NAS/NAE Committee on the Prospects for IFE Systems

San Ramon, CA29 January 2011

Technology for Polar-Drive Ignition on the NIF

J. D. ZuegelUniversity of RochesterLaboratory for Laser Energetics

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field

inte

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E19667

Polar-drive ignition could be tested on the NIF with a few modest modifications to the facility

• Beam-smoothingimprovements:

– Multi-FM 1-D smoothing for spectral dispersion (SSD) provides the required beam smoothing with simple modifications to the NIF facility

– Beamsmoothingisonlyrequiredatthebeginningofthelaserpulse, which minimizes stress on the laser

– Polar-drive phase plate and polarization-smoothing designs are underway

– A NIF PD beam-smoothing demonstration on OMEGA EP is planned in FY12

• Direct-drivetargettechnology:

– NIF-scale fill-tube targets have been demonstrated and are being optimized

– Concepts for a polar-drive ignition target insertion cryostat (PD-ITIC) are being developed

Summary

E19668

Implementing polar drive (PD) requires five changes on the NIF for an ignition demonstration

5

34

New PD phase plates (2~)and polarization plates (3~)in final optics assembly

New PD ignition target insertion cryostat (PD-ITIC)

2 Add new SSD grating to 48preamplifier modules (PAM)

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Add multi-FM fiberfront end and combinewith existing system

1

E19669

Laser nonuniformity imprint is minimized by optimizing smoothing by spectral dispersion (SSD)

• SSDdivergence(DiSSD) determines the asymptotic uniformity• Increasingtheinversecoherencetime(tc–1) allows the target to

experience a smoother spot for a longer period

100

10–1vrm

s (%

), ,

-mo

des

30:

60

t (ps)103102101

t (ps)103102101

Single-beam laser nonuniformity

Increasing tc ? , × Dm × Ncc–1

Ncc = 2

Ncc = 8

Ncc = 4

DiSSD, Dmheld fixed

Dm / Bandwidth Ncc / Color cycles fm / Modulator frequency

Increasing

DiSSD ?Dm × Nccfm

E19682

MultiFM 1-D SSD provides required beam smoothing performance with minimal impact on the facility

• TraditionalSSDsystemsusingsingle-frequencyphasemodulationhavelow smoothing rates for many important spatial modes (, < 150)

• MultiFM1-DSSDisanewapproachthat – provides better smoothing rates with lower total bandwidth

(esp. for PD pulse shapes with picket prepulses) – can be implemented on NIF with simple modifications

1000

800

600

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0100 101

t c

(GH

z)–1

102

, mode

MultiFM 1-D SSD(0.5 THz)

1-D SSD

2-D THz SSD

Inverse coherence time versus far-field spatial frequency

E19683

An optimized MultiFM configuration that achieves high gain in polar drive simulations has been identified

• MultiFM1-DSSDemploystechnologydevelopedforthetelecommunications industry

– 40-GHz phase modulators and drive electronics – UVbandwidth: Dftotal = 500 GHz (effective bandwidth) – SSDdivergence:DiSSD = 100 nrad (half angle at full beam)• DRACO2-Dsimulationswithallnonuniformitysources:Gain= 32

SSD beam divergence (DiSSD)

Do

2 (G

Hz)

Do1 (GHz)

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1000

102

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0.55

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0.40UV

div

erg

ence

1/2i

(n

rad

)

vre

ach

ed(%

); l-

mo

de

= (

80:2

00)

0Do1 (GHz)

5004003002001000

MultiFMoptimum

MultiFMoptimum

Laser uniformity (vrms,80:200)

E19671

DynamicBandwidthReduction(DBWR) minimizes stress on the laser with little affect on target gain

MultiFM 1-D SSD beam smoothing only needs to be applied to pickets in the polar-drive point design pulse shape.

