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