C. RenUniversity of RochesterLaboratory for Laser Energetics

46th Annual Anomalous Absorption Conference

Old Saybrook, CT1–6 May 2016

Simulation of Stimulated Brillouin Scattering and Stimulated Raman Scattering in Shock Ignition

1

Transmitted laser intensity at n = 0.17 nc (from HLIP)

11.0

1.5

2.0

2.5

3.0

3.5

5 9

Incident intensity (×1015 W/cm2)

Tran

smitt

ed in

ten

sity

(×

1015

W/c

m2 )

13 17 21

Te = 3.5 keV, Ti = 1.6 keVTe = 1.6 keV, Ti = 550 eV

Laser–plasma instabilities below the quarter-critical surface are important in shock ignition

Summary

TC12764

• Particle-in-cell (PIC) and fluid simulations find that stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in the low-density region can cause significant pump depletion of the ignition pulse in shock ignition

• SBS is reduced by the plasma flow

• New simulations with both realistic seed levels and nonlinear physics are needed

2

Collaborators

L. Hao, J. Li, W.-D. Liu, and R. Yan

University of RochesterLaboratory for Laser Energetics

We thank the UCLA-IST Consortium for the use of the particle-in-cell code OSIRIS*

3

*R. Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002).

The 40 + 20 spherical shock-ignition experiment on OMEGA used separate compression and ignition beams

TC12736

• 60 OMEGA beams were split into 40 low-intensity drive beams (~14 kJ) and 20 tightly focused, delayed beams (~5 kJ)

Target design and laser pulse shape*

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

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

4

Simulation parameters similar to the 40 + 20-beam shock-ignition (SI) experiments on OMEGA*

TC12737

• Ln = 170 nm

• Ux/c = 4.26 × 10–6 x (nm) – 0.00356

• Two temperatures:

Te/Ti = 3.5 keV/1.6 keV = 2.2 (HT)

Te/Ti = 1.6 keV/0.55 keV = 2.9 (LT)

• I = (2 to 20) × 1015 W/cm2

HT: high temperature LT: low temperature * W. Theobald, et al., Phys. Plasmas 19, 102706 (2012). ** R. Yan, J. Li, and C. Ren, Phys. Plasmas 21, 062705 (2014).

Typical plasma profile in shock ignition**

00.0

0.1

0.2

0.3

0.4

200 400

Plasma profile

ne

(nc)

ux

(×10

–3 c

)

x (nm)600 800

–4

–3

–2

–1

0neux

5

6

Ion-density fluctuation

4

GEx H2 GEx H[×10–4(mec~0/c)2]

2

1.51.0

0.5

1.5

1.0

0.5

0.000

0.004

0.008

dn (nc)

2

6

–0.05

0 to

5

5 to

10

10 to

25

25 to

50

50 to

100

100

to 1

50

150

to 2

00

200

to 2

50

250

to 3

00

0.00

0.05

0.10

0.15CollisionlessCollisional

4

2

200x (c/~0)

(keV)

400 600

200x (c/~0)

400 600 200x (c/~0)

400 600t

(ps)

t (p

s)

En

ergy

flu

x (i

np

ut

lase

r flu

x)

In a conventional inertial confinement fusion (ICF) scheme, laser–plasma interactions (LPI’s) at nc/4 reach a steady state

TC12738

• I = 6 × 1014 W/cm2 L = 150 nm Te = 3 keV Ti = 1.5 keV n = 0.21 to 0.27 nc

• Hot electrons are staged, accelerated from left to right

• Collisions can reduce hot electrons

R. Yan et al., Phys. Rev. Lett. 108, 175002 (2012).

6

Interplay of the modes at different densities leads to intermittent LPI activities at SI intensities

TC12739

• I = 2 × 1015 W/cm2 L = 170 nm n = 0.17 to 0.33 nc

– low T: Te = 1.6 keV Ti = 0.55 keV

– high T: Te = 3.5 keV Ti = 1.6 keV

• Significant pump depletion is seen at nc/4

R. Yan, et al., Phys. Plasma 21, 062705 (2014).

GEx H in x–t2

Low T

8

0.0000

0.0001

0.0002

0.0003

0.0004

6

4

2

0

20t (p

s)

