-heat shielding have the best calculated performance

Tolerance to ice roughness is also better (5 µm compared to 1 µm)

Implosion Dynamics

OptimizedUniformDopantBe(Cu)Design

High modemorphology

Graded Cu profile

18 µm 18 27 27 15

0.34% 0.65% 0.34% 0% 0% At % Cu

Sub-microngrains

1 mm

We have produced graded Cu-doped Becapsules at NIF-scale by sputter deposition

The surfaces of our firstgraded dopant Be capsulesdon’t meet specifications, butimprovements are planned

Target Fabrication

• We plan to investigate use of ion bombardment and othermethods as means to improve surface finish and coating density

• In FY05 LANL will study shell polishing

10-2

10-1

100

101

102

103

104

105

po

we

r (n

m2)

10 100 1000

mode number

Graded doped capsule

Tolerable surface with no margin for other imperfections

New NIF design spec for graded Cu-doped Be capsules

Polished Cu-doped Be flat

Original NIF Design Standard

The surfaces of our first graded dopant Be capsules don’tmeet specifications, but improvements are planned

Target Fabrication

•Two routes to filling beryllium capsules—Fill Tube—Drill, Fill and Plug (Strong Capsules)

2 µm Entrance

Exit2 µm

Laser:Ti:sapphire405 nm, <1µJ,120 fs, 3.5 kHzdrilling time:~40 sec

Fill tube

Epoxy

6µm OD

2mm

OD

Fill Tube

Current effort:minimizing the glue joint

Target Fabrication

Be is non-permeable, and requires a fill hole through the ~100-µm-thick ablator for filling

Glass tube, 10 µm OD, 3µm ID, inserted 20 µm,with 3 µm diameter holethrough beryllium ablator

Density contours at ignition time

Z along tube axis (cm)

Radi

us (c

m)

Simulations indicate that a 10 µm tube with a 3 µmhole has an acceptable impact on the implosion

• Uniformly doped Be(Cu) capsule• Calculation ignites and burns to same yield as 1D clean -- 21.7 MJ• Simulations have also beencarried out with graded dopedBe and CH capsules with filltubes up to 20 microns diameter.

Target Fabrication

The target is filled through the small fill-tube using aself-contained fuel reservoir

View of 2mm shell throughlaser entrance hole

View of 2mm shellthrough side hole

ShellLaserentrance

hole

Coolingrings

Fill tube8 µm ID

Hohlraum Fill tube Coolingrods

Fuelchamber

• Fuel pressure 2-3 atm• ~ 5 Ci DT

• Capsule filled in target inserter bytemperature control on fuelreservoir and hohlraum

Target Fabrication

The DT fuel layer in optically opaque beryllium has beenrecently characterized with x-ray refraction

Point projectionradiograph image

DT liquidmeniscus

Be(Cu)shell

Filltube

~ 1 hour

Point projection radiograph image

DT beta layer

Ice

• Rays tangent to surface areslightly deflected

• Other rays are very nearly un-deflected

• This method is many timesmore sensitive than absorption

x-rays

shell

Target Fabrication

The best cryo layers are formed near thetriple point

0

1

2

3

4

5

6

18.4 18.8 19.2 19.6

RM

S (

µm

)

Temp(K)DT triple point

IR enhanced layering is one technique shown tokeep the layer smooth at temperatures down to1.5 K below the triple point

Layer smoothness appears to degrade attemperatures less than 0.5 K below the triple point

DT in Be shell

DT in CH shell

Modes 4-128

Ignition designs

0.5 mg/cc}

0.3 mg/cc

}

Target Fabrication

CH Shell

CH Shell

Be Shell

The lower gas fill obtained with lower temperature cryofuel layers lowers the energy required for ignition

Target Fabrication

08-00-1098-1998pb01

Haan 2-4

Absorbed Energy(kJ)

Gas Fill (mg/cc)

0.5mg/cc DT+1 day He+0.025%H

Gas fill atDT triplepoint 19.6˘K

Minimum Capsule sizeversus gas fill

Current Baseline

Gas fill at18˘K

The ignition island is increased by improving targetfabrication

Target fabrication

Coupling efficiency = ηabsηHηabs = 1 - backscattered fractionηH = hohlraum coupling efficiency

The goal of targetfabrication and theimplosion dynamicscampaign is toachieve capsuleperformance whichis near the ideal limit

First ignitionExperiments1 MJ

IgnitionIsland

+η

Full NIFEnergy


Outline





NIF’s 192 beams are organized into “bays”,“clusters”, “bundles”, and “quads”

NIF Project

The National Ignition Facility

NIF Project

The beampath infrastructure for all 192 beams iscomplete and the first four beams have beenactivated for experiments

NIF Project

The NIF Early Light (NEL) commissioning of fourlaser beams has demonstrated all of NIF’s primaryperformance criteria on a per beam basis

21 kJ of 1w light (Full NIF Equivalent = 4.0 Mjoule) 11 kJ of 2w light (Full NIF Equivalent = 2.2 Mjoule - non-optimal

crystals) 10.4 kJ of 3w light (Full NIF Equivalent = 2.0 Mjoule) 25 ns shaped pulse < 5 hour shot cycle (UK funded shot rate enhancement program) Better than 6% beam contrast Better than 2% beam energy balance Beam relative timing to 6 ps

NIF Project

Firstcluster

Firstbundles

Key NIF ignition experiments begin following firstcluster completion and full system ignition experimentsstart in FY9-10 following project completion

Target Fabrication R&D•Capsules•Cryo layering in hohlraums

Diagnostic

activation

Confirm Ablator

Beams: 4

Drive Symmetry

Implosion Dynamics

0907FY04 05 06 08

Coupling and Trad diagnosticactivationHohlraum

Energetics

Firstquad

ActivateShock Timing

Chooselaser spotsize (CPPs)

Pre-Ignition

Cryo

1st

tune

48

Diag.techniqueactivation

Ignition TargetDesign

Ignition Implosions,Fabrication

TargetProduction

Beams: 16

10 11

2nd

tune

2nd

tune

symmetrytuning

shocktiming

1st

tune

Trtuning

Design1

D2 D3

Cryoignitionimplosions

non-cryoimplosion

s

Full NIF

192

Confirmfull NIFcouplin

g

Ablatortests

Cuts in the NIF Project funding for FY05 will delay near term objectives but we areworking with NNSA to develop a plan which will preserve the goal of Ignition in 2010

Summary/Conclusions

• Be targets are ~1/3 more efficient than earlier CH designs and morerobust to hydrodynamic instability.

• Major advances in Be capsules filled through a few micron fill tubehave opened up the possibility of implementing a greatly simplifiedcryo target support system on NIF for early ignition experiments

• Success with Be capsules, and the use of “cocktail” wall hohlraumswould result in a target which is about 2/3 more efficient than theoriginal baseline targets.

• This would enable successful ignition experiments at ~1 MJ in the firstcouple of years after completion of the NIF and allow much higheryields when NIF operates at its full design energy of 1.8 MJ.

• Additional optimization and hohlraum features such as laser entrancehole shine shields can yield further increases in coupling efficiencyand yields.

The NIF Ignition Program - PPPL

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