Mitigation of Cross-Beam Energy Transfer
in Direct-Drive Implosions on OMEGA
OMEGA cryogenic ignition
hydro-equivalent design
tR = 300 mg/cm2, Vimp = 3.7 × 107 cm/s
In-flight aspect ratio
35
30
m = 48 ng
a = 1.7
25
0.85 Rt (CH)
Zooming (CH)
20
15
10
Current stability
threshold
m = 65 ng
a = 3.2
Hydro-equivalent
curve
CBET
No CBET
140
180
230
Ablation pressure (Mbar)
D. H. Froula
Plasma and Ultrafast Physics Group Leader
University of Rochester
Laboratory for Laser Energetics
55th Annual Meeting of the
American Physical Society
Division of Plasma Physics
Denver, CO
11–15 November 2013
Summary
Reducing cross-beam energy transfer (CBET) on OMEGA
will allow for more stable ignition-relevant implosions
• CBET can be mitigated by reducing the diameter of the laser beams
• Mitigating CBET increases the ablation pressure, allowing for thicker
shelled targets and higher adiabats
• Two approaches are being investigated on OMEGA to reduce
the laser beams
– smaller laser spots—reduced beam-to-beam overlap
– two-state zooming—increased single-beam imprint
Experiments to validate these schemes are underway.
E22428
Collaborators
T. J. Kessler, I. V. Igumenshchev, V. N. Goncharov, H. Huang,
S. X. Hu, E. Hill, J. H. Kelly, D. T. Michel, D. D. Meyerhofer,
A. Shvydky, J. D. Zuegel, and R. Epstein
University of Rochester
Laboratory for Laser Energetics
CBET reduces the energy coupled
to the fusion capsule
k1
CBET is spatially
limited near M ~ 1
k2
Energy is transferred
between beams
by ion-acoustic
waves
ka
Target
E19971d
CBET reduces the most hydrodynamically
efficient portion of the incident laser beams.
CBET modeling is required to match the experimental
observables (scattered light, implosion velocity,
and bang time)*
1225 ps
1280 ps
1330 ps
1385 ps 1825 ps 1880 ps 1930 ps 1985 ps**
Simulations (Nonlocal +
500
25
CBET models)
20
400
20
15
200
10
5
100
5
0
0
0
3
10
0
1
2
t (ns)
3
0
1
2
t (ns)
V (km/s)
Data
300
20
300
P (TW)
15
Simulations (Nonlocal +
CBET models)
400
25
15
200
10
Data
100
0
0
1
2
t (ns)
P (TW)
Simulations (Nonlocal +
CBET models)
25
R (nm)
P (TW)
760 nm
5
0
3
CBET reduces the ablation pressure by ~45%.
E22473a
*I. V. Igumenshchev et al., Phys. Plasmas 19, 056314 (2012).
**D. T. Michel et al., Rev. Sci. Instrum. 83, 10E530 (2012).
Experiments have demonstrated that CBET can be
mitigated by reducing the radius of laser beams
0.3
CBET model
0.2
0.8
0.1
0.0
0.4
0.7
0.9
0.6
0.8
1.0
Rbeam/Rtarget
1.0
1.2
In-flight aspect ratio
35
Absorption fraction
Scattered-light fraction
OMEGA cryogenic hydro-equivalent design
tR = 300 mg/cm2, Vimp = 3.7 × 107 cm/s
30
m = 48 ng
a = 1.7
25
20
15
10
Current
stability
threshold*
0.85 Rt (CH)
Hydroequivalent
curve
CBET
m = 65 ng
a = 3.2
No CBET
140
230
180
Ablation pressure (Mbar)
Reducing the radius of the beams will allow the thickness of the
shell and the adiabat to be increased in a hydro-equivalent design
but the reduced overlap uniformity may increase the imprint.
E20145g
*D. H. Froula et al., Phys. Rev. Lett. 108, 125003 (2012).
*V. N. Goncharov, GI3.00001, this conference (invited).
Simulations suggest that reducing the beam diameters
by 20% (Rb/Rt = 0.8) will have minimal impact on the hotspot symmetry
2-D DRACO simulations
(low-order nonuniformities only)
40
Rb/Rt = 1.07
YOC = 1.0
r (nm)
30
20
10
0
0
10
20 30
z (nm)
Rb/Rt = 0.8
YOC = 0.94
t (g/cm3)
361
Rb/Rt = 0.7
YOC = 0.62
t (g/cm3)
352
t (g/cm3)
426
241
234
284
122
117
142
1
0
0
40
50
0
10
20 30
z (nm)
40
50
0
10
20 30
z (nm)
40
50
Reducing the beam diameters by more than 20% significantly
degrades the target performance.
TC9894
I. V. Igumenshchev et.al., Phys. Plasmas 19, 056314 (2012).
Reducing the diameter of the laser beams beyond 20%
after a sufficient conduction zone is generated (“zooming”)
is predicted to maintain good low-mode uniformity
v - 17 nm
20
10
0
0
10 20 30 40
r (nm)
Rb/Rt = 1.0
20
15
r (nm)
30
Power (×1012 W)
r (nm)
rms deviation from round (v)
Rb/Rt = 0.7
10
5
0
0
1
2
3
Time (ns)
4
30
v < 3 nm
20
360
10
0
0
t (g/cm3)
10 20 30 40
r (nm)
240
120
0
E21317l
*I. V. Igumenshchev et al., Phys. Rev. Lett. 110, 145001 (2013).
