Crossed-Beam Energy Transfer for Direct-Drive Implosions

I. V. IgumenshchevUniversity of RochesterLaboratory for Laser Energetics

53rd Annual Meeting of theAmerican Physical SocietyDivision of Plasma Physics

Salt Lake City, UT14–18 November 2011

Crossed-Beam Energy Transfer for Direct-Drive Implosions

Target

Beam 1

Center-beam ray

Beam 2

High-angle raysseed CBET

0.60.0S

catt

ered

-lig

ht

frac

tio

n

0.1

0.2

0.3

0.4

Simulationswith CBET

Simulationswithout CBET

Measurements

0.8Rbeam/Rtarget

1.0 1.2

Crossed-beam energy transfer (CBET) can reducethe performance of direct-drive ICF implosions

TC9760

Summary

• CBET is observed in time-resolved reflected-light spectra as a suppression of red-shifted light during the main laser drive

• CBET extracts energy from the center-beam incoming light and transfers it to outgoing light, reducing the laser absorption and hydrodynamic efficiency

• CBET can be reduced

– using beams smaller than the target diameter

– using laser beams with two or more colors

Mitigation strategies are being tested on OMEGA.

Collaborators

W. Seka, D. H. Edgell, D. H. Froula, V. N. Goncharov, R. S. Craxton, R. Follett,D. T. Michel, R. L. McCrory, A. V. Maximov, D. D. Meyerhofer, J. F. Myatt,

T. C. Sangster, A. Shvydky, S. Skupsky, and C. Stoeckl

Laboratory for Laser EnergeticsUniversity of Rochester

L. Divol and P. Michel

Lawrence Livermore National Laboratory

Outline

TC9761

• Introduction

• Modeling CBET

• CBET in symmetric OMEGA implosions

• Mitigation of CBET: experiments and simulations

• Conclusions

00

100

200

300

2 4Time (ns)

Po

wer

(T

W)

6 8 10 12

E = 1.5 MJ E = 20 kJ E = 16 kJ

DT gas

1700

nm

Symmetic NIF ignition design*(1-D gain = 50)

Cryo Warm

00 1

Time (ns)2 30 1

Time (ns)2 3

63912

2

4

6

8

10

1210 atmD2 gas

430 n

m

25-nm CH37-nm CH160-nm DT

16

12

8

4

0

DT gas

430 n

m

10-nm CH65-nm DT

OMEGA targets

Scaled-down implosion experiments on OMEGA are used to validate direct-drive NIF implosion designs

TC9858 *V. N. Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010).

Experiments on OMEGA have been modeled using hydrodynamic codes LILAC* and DRACO**

TC9881

* J. Delettrez et al., Phys. Rev. A 36, 3926 (1987). ** B. Radha et al., Phys. Plasmas 12, 032702 (2005). *** V.N. Goncharov et al., Phys. Plasmas 15, 056310 (2008).

• Radiation transport package

– multi-group diffusion

• Equation-of-state package

– SESAME

– QEOS

• Laser absorption package

– inverse bremsstrahlung

• Thermal transport package

– flux-limited transport

– nonlocal transport***

Hydrodynamic efficiency

Measured bang time is late by ~200 ps, indicating reduced laser coupling

TC9859

2.61018

1019

1020

1021

2.8 3.0Time (ns)

Neu

tro

n r

ate

(s–1

)

Po

wer

(T

W)

3.2 3.4

Measured

Bang-time measurements Scattered-light measurements

Measured6391263912 Simulated

f = 0.06

Simulatedf = 0.06

Simulatednonlocal

0

1

2

3

4

Simulatednonlocal200 ps

0.0 1.0 1.5 2.0 2.50.5Time (ns)

Simulations overpredict the red-shifted scattered light

E19908b

350.6

350.8

351.0

351.2

351.4

0.0 0.4 0.8

Time (ns)

1.20.0 0.4 0.8

Time (ns)

Wav

elen

gth

(n

m)

1.2

Experimental

Center-beam rays

52503

Simulated

Time-resolved scattered-light spectra from a spherical implosion

Simulations overpredict the red-shifted scattered light

E19908c

Without nearly radial rays(r = 0 to 150 nm)

0.0 0.4 0.8

Time (ns)

