Modeling Cross-Beam Energy Transfer for Polar-Drive Experiments

D. H. EdgellUniversity of RochesterLaboratory for Laser Energetics

54th Annual Meeting of theAmerican Physical SocietyDivision of Plasma Physics

Providence, RI 29 October–2 November 2012

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Scattered-lightcalorimeters

Beam ports

Predicted scattered-lightnormalized distribution at wall

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20Angle from pole (°)

Measured and predicted energycollected by calorimeters

Sca

tter

ed li

gh

t (k

J/sr

)

40 60 80

MeasuredPredicted with CBET

Modeling suggests that cross-beam energy transfer (CBET) is significant during polar-drive (PD) implosions, but mitigation strategies exist

E21536

Summary

• ModelingpredictsthatCBETwillreduceabsorptionofthelaserenergyby ~10% during PD implosions on OMEGA

– this level is similar to that in symmetric 60-beam implosions

– the equatorial ring is affected more than the other rings

• AreductioninabsorptionispredictedforthePDpointdesignontheNIF

• CBETmaybemitigatedbyreducingtheratioofthebeam/target radii

– smaller beam profiles reduce CBET but increase illumination nonuniformity

Collaborators

P. B. Radha, V. N. Goncharov, I. V. Igumenshchev, J.A.Marozas,J.F.Myatt,W.Seka,andD.H.Froula

University of RochesterLaboratory for Laser Energetics

CBET is modeled using our scattered-light simulation codewitha3-DPDgeometry

E21537

• Plasmaparameterstakenfrom2-D DRACO hydrocode calculations (withoutCBET)

• Beamprofilesaredividedintomanybeamletsandraytracing is used to calculate their paths and Doppler shifts

• CBETcalculated*in3-Dforallbeamletcrossingsusingallbeams

– intensity, absorption, and transfer along each beamlet solved iteratively

• IncorporationofCBETintoDRACOisanongoingeffort**

• ModelinghasbeenbenchmarkedusingPDimplosionsonOMEGA

*I.V.Igumenshchevet al., Phys. Plasmas 17, 122708 (2010). **J.A.Marozaset al., Bull. Am. Phys. Soc. 56, 241 (2011). FormoredetailsaboutCBETmodeling,seealsotalk by J. A. Marozas, UO5.00003, this conference.

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Scattered-lightcalorimeters

Beam ports

Predicted scattered-lightnormalized distribution at wall

0 20Angle from pole (°)

Measured and predicted energycollected by calorimeters

40 60 80

MeasuredPredicted no CBET

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tter

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Integrated scattered-light distribution from OMEGA PD implosions support the CBET predictions

E21281a

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Scattered-lightcalorimeters

Beam ports

Predicted scattered-lightnormalized distribution at wall

0 20Angle from pole (°)

Measured and predicted energycollected by calorimeters

40 60 80

MeasuredPredicted no CBETPredicted with CBET

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tter

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Integrated scattered-light distribution from OMEGA PD implosions support the CBET predictions

E21281b

Plannedscattered-lightcalorimetersonNIFwillbeessential measurements for PD implosions.

CBET modeling of the time-dependent scattered light spectrum reproduces the observed features

E21538

• DetailsofthepredictedspectrawilllikelymatchtheobservationsevenbetterwithCBETincorporatedinto the hydrocode plasma profile predictions

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Time (ns)

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abso

rbed

lig

ht

(TW

/Sr)

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1.0H17C at 75°

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Measured

Predicted350.6

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MeasuredPredicted

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log10(TW/Sr/A)

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A ~10% reduction in absorption caused by CBET is predicted forupcomingoptimizedPDimplosionsonOMEGA*

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• Thislevelissimilartothatinsymmetric60-beamimplosions

• Thisisconsistentwithobserved~180 ps bang-time delays in PD implosions

• EquatorialthirdringmostaffectedbyCBET

Ring Absorptionno CBET

AbsorptionwithCBET

1 85% 82%

2 85% 78%

3 84% 69%

Total 85% 75%

Rings 1 and 2 Ring 3Beam profiles (W/cm2)

PD distributed phase plates (DPP’s),optimized pointing

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(nm

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400 –400 00

1

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×1014

(nm)400

*P.B.Radha,NI2.00006,thisconference

at 2 ns

ThePDignitionpointdesign*fortheNIFhasbeenstudied using the CBET model

E21540 *T.J.B.Collinset al., Phys. Plasmas 19, 056308 (2012); T. J. B. Collins, JO4.00010, this conference

y (m

m)

x (mm)

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Normalizedintensity

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Polar region Mid-latitude region Equatorial region

Ring 1a (y0 = 29.5 nm)

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Ring 2b (y0 = 409.6 nm)

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Ring 3c (y0 = 1054 nm)

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(a) Polar (24.5°)Ring 1 (23.5°)

Ring 2 (30.58°)Ring 3 (44.5°)

Ring 4 (50°)

(b) Mid-latitude (44°)

(c) Equatorial (83°)

Irradiation regions ( )z

Ports ( )

Preliminary results suggest ~20% reduction in absorptionasaresultofCBETforPDontheNIF

E21541

• TheNIFPDimplosionsseemmoresensitivetoCBETthanOMEGA

– quadstreatedassinglebeamswithquadrupleintensity

– pointing offsets are larger

- somebeamsareoffsetazimuthallyaswellas towardtheequator

Ring Absorptionno CBET

AbsorptionwithCBET

1a 95% 88%

2b 95% 80%

3b 92% 90%

3c 90% 59%

4c 92%89%

55%60%

Total 92% 72%

CBET may be mitigated by using a smaller laser beam spot size

E21542

• InCBETtheedgeregionsofthe beam profiles steal energy from the centers

– ifweremovethisedgeregionbymakingthespotsmallerwemayreducetheeffects of CBET

• Experiments*insymmetric-driveimplosionswithreducedbeam/target radius support this

*I.V.Igumenshchevet al., Phys Plasmas 19, 056314 (2012). D.H.Froulaet al., Phys. Rev. Lett. 108, 125003 (2012).

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Arbitraryunits

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Energy transfer integratedalong beamlet

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nm

Ring 191% 88% 80%

Ring 2 Ring 3

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(nm

)

400 –400 0(nm)

400 –400 0(nm)

400

Rb/Rt = 0.7Absorption with CBET (85%)

at t = 2 ns0 1 2 3 4 5 6

W/cm2 (×1014)

Absorbed power

Using a smaller spot size recovers the 85% total absorption of the no-CBET predictions in PD

E21285a

This could be an effective CBET mitigation strategy for PD, but itmustbebalancedwithincreasingilluminationasymmetriesandpossibleLPIissueswithdecreasingbeamsize.

Modeling suggests that cross-beam energy transfer (CBET) is significant during polar-drive (PD) implosions, but mitigation strategies exist

E21536

Summary/Conclusions

• ModelingpredictsthatCBETwillreduceabsorptionofthelaserenergyby ~10% during PD implosions on OMEGA

– this level is similar to that in symmetric 60-beam implosions

– the equatorial ring is affected more than the other rings

• AreductioninabsorptionispredictedforthePDpointdesignontheNIF

• CBETmaybemitigatedbyreducingtheratioofthebeam/target radii

– smaller beam profiles reduce CBET but increase illumination nonuniformity