15

10

5

00 500 1000 1500

High T

(mec~0/e)2

x(c/~0)

0.0000

0.0001

0.0002

0.0003

0.0004

7

A single Maxwellian fit Thot = 29.5 keV was consistent with the experimental values Thot = 30 to 40 keV

TC12742

0

10–3

10–2

10–1

100

50 100

Electron energy (keV)

En

ergy

flu

x (l

aser

-en

ergy

flu

x)

150 200 250 300

Thot = 29.5 keV

High-T case f50 = 19%, f100 = 8%Experimental measurement: f50 # 12%

8

SBS in the n = 0.015 to 0.17 nc region can cause significant backscattering—plasma flow is important

TC12743

One-dimensional PIC simulations, I = 2 × 1015 W/cm2, high-T*

L. Hao et al., Phys. Plasmas 23, 042702 (2016).

Without flow With flow

S/S0 (Normalized Poynting flux)

00

5

10

15

20

200 400x (nm)

t (p

s)

600 800 0 200 400x (nm)

600 800–2

–1

0

1

2

9

Significant pump depletion is seen at n = 0.17 nc

TC12744

00.0

0.2

0.4

Poy

nti

ng

flu

x fr

acti

on

0.6

0.8

1.0

5 10

t (ps)

15 20

Without flowWith flow

10

I = 2 × 1015 W/cm2

Significant SRS is also seen at high intensities

TC12745

I = 5 × 1015 W/cm2, low T 100 PPC = 64% total, 50% SBS, 14% SRS 1000 PPC = 50% total, 30% SBS, 20% SRS

PPC: particles per cell

1.5

1.0

Ref

lect

ivit

y o

f S

RS

0.5

0.00 5

Time (ps)

10 15 20

6

4

Tota

l ref

lect

ivit

y2

00 5

Time (ps)

10 15 20

kR > 0.52 ~0/c 100 PPCkR < 0.52 ~0/c 100 PPCkR > 0.52 ~0/c 1000 PPCkR < 0.52 ~0/c 1000 PPC

11

Fluid simulations with HLIP see smaller reflectivities

TC12746 *L. Hao et al., Phys. Plasmas 21, 072705 (2014).

0.0

0.4

0.8

2 × 1015 W/cm2

5 × 1015 W/cm2

No

rmal

ized

inte

nsi

ty

HT LT

PumpSRSSBS

0

0.0

0.4

0.8

200 400x (nm)

600 800 0 200 400x (nm)

600 800

12

Compared to HLIP, OSIRIS has kinetic and nonlinear physics but also higher seed levels for convective SRS and SBS

TC12747

• High seed levels can lead to high saturation levels for convective modes

• HLIP lacks nonlinear physics such as density-modulation–induced absolute SRS*

–12

0 200 400

x (nm)

No

rmal

ized

see

ds

of

scat

tere

d li

gh

t (l

og

10)

600 800

–10

–8HT

LT

–6

–4

–2

–12

–10

–8

–6

–4

–2

SBS seeds in HLIPSRS seeds in HLIPFitted SBS seeds in PICFitted SRS seeds in PIC

13

*J. Li, this conference.

Laser transmittance may be limited by LPI

TC12748

Transmitted laser intensity at n = 0.17 nc (from HLIP)

11.0

1.5

2.0

2.5

3.0

3.5

5 9

Incident intensity (×1015 W/cm2)

Tran

smitt

ed in

ten

sity

(×

1015

W/c

m2 )

13 17 21

Te = 3.5 keV, Ti = 1.6 keVTe = 1.6 keV, Ti = 550 eV

14

Open questions

TC12750

• Modeling of LPI coupling in the entire coronal region

– computation challenge (1020 FLOPS in 2-D)

– seed levels for convective modes

• Coupling LPI and hydro simulations

• Integrated design for ICF

FLOPS: floating-point operations per second

15

TC12764

16

Summary/Conclusions

Laser–plasma instabilities below the quarter-critical surface are important in shock ignition

• Particle-in-cell (PIC) and fluid simulations find that stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in the low-density region can cause significant pump depletion of the ignition pulse in shock ignition

• SBS is reduced by the plasma flow

• New simulations with both realistic seed levels and nonlinear physics are needed