1.2

Simulated350.6

350.8

351.0

351.2

351.4

0.0 0.4 0.8

Time (ns)

1.20.0 0.4 0.8

Time (ns)

Wav

elen

gth

(n

m)

1.2

ExperimentalSimulated

Center-beam rays

52503

150 nm150 nmBlockedarea


• Blocking the central portion of the beam in the simulations reproduces the observed spectrum

CBET can be responsible for the discrepancy between experiments and simulations

E19905e

• CBET involves electromagnetic (EM)-seeded, low-gain stimulated Brillouin scattering

• EM seed is provided by edge-beam light

• Center-beam light transfers some of its energy to outgoing light*

• The transferred light bypasses the highest absorption region near the critical surface*

Target

Beam 2

Beam 1

Edge-beam ray

Center-beam ray

Cross-beamenergy transferis spatially limitednear M ~ 1

CBET reduces laser absorption and hydrodynamic efficiency.**

* D. H. Edgell et al., Bull. Am. Phys. Soc. 52, 195 (2007); 53, 168 (2008); 54, 145 (2009). ** I. V. Igumenshchev et al., Phys. Plasmas 17, 122708 (2010).

Outline

TC9761a

• Introduction

• Modeling CBET



• Conclusions

The CBET numerical algorithm considers pairwise interactions of light rays

TC9762

* C. J. Randall, J. R. Albritton, and J. J. Thomson, Phys. Fluids 24, 1474 (1981). † E. A. Williams et al., Phys. Plasmas 11, 231 (2004). ** I. V. Igumenshchev et al., Phys. Plasmas 17, 122708 (2010).

Probe i ray

Target

Pump j rays

–dd,I

I L–iij ij

1#= /

ie

i

– /

/

1mI IL

m ce k

n nn n

1–1

pe cr

crij j

L a

e

e e2 3

2

2

2#

m~ | |

| |= + +

+1^ h= G

–

–k k kprobe pump

probe pump

a

a

~ ~ ~=

=4

Three-wavematchingcondition

• The CBET model*† is implemented in LILAC absorption package assuming spherical symmetry**

An ion-acoustic wave saturation model is required to match the scattered-light power for intensities I L 4 × 1014 W/cm2

TC9763

• The amplitude of ion-acoustic waves is limited by clamping electron- density fluctuations*

• The value of the clamping parameter (dn/ne)clamp is determined by fitting the simulation results with the scattered-light measurements

– for CH ablators: (dn/ne)clamp ≈ 0.1%

, , , , ...dd,I

I IF k n T nn

e ee

ia aj

iji j#~ d= _ ci m/

, , , , ... I Inn G k n Te

e eij

a a i j#d ~=c _m i

,minnn

nn

nn

e e clamp eij ij

d d d=c c cm m m* 4R

*P. Michel et al., Phys. Rev. Lett. 102, 025004 (2009).

Outline

TC9761b

• Introduction

• Modeling CBET



• Conclusions


350.6

350.8

351.0

351.2

351.4

0.0 0.4 0.8

Time (ns)

1.20.0 0.4 0.8

Time (ns)

Wav

elen

gth

(n

m)

1.2

ExperimentalSimulatedwithout CBET

Center-beam rays

52503

0.0 0.4 0.8

Time (ns)

1.2

Simulatedwith CBET

Simulations including CBET agree well with scattered-light spectral measurements

E19972c

CBET extracts the energy from the center-beam incoming rays and transfers it to outgoing rays.

CBET reduces the absorption by ~10%, but the implosion hydrodynamic efficiency is reduced by ~20%

TC8628a

Energy deposition area is shifted outward, reducing hydrodynamic efficiency.

