W. TheobaldUniversity of RochesterLaboratory for Laser Energetics

44th Annual Anomalous Absorption Conference

Estes Park, CO8–13 June 2014

Strong-Shock Generation and Laser–Plasma Interactions for Shock-Ignition Inertial Fusion

High-intensitybeams

Shockwave

Solidtarget 1.0

100

200

300

Absorbed intensity at nc (1015 W/cm2)

Pea

k ab

lati

on

pre

ssu

re (

Mb

ar)

1.5 2.0

1-ns squareShaped pulse (2 ns)Shaped pulse (1.4 ns)

FSC

FSC

High-intensity shock-ignition (SI) experiments on OMEGA provide valuable data on hot-electron production and shock ablation pressure

Summary

• Multiple overlapping high-intensity beams generate an energetic hot-electron distribution (<100 keV) with a conversion efficiency of up to ~10%

• Turning off smoothing by spectral dispersion (SSD) in overlapping high-intensity beam experiments increased the hot-electron fraction and shock strength

• Stimulated Raman scattering (SRS) increases significantly (~5×) when SSD is turned off, while two-plasmon–decay (TPD) is unaffected

• Moderate hot electrons may be beneficial to shock ignition by coupling energy to the outer layer of the imploding capsule

E23174

The inferred ablation pressures of 270 Mbar at ~3 × 1015 W/cm2 approaches the minimum SI requirement of 300 Mbar.

FSCCollaborators

R. Nora,* M. Lafon, K. S. Anderson, M. Hohenberger, F. J. Marshall, D. T. Michel, T. C. Sangster, W. Seka, A. A. Solodov, C. Stoeckl, B. Yaakobi, and R. Betti*

University of RochesterLaboratory for Laser Energetics

*also Departments of Mechanical Engineering and Physics

A. Casner and C. ReverdinCEA, DAM, DIFArpajon, France

X. Ribeyre and A. ValletCELIA

University of Bordeaux, France

J. PeeblesUniversity of California, San Diego

La Jolla, CA

M. S. WeiGeneral AtomicsSan Diego, CA

Sufficient ablation pressure and Thot < 150 keV must be demonstrated for shock ignition during the ignitor spike

TC8918d

FSC

*R. Betti et al., Phys. Rev. Lett. 98, 155001 (2007).

00

100

200

300

400

5 10Time (ns)

Po

wer

(T

W)

Ignitor spike~5 × 1015 W/cm2

Standardpulse

Assembly

Spikeshock wave

Returnshock

• Critical issues for shock ignition

– demonstrate ~300- to 400-Mbar spike-generated ablation pressure

– demonstrate hot-electron temperatures of ≤150 keV generated by spike

SI requires minimum ablation pressures of ~300 Mbar; up to ~600 Mbar is required for large ignition margins

E23176

Minimum ablation pressure for SI ~ 2× (PA main drive) ~300 Mbar.Ablation pressure for the National Ignition Facility (NIF) 700-kJ polar-drive (PD) SI design with an ignition threshold factor (ITF) = 4 ~ 600 Mbar.

0.00 0

20

40

60

80

100

120

140PA

1

2

Den

sity

(g

/cm

3 )

3

4

5

6

7

0.5Mass coordinate (mg)

0.00 0

200

400

600

800

PA

20

15

10

5

0.5Mass coordinate (mg)

Main drive beforepower spike

(NIF 700-kJ SI design)*During power spike

Iabs = 6.7 × 1015 W/cm2cr

Pre

ssu

re (

MB

ar)

PressureDensity

FSC

*K. S. Anderson et al., Phys. Plasmas 20, 056312 (2013).

Laser–plasma interaction during the spike pulse and hot-electron generation are important issues for shock ignition

TC7870i

If the tR is high enough when hot electrons are produced, they will be stopped in the outer regions of the shell.