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r (nm)

z (n

m)

80 100 1200.0 0.0

0.4

0.8

1.2

1.6

2.0

0.5

1.0

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2.0

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Time (ns)

Relative gain with DBWRDensity at peak compression

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rmal

ized

tar

get

gai

n

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wer

/bea

m (

TW

)

6

Gain = 32

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300

150

0

t

24

6

8

Gain = 32

Integrate NIF PAM into OMEGA EP

Verify laser imprint with foil-targetexperiments on OMEGA EP**

In progress

Early FY12

Verify smoothing with equivalent target plane measurements*

No SSD With SSD

300 nm

Simulationof method

101

10–1

10–3

Am

plit

ud

e (n

m)

0.0 0.5 1.0 1.5Time (ns)

Rippled target

Laserimprint

Equivalent surfaceperturbation

Demonstrate MultiFM Seed Source

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E19672

A MultiFM 1-D SSD beam-smoothing demonstration on OMEGA EP will validate laser imprint performance

**T.R.Boehlyet al.,LaserPart.Beams18, 11 (2000).*S. P. Regan et al.,J.Opt.Soc.Am.B17, 1483 (2000).

E19673

The focal-spot conditioning strategy for polar-drive ignition includes phase and polarization plates

The NIF final optics assembly (FOA)willinclude: • Phaseplatebetweenthefrequencyconversioncrystals(2~) • Polarizationplate(3~)

Targetchamber

3~beam

1~beam

FOA

To target

Debris shieldsWedged focus lens

THG

SHGVacuum window

2~ phase plate

3~ polarization plateIntegratedOpticsModule(IOM)

Phase plates and polarization smoothing are being designed to efficiently and uniformly couple energy to polar-drive targets

E19681

• Phaseplatesefficientlydeliverlaserenergywithadesiredfocal pattern to achieve required irradiation uniformity.

• Polarizationsmoothinginstantaneouslyimprovestargetedmodes of focal-spot irradiance modulation.

x Targetplane

Lens

Phaseplate

“o”

“e”

Dx

x

Io e

Polarization smoothingFocal spots for polar-drive beams

Polar and mid-latitude(Rings 1 to 3)

xff (mm)–2

–2

2

2

0xff (mm)

–2 20

0

y ff (

mm

)

Equatorial(Rings 4 to 5)

Wedge of KDPid

Did

E19674

A NIF fill-tube target has been demonstrated that will be optimized to meet polar-drive ice layer specifications

Facility renovations and equipment upgrades are underway at LLE to demonstrate NIF PD cryogenic layering with DT targets.

Target: 2.95-mm OD, 20-nm wallFill tube: 30-nm OD at shell wall

Cryogenic layering sphere

D2 layer (297 nm) with mid-IR laser layering

Inner ice layer surfaceShadowgraph X-ray phase contrast

–20 –10 0 10 20 30

Deviations from sphericity in nm

E19675

A polar-drive ignition target insertion cryostat (PD-ITIC) will minimize the impact on the NIF facility

• Apolar-drivetargetthatsurvives>3-s exposure to the target chamber is required to use existing “clam-shell” shroud design

• UseexistingNIFspaceenvelopeandcryogenicsupportsystems(TAS, LLCS, TARPOS)

Existing TAS and LLCS place challenging constraints on a PD-ITIC design that will limit size of the cryogenic shroud.

Load, Layer, and Characterization Station (LLCS)

Target Alignment System (TAS)

E19667

Polar-drive ignition could be tested on the NIF with a few modest modifications to the facility

Summary/Conclusions

• Beam-smoothingimprovements:

– Multi-FM 1-D smoothing for spectral dispersion (SSD) provides the required beam smoothing with simple modifications to the NIF facility

– Beamsmoothingisonlyrequiredatthebeginningofthelaserpulse, which minimizes stress on the laser

– Polar-drive phase plate and polarization-smoothing designs are underway

– A NIF PD beam-smoothing demonstration on OMEGA EP is planned in FY12

• Direct-drivetargettechnology:

– NIF-scale fill-tube targets have been demonstrated and are being optimized

– Concepts for a polar-drive ignition target insertion cryostat (PD-ITIC) are being developed