550500

nc

Targetcenter

M = 1nc/4

With CBET

Without CBET

Absorption and transferredenergy rates at t = 0.5 ns

450 600

•d

E/d

V (

rela

tive

un

its)

R (nm)

0.2

0.0

1.0

0.8

0.6

0.4

Laser

Energy transferredfrom incomingto outgoing light

Laser coupling at intensities up to I ~ 6 × 1014 W/cm2 is accurately predicted by the CBET model

TC9764

2.61018

1019

1020

1021

2.8 3.0

Time (ns)

Neu

tro

n r

ate

(s–1

)

3.2 3.4

Bang-time measurements

63912Simulated with CBET

SimulatedwithoutCBET Measured

00 1 2 3

100

200

300

Time (ns)

400

Rad

ius

(nm

)Ablation-front trajectory

inferred from x-ray images

0

4

8

12

Po

wer

(T

W)

Laser

SimulatedwithoutCBET

Simulatedwith CBET

63912

Measured


TC9764a

Po

wer

(T

W)

Scattered-light measurements

Measured63912

Simulatedwith CBET

0

1

2

3

4

Simulatedwithout CBET

0.0 1.0 1.5 2.0 2.50.5

Time (ns)


TC9764b

The accuracy of the CBET model was demonstrated using OMEGA implosions with different pulse shapes and targets.

Bang-time measurements Scattered-light measurements

2.81019

1020

1021

1022

3.0Time (ns)

Simulatedwith CBET

Neu

tro

n r

ate

(s–1

)

3.2 3.4

Simulatedwithout CBET

Measured

130 ps

0.00 1

Time (ns)2

0.2

0.4

0.6

0.8

1.0Shot 60000

Rel

ativ

e p

ow

ers

3/2~

CBETmodelExp.scattered

light

Exp. scattered light + f × (3/2~)

High-intensity implosions (I ~ 1015 W/cm2) show disagreements with the CBET model

TC9864

The missing scattered light may be caused by– two-plasmon-decay instability*– enhanced absorption in laser hot spots**

* W. Seka, U06.00005 ** A. V. Maximov, UO6.00007

Outline

TC9761c

• Introduction

• Modeling CBET



• Conclusions

Target

Beam 1

Beam 2

Edge-beam ray

CBET region

Center-beam ray

CBET can be mitigated in symmetric direct-drive implosions by reducing the energy in beam edges

TC9767

Target

Beam 1

Center-beam ray

Beam 2

Edge-beam ray

CBET region


TC9767a

Target

Beam 1

Center-beam ray

Beam 2

Edge-beam ray

10

20

30

00.7

CBET

No CBET

1.00.4

Sca

tter

ed e

ner

gy (

%)

No

min

al

CBET region

Rbeam*/Rtarget


TC9767b *Rbeam defined at 95% energy

Target

Beam 1

Center-beam ray

Beam 2

Edge-beam ray

10

20

30

00.7

CBET

No CBET

1.00.4

Sca

tter

ed e

ner

gy (

%)

2

4

6

8

0

No

nu

nifo

rmit

ies

(%)

No

min

al

CBET region

Rbeam*/Rtarget


TC9767c

Simulations suggested an optimum neutron yield can be achieved on OMEGA by reducing the laser beam to Rbeam/Rtarget ~ 0.8.

*Rbeam defined at 95% energy

Experiments* on OMEGA are investigating the optimum laser-beam diameter by balancing CBET with nonuniformities in low-adiabat implosions

TC9768*D. Froula, UO6.00009

Small phase plates

Beam profiles for different defocusing

860 nm

950 nm

1000 nm

Fixed targets Fixed beamsStandard SG4 phase plates

Rbeam/Rtarget = 0.97



10–2

10–1

100

10–30.4 1.00.7

No

rmal

ized

inte

nsi

ty

Rbeam/Rtarget

2000 600400Rbeam(nm)

Rbeam/Rtarget= 0.9

0.70.5

Not optimal single-beam uniformity Optimal single-beam uniformity

I á 4.5 × 1014 W/cm2

Experiments with small beams recover the red-shifted part of the spectrum

E20481b

Rbeam/Rtarget = 1.0 Rbeam/Rtarget = 0.5

–0.40 1 2 3

–0.2

0.0

0.2

0.4

Wav

elen

gth

(n

m)

–0.4

–0.2

0.0

63178 63183

0.2

0.4W

avel

eng

th (

nm

)

Time (ns)0 1 2 3

Time (ns)

Maximum CBET Reduced CBET

Experiment

Simulations

2.2

1.4

log10 (I)

0.6

Red-shiftedlight frombeam center

The scattered light decreases rapidly with reduced beam size

TC9770

0.60.0

Sca

tter

ed-l

igh

t fr

acti

on

0.1

0.2

0.3

0.4



Measurements

0.8

Rbeam/Rtarget

1.0 1.2

Less CBET

The increased absorption results in earlier bang time

TC9860

0.6

2.8B

ang

tim

e (n

s)

3.0

3.2

3.4

3.6



Measurements

0.8Rbeam/Rtarget

1.0 1.2

Bang times inferred fromneutron timing dataBang-time measurements

3.01018

1019

1020

1021

3.2 3.4Time (ns)

Neu

tro

n r

ate

(s–1

)

3.6 3.8

SimulatedwithoutCBET

63178 Simulatedwith CBET

Measured

Rbeam/Rtarget = 1

Bang time shifts ~20% earlier, indicating increasing hydro efficiency.