10.00

2

3

1

4

5

0

40

60

20

80

100

10.5 11.0

Time (ns)

Inte

nsi

ty (

1015

W/c

m2 )

tR

(m

g/c

m2 )

10.20

10

40

30

20

50

60

10.4 10.6

Shock-launching time (ns)

Marginallyigniting

(no hot e–)

Gai

n

tR

tR range of 100-keV e–

IL

Shock-ignition targetwith 350-kJ total energy

Thot = 170 keVEhot/Espike = 0.17

Boostedmargin

(with hot e–)

R. Betti et al., J. Phys., Conf. Ser. 112, 022024 (2008);S. Gus’kov et al., Phys. Rev. Lett. 109, 255004 (2012).

FSC

The 40 + 20 configuration was used to study the interaction with single high-intensity beams on OMEGA

E20035c

• Single-beam shock-ignition studies in spherical geometry up to ~8 × 1015 W/cm2

• 40 drive beams: distributed phase plate (DPP), distributed phase rotator (DPR) (no SSD), ~14 kJ

• 20 spike beams: no DPP, ~5 kJ

0.1

0.2

0.3

0.4

0.5

0.01 2 3 4

Variable delay

40 drivebeams

20 “spike” beams,tight focus

0

Bea

m p

ow

er (

TW

)

Time (ns)

CH

390 nm

34 nm

0.1 nm Al

25 atmD2 gas

FSC

W. Theobald et al., Phys. Plasmas 19, 102706 (2012).

Up to ~12% of the spike energy was converted into a moderate (~30-keV) hot-electron distribution

E23179

• Hot-electron generation in the 40 + 20 experiment is dominated by SRS

• The TPD instability is strongly suppressed

1

0

2

0.5

0.0

1.0 0.7

0.5

0.3

Focus ~140 nm Focus ~220 nm Focus ~580 nm

2.00

20

40

60

2.2Spike onset (ns)

T e (

keV

)

2.4 2.6 2.8 2.00

5

10

20

15

2.2Spike onset (ns)

Ho

t el

ectr

on

s (%

)

2.4 2.6 2.8

~0.9 × 1015 W/cm2

~1.5 × 1015 W/cm2

~1.5 × 1015 W/cm2 (DPP)~3.0 × 1015 W/cm2

Inte

nsi

ty o

n t

arg

et(×

1015

W/c

m2 )

Inte

nsi

ty o

n t

arg

et(×

1015

W/c

m2 )

Inte

nsi

ty o

n t

arg

et(×

1015

W/c

m2 )

FSC

At high laser intensity, stimulated Brillouin scattering (SBS) signal is emitted near nc /4 and TPD is suppressedFSC

1.5 × 1015 W/cm2

ne /nc0.23

0.10

0.040.03

349.5

350.0

350.5

351.0

351.5 8,000

Wav

elen

gth

(n

m)

100

200

300

400

02 84 60~

/2 s

ign

al (

arb

itra

ry u

nit

s)

Laser intensity (×1015 W/cm2)

TPD signature

0 1 2 3Time (ns)

6,000

4,000

2,000

7.6 × 1015 W/cm2

ne /nc0.23

0.10

0.040.03

12,000

0 1 2 3Time (ns)

10,000

6,000

8,000

2,000

4,000

2.9 ns2.7 ns2.5 ns2.3 ns

E20064a

Areal-density measurements show implosion degradation with increasing hot-electron productionFSC

120

140

100

80

60

40

0400300200 500 600 700 800 900

GtRH n

(m

g/c

m2 )

Ehot (J)exp

The areal density of OMEGA targets is too lowto stop hot electrons on the ablation surface.

TC10350a

A laser–plasma interaction experiment was performed in planar geometry with overlapping beams to infer ablation pressure and to study hot-electron generationFSC

Time (ns)

OMEGA pulse shape15

10

5

00 1 2 3

Pea

k in

ten

sity

(×10

14)

W/c

m2

Cone 3

Cone 1Cone 2

40 nmCH 30 nm

Mo138 nmquartz

Hardx rays

VISAR*SOP**

17.5 keVMo Ka

Highintensity

Lowintensity

Laserbackscatter

• Phase plates and DPR’s with ~900-nm focal spots were used in plasma-generating beams (cone 2 and cone 3)

• Phase plates with an ~600-nm focal spot were used in six high-intensity beams (cone 1)

• Ln ~ 350 nm M. Hohenberger et al., Phys. Plasmas 21, 022702 (2014). * Velocity interferometer system for any reflector ** Streaked optical pyrometerE20455e

The hot-electron temperature and hot-electron fraction increase with spike laser intensity

E20783

Co

nver

sio

n e

ffici

ency

(%

)

Spike intensity (×1014 W/cm2)5

1

0

2

10 15

T ho

t (ke

V)

Spike intensity (×1014 W/cm2)5

20

40

80

60

10 15

Bac

ksca

tter

wit

hin

the

foca

l co

nes

(%

)

