V. N. Goncharov
University of Rochester
Laboratory for Laser Energetics
Hydroefficiency (Ekin /EL, %)
80 nm
Cross-Beam Energy Transfer Mitigation
in Cryogenic Implosions on OMEGA
3-D ASTER simulation
mass-density map
7
6
5
4
3
0.80
0.85
0.90
Rb /Rt
0.95
1.00
57th Annual Meeting of the
American Physical Society
Division of Plasma Physics
Savannah, GA
16–20 November 2015
1
Summary
Increased hydrodynamic efficiency by mitigating cross-beam energy transfer
(CBET) has been demonstrated in cryogenic implosions on OMEGA
• Target illumination with a focal spot size smaller than the target size (Rb /Rt < 1)
was used to mitigate CBET; the target size varied from Rt = 400 nm to 500 nm
to reduce Rb /Rt
• Current cryogenic implosions on OMEGA have reached Phs = 56!7 Gbar
ign
(Phs > 120 Gbar); implosions with convergence ratio (CR) < 17 and a > 3.5
proceed close to 1-D prediction (CRign > 22)
• Improving target performance with Rb /Rt < 1 on OMEGA will require reducing
long-wavelength nonuniformity seeded by power imbalance and target offset
TC12309a
2
Collaborators
S. P. Regan, T. C. Sangster, R. Betti, T. R. Boehly, M. J. Bonino,
E. M. Campbell, T. J. B. Collins, R. S. Craxton, A. K. Davis, J. A. Delettrez,
D. H. Edgell, R. Epstein, C. J. Forrest, D. H. Froula, V. Yu. Glebov, D. R. Harding,
S. X. Hu, I. V. Igumenshchev, R. T. Janezic, J. H. Kelly, T. J. Kessler, T. Z. Kosc,
S. J. Loucks, J. A. Marozas, F. J. Marshall, R. L. McCrory, P. W. McKenty,
D. T. Michel, J. F. Myatt, P. B. Radha, W. Seka, W. T. Shmayda, A. Shvydky,
S. Skupsky, C. Stoeckl, W. Theobald, F. Weilacher, and B. Yaakobi
University of Rochester
Laboratory for Laser Energetics
D. D. Meyerhofer
Los Alamos National Laboratory
J. A. Frenje, M. Gatu Johnson, and R. D. Petrasso
Massachussets Institute of Technology, Plasma Science and Fusion Center
S. P. Obenchain and M. Karasik
Naval Research Laboratory
3
The hot-spot pressure in an ignition design must exceed a threshold value
Current high-foot indirect drive
Required: Phs = 350 to 400 Gbar
Achieved: Phs = 230 Gbar
|no a ~ 0.7
500
Pth (Gbar)
400
• Pressure threshold for ignition
Energy-scaled current OMEGA (Ehs = 0.44 kJ)
Required: 140 Gbar
Achieved: 56!7 Gbar
|no a (energy scaled) ~ 0.7
300
200
• Generalized Lawson criterion*
| = Px/Px ign = ^tRh0.61 ^0.24 Y 16 /Mh0.34
Mitigated CBET
100
0
Pth ~ 1/ E hs
| X " NIF ~ E 0.37
0
20
80
60
40
Hot-spot energy Ehs (kJ)
100
Direct-drive designs are in a less-challenging hydrodynamic regime with CR K 22 and
Phs > 120 Gbar; indirect-drive–ignition targets require CR = 30 to 40 and Phs > 350 Gbar.
TC12311e
*R. Betti et al., Phys. Plasmas 17, 058102 (2010).
4
The cryogenic implosion campaign on OMEGA was designed to demonstrate
enhanced laser coupling by mitigating CBET
Beam 2
350
Target
Beam 1
Beam 2
CBET
Ablation pressure (Mbar)
CBET
300
Rb = Rt, no CBET
250
200
150
Rb = Rt
100
Rb /Rt = 0.5
Rb /Rt = 0.7
Rb /Rt = 0.8
50
0
1.0
Dt
1.5
2.0
2.5
3.0
3.5
Shell convergence ratio during laser drive
Db
Beam 1
TC12317a
5
Implosions with Rb /Rt < 1 reach a larger hydrodynamic efficiency
25
500
20
400
15
300
10
Data
200
5
100
0
0.0
0.5
1.0 1.5 2.0
Time (ns)
2.5
0
3.0
Power (TW)
Distance (nm)
600
Hydroefficiency (Ekin/EL, %)
Simulations
Rb = 380 nm Rb/Rt = 0.77: experimental condition
Rb = 500 nm Rb/Rt = 1.00: test run
7
6
5
4
3
0.80
0.85
0.90
Rb /Rt
0.95
1.00
TC12318a
6
Current cryogenic implosions on OMEGA have reached Phs = 56!7 Gbar
Typical error bar
(!300 eV Ti, !6 to 10 ps Dtburn, !0.2 to 1 nm R17)
70
Yield =
Phs (Gbar)
60
50
40
$ Dt
dt
burn
Yield ~ n D n T
30
20
Measured
yield
10
0
700
750
800
850
900
T2 f
Phs2
#V
hs
$V
hs
n D n T vv dV
vv
dV p Tt burn
2
T
Depends on
measured Ti
and Vhs
Measured
burnwidth
950 1000
Target diameter (nm)
• Target yield and hot-shot pressure degrade (relative to 1-D predictions)
with an increase in target diameter and a reduction in Rb /Rt*
TC12319a
*S. P. Regan et al., CI3.00005, this conference (invited).