Higher implosion velocities are achieved with smaller beams

TC9771

0.60.4150

Vim

p (

km/s

)165

180

195

210

0.8

Implosion velocitiesShell trajectories

Rbeam/Rtarget

1.0 1.2

Measurements

10100

200

300

400

2

Time (ns)

Rad

ius

(nm

)

3


Rbeam/Rtarget = 1.0



Predicted effects of small beams are consistent with scattered-light, bang-time, and shell trajectory measurements.

Rb/Rt = 0.50 0.65 0.74 0.880.88 1.001.00 1.101.10

X-ray framing-camera images at the same target radius

1.20

2

4

6

0.8

Noise

Am

plit

ud

e (n

m)

Pre

dic

ted

dep

osi

tio

n v

rms

(%)

Rbeam/Rtarget

0.40

10

Ablationsurface

20

30

More perturbations

Ablationsurface

Smaller beams introduce more nonuniformities caused by the laser-beam geometry

TC9772

• For beam radii < 70% to ~80% of the target radius, significant nonuniformities develop

• Neutron yields in these experiments are affected by single-beam nonuniformities

Experiments* on OMEGA are investigating the optimum laser-beam diameter by balancing CBET with nonuniformities in low-adiabat implosions

TC9884*D. Froula, UO6.00009

860 nm

950 nm

1000 nm

Fixed beamsStandard SG4 phase plates




10

20

30

00.7

CBET

1.00.4

Sca

tter

ed e

ner

gy (

%)

2

4

6

8

0

No

nu

nifo

rmit

ies

(%)

No

min

al

Rbeam/Rtarget

Optimal single-beam uniformity

Neutron yield sensitivity was addressed in experiments with varying target size

TC9861

Experiments demonstrate beneficial effects of reducing beam sizes.

0.24

0.00

0.08Rel

ativ

e yi

eld

0.16

Measured yield normalized tosimulations without CBET

assuming Rbeam/Rtarget = 1

Rbeam/Rtarget

0.80 0.85 0.90 0.95 1.00

Measured yield normalized to simulations with CBET

Rbeam/Rtarget

0.80 0.85 0.90 0.95 1.00

Similarnonuniformities

CBET can be mitigated by using multiple-color laser beams

TC9774

Separation of the wavelengths by Dm > mL(ca/c) ~ 5 Å(for a 351-nm laser) reduces the CBET by a factor of 2.

I

3510 m (Å)

DmI

3510 m (Å)

Two-color split

Same wavelengthbeams are stronglycoupled

Beams with separatedwavelengths weakly interact with each other

0A

bso

rpti

on

fra

ctio

n0.70

0.75

0.80

0.85

10

SSD

1/2 CBET

Dm (Å)

20

~10% increase

Future work

TC9856

• Implementation of the CBET model in 2-D* to simulate polar-drive designs

• Using truncated phase plates to mitigate CBET

• Optimization of phase plates for polar drive when including CBET

*J. A. Marozas, PO8.00003

TC9760

Mitigation strategies are being tested on OMEGA.

Summary/Conclusions

Crossed-beam energy transfer (CBET) can reducethe performance of direct-drive ICF implosions

• CBET is observed in time-resolved reflected-light spectra as a suppression of red-shifted light during the main laser drive

• CBET extracts energy from the center-beam incoming light and transfers it to outgoing light, reducing the laser absorption and hydrodynamic efficiency

• CBET can be reduced

– using beams smaller than the target diameter

– using laser beams with two or more colors

Crossed-Beam Energy Transfer for Direct-Drive Implosions

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Transcript of Crossed-Beam Energy Transfer for Direct-Drive Implosions