Spike intensity (×1014 W/cm2)5

0

2

3

1

4

10 15

FSC

FSC

The shock propagation in quartz was observed with SOP and VISAR*

40

30

202 6 84

Time (ns)

Time (ns)

2

6

4

2

04 6 8Te

mp

erat

ure

(eV

)

Sh

ock

vel

oci

ty(n

m/n

s)8

4

0

2000

–200

0 2 4 6 8

VISAR

Laser

2000

–200

Inte

nsi

ty(×

1014

W/c

m2 )

y (n

m)

y (n

m)

SOP

*J. E. Miller et al., Rev. Sci. Instrum. 78, 034903 (2007); P. M. Celliers et al., Rev. Sci. Instrum. 75, 4916 (2004); M. Hohenberger et al., Phys. Plasmas 21, 022702 (2014).E20451a

Two-dimensional DRACO simulations reproduce well the shock dynamics over a range of spike intensities

E20784

• The ablation pressure reaches ~75 Mbar at 1.2 × 1015 W/cm2

Sh

ock

rea

r b

reak

ou

t (n

s)

Spike intensity (×1014 W/cm2)

646

7

8

108

ExperimentSimulation

12 14 Pea

k ab

lati

on

pre

ssu

re (

Mb

ar)

Spike intensity (×1014 W/cm2)

64

40

20

60

80

108 12 14

Simulation

FSC

Ablation pressures in planar target experiments are limited by lateral heat losses

E23180

Higher ablation pressures can be achieved in spherical geometry.

Heat flux: Q = –lthdT

Target surface

Planar target

z (nm)

r (n

m)

00

200

400

600

800

1000

2000

1500

1000

500

3500

3000

2500

2000

1500

1000

500

y (n

m)

–1000

–500

0

500

1000

200 400 600 800 1000x (nm)

–1000 –500 0 500 1000

Temperature (eV) Temperature (eV)Spherical target

Laser beam

FSC

A new OMEGA platform has been developed to study the generation of strong shocks in spherical targets

E22457a

CH Ti (5%)

CH

OMEGA 60 Framing camera

Streaked x-rayspectrometer

Ti x ray

Ti x ray

430 nm

FSC

The small solid target was irradiated by 60 high-intensity beams equipped with small-spot phase plates

–400 –200

0.00.0

0.2

0.4

0.6

0.5

Time (ns)B

eam

po

wer

(T

W)

1.0 1.5

0

x (nm)

400200

Vertical

DataFit

–400 –200 0

x (nm)

y (n

m)

400

200

0

–200

–400

400200

ETP* with DPR and SSD

–4000.0

1.0

2.0

3.0

Sig

nal

(ar

bit

rary

un

its)

(×10

4 )

–200 0

x (nm)

400200

442 nm

Horizontal

492 nm

1-ns square pulseShaped pulse

E22458a

• Small-spot phase plates

• DPR’s

• SSD

• GIH ~ 3 × 1015 W/cm2

• Density scale length Ln 4c ~120 nm

*Equivalent target plane

FSC

One-dimensional LILAC simulations predict a strong spherical shock wave that converges in the center of the solid target

E22459a

00.1

1.0

10.0

100.0

100Radius (nm)

Mas

s d

ensi

ty (

g/c

m3 )

200

ns1.441.060.64

010

100

1000

100Radius (nm)

200

0100

1,000

10,000

100Radius (nm)

Sh

ock

pre

ssu

re(M

bar

)

Tem

per

atu

re (

eV)

200

FSC

An x-ray framing camera captured a short x-ray flash at the time when the shock converged in the center

E23233

0

0

200

Corona x-raysfrom laser

400

0

200

y (n

m)

x (nm)

400

0

200

400

1.13 ns1.13 ns 1.18 ns1.18 ns

x-ray flashfrom center of CH ball

1.28 ns

1.34 ns 1.39 ns 1.42 ns

1.44 ns 1.48 ns 1.54 ns

200 400 0 200 400 0 200 400

FSC

The x-ray flash was measured with a streaked x-ray spectrometer

E22461a

FiducialFiducial

1.2

3.4

3.0

2.6

2.2

1.6 2.0Time (ns)

Wav

elen

gth

(A

)

2.4 2.8 2.0 2.5 3.0 3.5

Shaped pulse

1.7

1.9

1.5

1.3

Intensity (1015 W/cm2)

X-r

ay fl

ash

(n

s)

