Mitigation of Cross-Beam Energy Transfer
in Direct-Drive–Implosions on OMEGA
In-flight aspect ratio
OMEGA constant hot-spot pressure
(180 Gbar 1-D), areal density (tR = 300 mg/cm2)
implosion velocity, Vimp = 3.7 × 107 cm/s)
35
9 × 1014 W/cm2
Current hydro-equivalent design
(26 kJ, m = 48 ng, a = 1.7)
30
Increasing shell thickness
(shell stability)
25
Current
stability threshold
20
15
10
No CBET
(26 kJ, m = 65 ng, a = 3.2)
140
180
220
Ablation pressure (Mbar)
D. H. Froula
University of Rochester
Laboratory for Laser Energetics
44th Annual Anomalous
Absorption Conference
Estes Park, CO
8–13 June 2014
Summary
Hydrodynamic equivalence on OMEGA will require
reducing cross-beam energy transfer (CBET) and/or
improving the stability threshold
• CBET reduces the ablation pressure by over 50% in hydroequivalent designs
• Experiments have demonstrated increased hydroefficiency
with reduced focal-spot size
• Calculations suggest that zooming can recover all of the ablation
pressure lost to CBET without negatively impacting the hydro stability
• A full-aperture zooming scheme is being developed that uses
bandwidth to control the focal-spot size and could provide more
on-target energy with full laser-beam smoothing
E23238
Collaborators
T. J. Kessler, A. K. Davis, G. Fiksel, R. K. Follette, D. H. Edgell,
I. V. Igumenshchev, V. N. Goncharov, R. J. Henchen, H. Huang,
S. X. Hu, J. H. Kelly, D. T. Michel, T. C. Sangster, A. Shvydky,
W. Seka, C. Stoeckl, and B. Yaakobi
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 +
25
CBET models)
500
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).
CBET reduces the ablation pressure by over 50%
in hydro-equivalent OMEGA designs
In-flight aspect ratio
OMEGA constant hot-spot pressure
(180 Gbar 1-D), areal density (tR = 300 mg/cm2)
implosion velocity, Vimp = 3.7 × 107 cm/s)
35
9 × 1014 W/cm2
Current hydro-equivalent design
(26 kJ, m = 48 ng, a = 1.7)
30
25
Current
stability threshold**
20
15
10
No
CBET
140
180
220
Ablation pressure (Mbar)
E22915e
*Goncharov PoP (2014).
**Sangster PoP (2013).
Experiments have demonstrated that CBET
can be mitigated by reducing the energy that
propagates past the target
0.7
0.2
0.8
0.1
0.9
0.0
0.4
E20145i
0.6
0.8
Rb/Rt
1.0
1.0
1.2
2.9 ns
195
Bang time
0.3
Vimp (km/s)
210
Absorption fraction
Scattered-light fraction
CBET model
Measurements
180
165
150
0.4
3.6 ns
0.6
0.8
Rb/Rt
1.0
1.2
D. H. Froula et al., Phys. Rev. Lett. 108, 125003 (2012);
D. T. Michel et.al., Rev. Sci. Instrum. 83, 10E530 (2012).
0.7
Convergence ratio = 2.5
0.9
0.6
0.8
Rb/Rt =0.5
30
6
25
20
4
15
10
2
5
0
0.4
0.8
Rb/Rt
Illumination
nonuniformity (%)
1.0
rms deviation from round (nm)
The reduced-beam overlap results in nonuniformities
on the imploding shell
0
1.2
Reducing the beam diameters is a trade-off between improved
coupling (thicker shells) and increased low-mode nonuniformity.
U1265g
D. T. Michel et al., Rev. Sci. Instrum. 83, 10E530 (2012).
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
Rb/Rt = 0.8
YOC = 0.94
Rb/Rt = 0.7
YOC = 0.62
t (g/cm3)
285
20
140
10
0
425
0
0
10
20 30
z (nm)
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.
TC9894a
I. V. Igumenshchev et al., Phys. Plasmas 19, 056314 (2012).
• The new distributed phase plates (DPP’s)
(R95 = 400 nm, nSSD = 5) will have
improved azimuthal symmetry and
reduced tails
Intensity
New phase plates are currently being fabricated for
OMEGA that will provide the flexibility to vary the target
diameter while maintaining relevant intensities
10–2
10–3
• Experiments will scale the target
radius to test a range of CBET
reduction options
10–4
y far field (nm)
Experiments with Rt = 480 nm
(Rb/Rt = 0.8) will recover half
of the ablation pressure lost
to CBET.
