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NAS/NAE Committee on the Prospects for IFE Systems
San Ramon, CA29 January 2011
Technology for Polar-Drive Ignition on the NIF
J. D. ZuegelUniversity of RochesterLaboratory for Laser Energetics
00
1
4
3
2
Divergence (nrad)
Far-
field
inte
nsi
ty (
×10
8 )
–100 100
E19667
Polar-drive ignition could be tested on the NIF with a few modest modifications to the facility
• Beam-smoothingimprovements:
– Multi-FM 1-D smoothing for spectral dispersion (SSD) provides the required beam smoothing with simple modifications to the NIF facility
– Beamsmoothingisonlyrequiredatthebeginningofthelaserpulse, which minimizes stress on the laser
– Polar-drive phase plate and polarization-smoothing designs are underway
– A NIF PD beam-smoothing demonstration on OMEGA EP is planned in FY12
• Direct-drivetargettechnology:
– NIF-scale fill-tube targets have been demonstrated and are being optimized
– Concepts for a polar-drive ignition target insertion cryostat (PD-ITIC) are being developed
Summary
E19668
Implementing polar drive (PD) requires five changes on the NIF for an ignition demonstration
5
34
New PD phase plates (2~)and polarization plates (3~)in final optics assembly
New PD ignition target insertion cryostat (PD-ITIC)
2 Add new SSD grating to 48preamplifier modules (PAM)
00
1
4
3
2
Divergence (nrad)
Far-
field
inte
nsi
ty (
×10
8 )
–100 100
Add multi-FM fiberfront end and combinewith existing system
1
E19669
Laser nonuniformity imprint is minimized by optimizing smoothing by spectral dispersion (SSD)
• SSDdivergence(DiSSD) determines the asymptotic uniformity• Increasingtheinversecoherencetime(tc–1) allows the target to
experience a smoother spot for a longer period
100
10–1vrm
s (%
), ,
-mo
des
30:
60
t (ps)103102101
t (ps)103102101
Single-beam laser nonuniformity
Increasing tc ? , × Dm × Ncc–1
Ncc = 2
Ncc = 8
Ncc = 4
DiSSD, Dmheld fixed
Dm / Bandwidth Ncc / Color cycles fm / Modulator frequency
Increasing
DiSSD ?Dm × Nccfm
E19682
MultiFM 1-D SSD provides required beam smoothing performance with minimal impact on the facility
• TraditionalSSDsystemsusingsingle-frequencyphasemodulationhavelow smoothing rates for many important spatial modes (, < 150)
• MultiFM1-DSSDisanewapproachthat – provides better smoothing rates with lower total bandwidth
(esp. for PD pulse shapes with picket prepulses) – can be implemented on NIF with simple modifications
1000
800
600
400
200
0100 101
t c
(GH
z)–1
102
, mode
MultiFM 1-D SSD(0.5 THz)
1-D SSD
2-D THz SSD
Inverse coherence time versus far-field spatial frequency
E19683
An optimized MultiFM configuration that achieves high gain in polar drive simulations has been identified
• MultiFM1-DSSDemploystechnologydevelopedforthetelecommunications industry
– 40-GHz phase modulators and drive electronics – UVbandwidth: Dftotal = 500 GHz (effective bandwidth) – SSDdivergence:DiSSD = 100 nrad (half angle at full beam)• DRACO2-Dsimulationswithallnonuniformitysources:Gain= 32
SSD beam divergence (DiSSD)
Do
2 (G
Hz)
Do1 (GHz)
500
500
400
400
300
300
200
200
100
1000
102
98
94
90
86
82
0.55
0.50
0.45
0.40UV
div
erg
ence
1/2i
(n
rad
)
vre
ach
ed(%
); l-
mo
de
= (
80:2
00)
0Do1 (GHz)
5004003002001000
MultiFMoptimum
MultiFMoptimum
Laser uniformity (vrms,80:200)
E19671
DynamicBandwidthReduction(DBWR) minimizes stress on the laser with little affect on target gain
MultiFM 1-D SSD beam smoothing only needs to be applied to pickets in the polar-drive point design pulse shape.