X-ray flashX-ray flashStreak spectrometerFraming camera

FSC

Time (ns)

Ti Hea flash fromstreak camera

Framing-camera imagesfrom target center

Sig

nal

(ar

bit

rary

un

its)

1.3

0

100

200

1.4

53 ps

1.5 1.6 1.7

15 nm

HorizontalFitted curve

–20–40

–40

–20

0

0

x position (nm)

y p

osi

tio

n (n

m)

20

20

40 –400

1

2

Sig

nal

(×

104 )

(ar

bit

rary

un

its)

–20 0 20 4040

The x-ray flash was emitted from a small volume of ~103 nm3 in less than 50 ps

E22462a

FSC

–

–

m m m

ps ps

x 15 12 9

53 40

2 2

2 2

. .n n nD

pst 35. .D

^ ^^ ^

h hh h

Time integrated x-ray spectra reveal strong 1s–2p absorption features from Ti ions of various charge states

E23182

40

3

5

81s–2p absorptionfrom Be- to Ne-like Ti ions

Ti Hea Ti Lya

5Energy (keV)

Inte

nsi

ty (

×10

15 k

eV/k

eV)

6 7

Te ~ 10 to 100 eVSolid density Be- to Ne-like Ti ions

FSC

FSC

One-dimensional simulations reproduce the measured x-ray flash time when the flux limiter is adjusted to match the absorbed laser-power time history

E23184

0.00

5

10

15

20

25

30

0.2 0.4 0.6

Time (ns)

Incident

Measuredabsorbed

Simulated absorbedflux limiter = 0.068

Measured x-ray flash at 1.42 ns

Po

wer

(T

W)

0.8 1.0 1.2 1.4

Shot #69140

The last 400 ps of the laser pulse does not affect the x-ray flash time

E23187

Simulated P(t < t*) constrained by the measured laser absorption and x-ray flash.Simulated Pmax is constrained only by measured absorption.

0.00

50

100

150

200

250 Pmax

0.5

P

1.0

t*

Time (ns)

Ab

lati

on

pre

ssu

re (

Mb

ar)

Ab

lati

on

inte

nsi

ty (

×10

15 W

/cm

2 )

1.5

Shot 69140

2.00.0

0.5

1.0

1.5

2.0Determines x-ray flash

Absorbed laser intensity(absorbed power/critical surface)Simulated ablation pressure compatible with x-ray flash time

FSC

Peak ablation pressures of up to 270 Mbar were inferred from simulations constrained by the observables

E23188

FSC

1.0

100

200

300

Absorbed intensity at nc (1015 W/cm2)

Pea

k ab

lati

on

pre

ssu

re (

Mb

ar)

1.5 2.0

1-ns squareShaped pulse (2 ns)Shaped pulse (1.4 ns)

A plot of the ablation pressure versus the absorbed intensity shows the extrapolation required for theSI 700-kJ NIF point design

E22650a

The ablation pressure in the OMEGA experiments approaches the minimum requirements for SI of 300 Mbar. Demonstration of ~600 Mbar for the SI NIF design may require experiments on the NIF.

Pmin for SI

P ~ Iabs

P ~ Iabs2/3

0

200

400

600

800

1 2

Absorbed laser intensity (×1015 W/cm2)(absorbed power/critical surface area)

Ab

lati

on

pre

ssu

re (

Mb

ar)

3 4 5 6 7

*Spherical experiments PSpherical experiments Pmax

700-kJ NIFSI design

FSC

*K. S. Anderson et al., Phys. Plasmas 20, 056312 (2013).

The two-plasmon–decay (TPD) instability is important for the hot electron generation in the spherical strong-shock experiments

E23189

FSC

0.00.0

0.2

0.4

0.6

0.8

1.0

0.5Time (ns)

No

rmal

ized

val

ues

1.0 1.5

Laser powerHard x-ray signal3/2~ spectral power

~0, k0"

~e1, ke1" "

~e2, ke2

~e1 ~ ~e2 ~ ~0/2ne ~ nc /4

"

~0, k0"

3/2~0, k3/2

~e2, ke2"

Switching the SSD bandwidth off has a significant impact on the laser–plasma interaction

E23190

FSC

• Measured with nine channel imaging-plate diagnostic

• A hotter electron distribution and more hot electrons were produced when SSD was turned off

• Up to ~10% of the laser energy was converted into hot electrons

Off40

50

60

70

SSD

~3 × 1015 W/cm2

T ho

t (ke

V)