7
Long-wavelength modes (1 < , < 5) cause a reduction
in peak pressure and burn truncation
On-target nonuniformities
caused by beam geometry,
power imbalance, beam mispointing
Illumination nonuniformity
3-D solid-sphere projection
ASTER* 3-D simulation of a CR = 20 cryogenic implosion Rb /Rt = 0.75
(10-nm offset, 15% power imbalance, 10-nm rms mispointing)
Density
g/cm3
240
80 nm
180
120
Peak neutron production
in 3-D
Density
g/cm3
375
300
225
150
60
75
0
0
Time of peak neutron production
in 1-D; bubble burst causes drop
in Phs and burn truncation
The nonuniformity spectrum shifts to more-damaging shorter wavelengths for smaller Rb /Rt (larger Rt).
TC12322a
*I. V. Igumenshchev et al., “Three-Dimensional Modeling of Direct-Drive
Cryogenic Implosions,” to be submitted to Physics of Plasmas.
I. V. Igumenshchev et al., UO4.00015, this conference.
8
Three-dimensional simulations predict an early burn truncation
because of long-wavelength, hot-spot distortion growth
Neutron rate (1/s)
1
ASTER,
1025
no perturbations
ASTER,
3-D perturbations
2
Density
1024
1023
2
1022
1021
2.4
1
2.5
2.6
2.7
Time (ns)
TC12376a
9
Measurements show earlier peak burn and burn truncation
Shot 76828
1-D bang time
1-D simulation
1023
Measurement
1022
1021
2.7
2.8
2.9
3.0
3.1
Time (ns)
3.2
3.3
Phs, 1-D (Gbar)
Neutron (1/s)
1024
Dt = 34 ps
70
60
50
40
30
20
10
0
1-D
bang time
Experimental
bang time
2.90
2.95
3.00
3.05
Time (ns)
Pressure evolves on an ~100-ps time scale; a tens of picoseconds shift in the temporal
sampling region makes a significant difference in the inferred pressure.
TC12324a
10
When the measured burn rate is included in the analysis, inferred Phs and tR
agree with 1-D predictions in implosions with CR < 17 and a > 3.5
1-D tR averaged over experiment burn
(with burn truncation)
1.1
1.0
1000 nm
960 nm
900 nm
860 nm
800 nm
Phs, 3-D Phs, 1-D
0.9
0.8
0.7
0.6
0.5
0.4
0.3
14
15
16
17
18
Convergence ratio
19
20
GtRHn, exp GtRHn,1-D
Inferred from experiment
1.0
0.9
0.8
0.7
0.6
0.5
0.4
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Adiabat
Because of a reduced beam overlap, long-wavelength nonuniformity increases with a reduction
in Rb /Rt, truncating burn earlier and reducing the observed Phs. Reducing beam power imbalance
and target offset are required to improve the target performance with Rb /Rt < 1.
TC12393
11
Summary/Conclusions
Increased hydrodynamic efficiency by mitigating cross-beam energy transfer
(CBET) has been demonstrated in cryogenic implosions on OMEGA
• Target illumination with a focal spot size smaller than the target size (Rb /Rt < 1)
was used to mitigate CBET; the target size varied from Rt = 400 nm to 500 nm
to reduce Rb /Rt
• Current cryogenic implosions on OMEGA have reached Phs = 56!7 Gbar
ign
(Phs > 120 Gbar); implosions with convergence ratio (CR) < 17 and a > 3.5
proceed close to 1-D prediction (CRign > 22)
• Improving target performance with Rb /Rt < 1 on OMEGA will require reducing
long-wavelength nonuniformity seeded by power imbalance and target offset
TC12309a
12