000
20
40
60
80
100
120
20 40 60
r (nm)
z (n
m)
80 100 1200.0 0.0
0.4
0.8
1.2
1.6
2.0
0.5
1.0
1.5
2.0
2 4
Time (ns)
Relative gain with DBWRDensity at peak compression
No
rmal
ized
tar
get
gai
n
Po
wer
/bea
m (
TW
)
6
Gain = 32
8
300
150
0
t
24
6
8
Gain = 32
Integrate NIF PAM into OMEGA EP
Verify laser imprint with foil-targetexperiments on OMEGA EP**
In progress
Early FY12
Verify smoothing with equivalent target plane measurements*
No SSD With SSD
300 nm
Simulationof method
101
10–1
10–3
Am
plit
ud
e (n
m)
0.0 0.5 1.0 1.5Time (ns)
Rippled target
Laserimprint
Equivalent surfaceperturbation
Demonstrate MultiFM Seed Source
00
1
4
3
2
Divergence (nrad)
Far-
field
inte
nsi
ty (
×10
8 )
–100 100
E19672
A MultiFM 1-D SSD beam-smoothing demonstration on OMEGA EP will validate laser imprint performance
**T.R.Boehlyet al.,LaserPart.Beams18, 11 (2000).*S. P. Regan et al.,J.Opt.Soc.Am.B17, 1483 (2000).
E19673
The focal-spot conditioning strategy for polar-drive ignition includes phase and polarization plates
The NIF final optics assembly (FOA)willinclude: • Phaseplatebetweenthefrequencyconversioncrystals(2~) • Polarizationplate(3~)
Targetchamber
3~beam
1~beam
FOA
To target
Debris shieldsWedged focus lens
THG
SHGVacuum window
2~ phase plate
3~ polarization plateIntegratedOpticsModule(IOM)
Phase plates and polarization smoothing are being designed to efficiently and uniformly couple energy to polar-drive targets
E19681
• Phaseplatesefficientlydeliverlaserenergywithadesiredfocal pattern to achieve required irradiation uniformity.
• Polarizationsmoothinginstantaneouslyimprovestargetedmodes of focal-spot irradiance modulation.
x Targetplane
Lens
Phaseplate
“o”
“e”
Dx
x
Io e
Polarization smoothingFocal spots for polar-drive beams
Polar and mid-latitude(Rings 1 to 3)
xff (mm)–2
–2
2
2
0xff (mm)
–2 20
0
y ff (
mm
)
Equatorial(Rings 4 to 5)
Wedge of KDPid
Did
E19674
A NIF fill-tube target has been demonstrated that will be optimized to meet polar-drive ice layer specifications
Facility renovations and equipment upgrades are underway at LLE to demonstrate NIF PD cryogenic layering with DT targets.
Target: 2.95-mm OD, 20-nm wallFill tube: 30-nm OD at shell wall
Cryogenic layering sphere
D2 layer (297 nm) with mid-IR laser layering
Inner ice layer surfaceShadowgraph X-ray phase contrast
–20 –10 0 10 20 30
Deviations from sphericity in nm
E19675
A polar-drive ignition target insertion cryostat (PD-ITIC) will minimize the impact on the NIF facility
• Apolar-drivetargetthatsurvives>3-s exposure to the target chamber is required to use existing “clam-shell” shroud design
• UseexistingNIFspaceenvelopeandcryogenicsupportsystems(TAS, LLCS, TARPOS)
Existing TAS and LLCS place challenging constraints on a PD-ITIC design that will limit size of the cryogenic shroud.
Load, Layer, and Characterization Station (LLCS)
Target Alignment System (TAS)
E19667
Polar-drive ignition could be tested on the NIF with a few modest modifications to the facility
Summary/Conclusions
• Beam-smoothingimprovements:
– Multi-FM 1-D smoothing for spectral dispersion (SSD) provides the required beam smoothing with simple modifications to the NIF facility
– Beamsmoothingisonlyrequiredatthebeginningofthelaserpulse, which minimizes stress on the laser
– Polar-drive phase plate and polarization-smoothing designs are underway
– A NIF PD beam-smoothing demonstration on OMEGA EP is planned in FY12
• Direct-drivetargettechnology:
– NIF-scale fill-tube targets have been demonstrated and are being optimized
– Concepts for a polar-drive ignition target insertion cryostat (PD-ITIC) are being developed