On Off40

500

1000

1500

2000

2500

SSDE

ho

t (J)

On

SRS increases significantly (~5× in FABS) when SSD is turned off*

E23226

700

Wav

elen

gth

(n

m)

650

600

5500 1

Time (ns)

E (arbitrary units) = 291 E (arbitrary units) = 1563

SSD

SRS

72676, FABS30

2

3

log10 (I)

2

1

3

log10 (I)

2

1

0 1Time (ns)

No SSD

72678, FABS30

2

0.210.21

0.170.17

0.130.13

ne /nc

0.210.21

0.170.17

0.130.13

ne /nc

*W. Seka et al., this conference

TPD is largely unaffected by SSD in contrast to backscattered SRS

E23227

720

700

680

238

Wav

elen

gth

(n

m)

Wav

elen

gth

(n

m)

236

234

232230

1.4 1.8 1.4Time (ns) Time (ns)

1.81.0

1.5

1

2

1

2

1.0 1.5

SSD No SSD

~/2 emission

3~/2 emission

1.0

1.0

FABS30 72676FABS30 72676 FABS30 72678FABS30 72678

7267672676 7267872678

1

2

3

1

2

3

E ~/2 (arbitrary units) = 483

ETPD (arbitrary units) = 420 ETPD (arbitrary units) = 328

E ~/2 (arbitrary units) = 445

The SBS backscatter signal is insensitive to SSD

E23228

Wav

elen

gth

(n

m)

351.4

351.0

350.6

0 1Time (ns)

SBBS = 10% (instant) SBBS = 12% (instant)

SSD

SBS

72676, SBS25

351 nm

I14 = 39

2

3

log10 (I)

2

1

3

log10 (I)

2

1

0 1Time (ns)

No SSD

72678, SBS25

2

I14 = 44

The increase in electron production correlates with an increase in the x-ray emission from the target center

E23191

FSC

*F. J. Marshall et al., Phys. Plasmas 5, 1118 (1998).

Time integrated x-ray microscope* data from the core center

Filter: 6.5 mils Be200 × 200-nm region Instrument spatial resolution ~7 nm

GMXI a image Median = 1Gamma = 2

7.5

6.0

4.5

3.0

1.5 0

0

2

–2

4

6

50 100

Ehot = 0.8 kJ

Ehot = 2.2 kJ

Position (nm)A

DU

/100

0150

ImageFit

The x-ray emission strongly increased when SSD was turned off

E23192

FSC

Off10

100

1000

10,000

25×

SSD

I ~ 3 × 1015 W/cm2

Noise floor

GM

XI s

ign

al (

arb

itra

ry u

nit

s)

On

An earlier flash time was measured when SSD was turned off

E23193

FSC

400 500

SSD off

(70±30) ps

SSD on

600

2.0

2.2

2.4

2.6

2.8

3.0

Diameter (nm)

X-r

ay fl

ash

(n

s)

E23174

Summary/Conclusions

FSC

High-intensity shock-ignition (SI) experiments on OMEGA provide valuable data on hot-electron production and shock ablation pressure

• Multiple overlapping high-intensity beams generate an energetic hot-electron distribution (<100 keV) with a conversion efficiency of up to ~10%

• Turning off smoothing by spectral dispersion (SSD) in overlapping high-intensity beam experiments increased the hot-electron fraction and shock strength

• Stimulated Raman scattering (SRS) increases significantly (~5×) when SSD is turned off, while two-plasmon–decay (TPD) is unaffected

• Moderate hot electrons may be beneficial to shock ignition by coupling energy to the outer layer of the imploding capsule

The inferred ablation pressures of 270 Mbar at ~3 × 1015 W/cm2 approaches the minimum SI requirement of 300 Mbar.

LILAC simulations indicate that hot electrons greatly enhance the shock pressure

E23271

Laser pulseAblation pressure without ThotAblation pressure with ThotShock pressure without ThotShock pressure with Thot

600

500

400

300

200

100

00.0 0.5 1.0

Time (ns)

Pre

ssu

re (

Mb

ar)

1.5 2.0

The simulated maximum ablation pressure with hot-electron deposition increases more than linear with the laser intensity

E23272

1100

200

300

400

500

2

Absorbed intensity at nc (×1015 W/cm2)

Max

PA

(M

bar

)

3 4

1.3~ Mbar IP 80maxA : abs^ h