Reducing the diameter of the laser beams beyond 20%
after a sufficient conduction zone is generated (“zooming”)
is predicted to maintain good low-mode uniformity
20
10
0
0
10 20 30 40
r (nm)
20
Rb/Rt = 1.0
Rb/Rt = 0.7
15
10
5
0
Rb/Rt = 1.0
r (nm)
v - 17 nm
30
Power (×1012 W)
r (nm)
rms deviation from round (v)
Rb/Rt = 0.7
0
1
2
3
Time (ns)
4
30
v < 3 nm
20
360
10
0
0
t (g/cm3)
10 20 30 40
r (nm)
240
Zooming from Rb/Rt = 1.0 to 0.7
r (nm)
30
120
20
0
10
0
0
E21317m
v - 12 nm
10 20 30 40
r (nm)
*I. V. Igumenshchev et al., Phys. Rev. Lett. 110, 145001 (2013).
Reducing the diameter of the laser beams beyond 20%
after a sufficient conduction zone is generated (“zooming”)
is predicted to maintain good low-mode uniformity
20
10
0
0
10 20 30 40
r (nm)
20
Rb/Rt = 1.0
Rb/Rt = 0.7
15
10
5
0
Rb/Rt = 1.0
r (nm)
v - 17 nm
30
Power (×1012 W)
r (nm)
rms deviation from round (v)
Rb/Rt = 0.7
0
1
2
3
Time (ns)
4
30
v < 3 nm
20
360
10
0
0
t (g/cm3)
10 20 30 40
r (nm)
240
Zooming from Rb/Rt = 1.0 to 0.7
r (nm)
30
v - 3.5 nm
120
20
0
10
0
0
E21317n
v - 12 nm
10 20 30 40
r (nm)
0
10 20 30 40
r (nm)
*I. V. Igumenshchev et al., Phys. Rev. Lett. 110, 145001 (2013).
Reducing the diameter of the laser beams beyond 20%
after a sufficient conduction zone is generated (“zooming”)
is predicted to maintain good low-mode uniformity
20
10
0
0
10 20 30 40
r (nm)
20
Rb/Rt = 1.0
Rb/Rt = 0.7
15
10
5
0
Rb/Rt = 1.0
r (nm)
v - 17 nm
30
Power (×1012 W)
r (nm)
rms deviation from round (v)
Rb/Rt = 0.7
0
1
2
3
Time (ns)
30
20
t (g/cm3)
360
10
0
0
4
v < 3 nm
10 20 30 40
r (nm)
240
Zooming from Rb/Rt = 1.0 to 0.7
r (nm)
30
v - 3.5 nm
v < 1.5 nm
120
20
0
10
0
0
E21317o
v - 12 nm
10 20 30 40
r (nm)
0
10 20 30 40
r (nm)
0
10 20 30 40
r (nm)
*I. V. Igumenshchev et al., Phys. Rev. Lett. 110, 145001 (2013).
Power (×1012 W)
Zooming can be implemented on OMEGA using a radially
varying phase plate and a dynamic near field
20
15
10
5
0
0
1
2
3
Time (ns)
4
eld
i
f
r
a
am
n
Dy
e
ic n
Zooming phase plate (ZPP)
Picket pulse
Main pulse
E22039b
D. H. Froula et al., Phys. Plasmas 20, 082704 (2013).
OMEGA cryogenic ignition
hydro-equivalent design
tR = 300 mg/cm2, Vimp = 3.7 × 107 cm/s
35
25
20
m = 65 ng
Hydro-equivalent
a = 3.2
curve
CBET
No CBET
140
180
230
Ablation pressure (Mbar)
25
20
15
10
e
bl
10
0.85 Rt (CH)
Zooming (CH)
30
ta
15
Current
stability
threshold
In-flight aspect ratio
30
m = 48 ng
a = 1.7
Hydro-equivalence for
current implosions
s
Un
In-flight aspect ratio
35
C
th ur
re re
sh nt
ol
d
Implementing zooming on OMEGA will provide
a more-robust implosion to hydrodynamic instabilities
1.5 2.0 2.5 3.0 3.5 4.0 4.5
Adiabat
tR/tR1-D
>0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
<0.50
Data
Both CBET mitigation strategies on OMEGA will allow the mass of the shell
and the adiabat to be increased while maintaining ignition-relevant conditions.
E22229b
T. C. Sangster et al., Phys. Plasmas 20, 056317 (2013).
Summary/Conclusions
Reducing cross-beam energy transfer (CBET) on OMEGA
will allow for more stable ignition-relevant implosions
• CBET can be mitigated by reducing the diameter of the laser beams
• Mitigating CBET increases the ablation pressure, allowing for thicker
shelled targets and higher adiabats
• Two approaches are being investigated on OMEGA to reduce
the laser beams
– smaller laser spots—reduced beam-to-beam overlap
– two-state zooming—increased single-beam imprint
Experiments to validate these schemes are underway.
E22428