DPP far field, no smoothing by
spectral dispersion (SSD)
1
10
SG4 (n = 7)
100
Analytic (n = 9)
Design (n = 9)
10–1
0
200
400
Radius (nm)
400
0
–400
–400
E22925
0
400
x far field (nm)
600
To reduce the laser spot without introducing nonuniformities,
the diameter of the laser beams must be reduced after
a sufficient conduction zone has been developed
20
10
0
0
10
20 30
r (nm)
40
20
Rb /Rt = 1.0
Rb /Rt = 0.7
v < 3 nm
30
15
10
t(g/cm3)
20
360
10
5
0
Rb /Rt = 1.0
r (nm)
v - 17 nm
30
r (nm)
Power (×1012 W)
rms deviation from round (v)
Rb /Rt = 0.7
0
1
2
Time (ns)
3
4
0
0
10
20 30
r (nm)
40
240
Zooming from Rb /Rt = 1.0 to 0.7
30
r (nm)
120
v < 1.5 nm
20
0
10
0
0
10
20 30
r (nm)
40
Zooming after the third picket is predicted to maintain good low-mode uniformity.
E21317t
I. V. Igumenshchev et al. Phys. Rev. Lett. 110, 145001 (2013).
Power (TW)
Zooming could 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
320 mm
A ZPP design has a region of high-spatial-frequency phase to produce
a large spot and a region of low-frequency phase to produce a small spot.
E22039d
*D. H. Froula et al., Phys. Plasmas 20, 082704 (2013).
The smaller-diameter laser beams used during the
pickets increase the power spectrum over the modes
with the highest Rayleigh–Taylor growth rates
Single-beam power spectrum
Analytic calculations*
Sub-aperture imprint
experimental setup
10–1
Half aperture
Full aperture
100-ps imprint beam
1-ns drive beams
Aperture
10–2
Full-diameter
beam
10–3
0
40
80
Mode ,
120
DPP
Half-apertured
beam
Focusing lens
15-nm
CH target
X-ray framing
camera (XRFC)
The effect of increased power spectrum resulting from the
reduced beam diameters was tested in planar experiments.
E22168a
*R. Epstein, J. Appl. Phys. 82, 2123 (1997).
540 nm
CH full aperture
540 nm
CH half aperture
E22927b
Optical density spectral power (×104)
The increased power spectrum was measured
to produce increased imprint levels over the
mid-frequency modes
Measured imprint*
CH full aperture
CH half aperture
10
8
6
4
2
0
0
40
80
Mode ,
120
160
*Experiments by G. Fiksel.
540 nm
CH full aperture
540 nm
CH half aperture
540 nm
CH-Pd* half aperture
E22927a
Optical density spectral power (×104)
X rays from a thin, high-Z layer (600-Å Pd)
were used to reduce the imprint*
Measured imprint**
CH full aperture
CH half aperture
CH-Pd half aperture
10
8
6
4
2
0
0
120
160
80
Mode ,
Pd mitigates the high frequency growth (>80) but the
increased mode ~30 perturbation is still a concern.
40
*M. Karasik et al., Bull. Am. Phys. Soc. 58, 370 (2013).
**Experiments by G. Fiksel.
A multipulse driver line is currently being implemented
on OMEGA to support CBET mitigation projects
Dynamic bandwidth reduction
Power (TW)
20
Bandwidth
on
Dynamic near field
Bandwidth
off
15
Picket pulse
10
5
0
Main pulse
0
1
2
3
Time (ns)
4
• The reduced bandwidth during the main drive leads to ~10% higher
frequency conversion (~28-kJ total energy)
• The near-field laser profiles will be independent, allowing spherical
experiments to test imprint mitigation schemes
Mitigation of imprint from sub-aperture beams
could lead to coaxial zooming in FY16.
E22928
A full-aperture zooming scheme is being developed
that uses bandwidth to control the focal-spot size
and could provide more on-target energy (28 kJ)
with full laser-beam smoothing
• A new optic is under development that
uses dynamic bandwidth reduction to
control the spot size of the laser
• A model of the new zooming/
smoothing scheme will be
– integrated into our hydrocodes
to assess their performance
– used to optimize the design
of the new optics
OMEGA hydro-equivalent curve
(tR = 300 mg/cm2, Vimp = 3.7 × 107 cm/s)
35
9 × 1014 W/cm2
Current design
30
Rb/Rt
In-flight aspect ratio
• Full-aperture zooming will provide the
flexibility to increase target diameter
– larger hot spot for improved
stability threshold
– reduced hot-electron fraction
25
Rb/Rt
20
15
10
Full-aperture zooming
(26 kJ, m = 65 ng, a = 3.2)
140
180
220
Ablation pressure (Mbar)
Full-aperture zooming provides a viable path to hydro-equivalence
but will likely require multilayer targets to mitigate TPD.
E22929a
Summary/Conclusions
Hydrodynamic equivalence on OMEGA will require
reducing cross-beam energy transfer (CBET) and/or
improving the stability threshold
• CBET reduces the ablation pressure by over 50% in hydroequivalent designs
• Experiments have demonstrated increased hydroefficiency
with reduced focal-spot size
• Calculations suggest that zooming can recover all of the ablation
pressure lost to CBET without negatively impacting the hydro stability
• A full-aperture zooming scheme is being developed that uses
bandwidth to control the focal-spot size and could provide more
on-target energy with full laser-beam smoothing
E23238