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High-Temporal-Contrast Experiments Using a Hybrid OPCPA-Nd:Glass Multi-Terawatt (MTW) Laser System
International Conference onUltrahigh Intensity Lasers
Shanghai-Tongli, China27–31 October 2008
J. D. Zuegel University of RochesterLaboratory for Laser Energetics
1019 W/cm2
2 to 20 nm
20 to500 nm
KaPBR
Kb
-20
-40
-60
-80
-500 -300 -100 100
Inte
nsi
ty (
dB
)
Time (ps)
The MTW laser provides a unique facility to study ultrahigh-intensity laser and target physics
E17217
• TheMTWlaserenablesimportantexperimentalcampaigns:
– high-temporal-contrast laser enhancements and diagnostic development
– high temporal contrast enables solid-target experiments
– high pointing stability makes it possible to use small, low-mass targets (Vtarget ≈ 10-6 mm3)
– high availability facilitates parametric studies with good statistics required to study important physics issues
• Laser-to-fast-electronconversionefficiencyis23%±8%whenintensepulses are energy scaled with no observed pulse width dependence
• Low-densityplasmagenerationfromlaserprepulsesplaysanimportantrole in fast electron generation, so measuring and controlling temporal contrast is important
Summary
Collaborators
High-Contrast Laser Team Target Physics Team C. Dorrer P. M. Nilson I. V. Begishev W. Theobald J. Bromage V. Ovchinnikov* R. Brown J. Myatt A. V. Okishev B. Eichman S. Ivancic M. Storm O. V. Gotchev C. Stoeckl T. C. Sangster R. Betti D. D. Meyerhofer
University of RochesterLaboratory for Laser Energetics
*Ohio State University
Outline
E17218
• Introduction:MTWlaserdescription
• MTWcontrastmeasurements
• ContrastenhancementstestedontheMTW
– ultrafast optical parametric amplifier
– OPCPA pump filter with Volume Bragg grating
• MTWtargetexperiments
– isochoric heating of solid-density targets by relativistic electrons
– laser-to-electron conversion efficiency for “bookend” targets
The front-end prototype for OMEGA EP is used for diagnostic development and target experiments
E16719a
• TheMTWlaserisasingle-beam,short-pulselaserfacility.
• Pulsewidths(0.5 to 100 ps) are adjustable using an Öffner grating pulse stretcher.
• OPCPAamplificationprovideshightemporalcontrast(C > 108).
• Afour-passNd:phosphateglassdiskamplifierdelivers10Jpershot(limited by compressor grating damage).
• An ~f/3 off-axis parabola focus (~4-nm FWHM) yields up to 4 # 1019 W/cm2.• Shotcycletimeasshortas10min;over2300systemshots!
Glass amplifier
PreamplifierOscillator table
Compressor
Targetarea
1m 2m 3m 4m 5m
Producing and measuring ultrahigh pulse contrast for future OMEGA EP experiments is a significant challenge
E15837a
Known limits to CPA-system pulse contrast
• Seed pulse contrast – typical ML oscillator contrast is ~106:1• Spectral phase distortions produce “picosecond pedestals”• Incoherent noise on OPCPA pump pulse imprints on chirped-pulse spectrum
that prevents full compression below ~100 ps• OPGand/or ASE – incoherent radiation does not compress• Discrete prepulses from ML pulse train and multipass amplification
Seed pulse contrast +Spectral phase distortions (~10 ps)
OPCPA pump-pulse-induced noise (~100 ps)
Optical parametric generation (OPG)+ Amplified stimulated emission (ASE)
Discrete prepulses
10–9
10–6
10–3
Time
Las
er p
ow
er (
arb
itra
ry u
nit
s)
1
OPG (2.4 ns)ASE (>100 ns)
Scanning and single-shot cross correlators are used to measure temporal contrast
E17219
• 660-psrange(–530/+130 ps)
• Estimateddynamicrange=91dBfora 30 nJ Fourier-transform-limited pulse on MTW
• 85replicaswith~6-ps delay between replicas yields ~ 510 ps temporal range
A prototype HCD is now deployed on the MTW laser for target shots.
see C. Dorrer talk session 10-4.
Filters3~ HR
2~ HR
2~ pulse
Opticalpulsereplicator
Pulse replicator output (2~)
Signal (3~)
Spatialattenuationstage
Densityfilters
2~ PR
1~ HR
1~ pulseunder test
Lens
THGcrystal
Detection plane
Sequoia* scanning cross correlator Single-shot high-contrast diagnostic* (HCD)
“Baseline” MTW contrast is measured using the 5-Hz OPCPA front end with the Sequoia cross-correlator
E17220
-20
-40
-60
-80
-500 -300 -100 100
0
Inte
nsi
ty (
dB
)
Time (ps)
Parametricfluorescence
(-83 dB)
~4 ps pulse(-63 dB)
Two-component pedestal
†
*C. Dorrer et al., JOSA B 24, 3048 (2007). †C. Dorrer et al., Opt. Express 16, 3058 (2008)
High-contrast technologies are being tested in the MTW laser system before being deployed in OMEGA EP.
E15838b
• Maximizinginputsignal-to-noiseratio(contrast) at every amplification stage minimizes degradation from OPG and ASE.
• Spectralamplitudeandphaseofamplifiedpulsemustbemaintained for best pulse compression.
Contrast levels greater than 1010 within ~10 ps have been achieved in the MTW seed source
E15839b
Ultrafast OPA using a seed pulse with zero dispersion (no stretch)and ~2-ps (FWHM) pump pulse at 2~
–100 –50 0 50 100
No
rmal
ized
inte
nsi
ty
Time (ps)
10–10
>1010
contrast
Noise floor
10–4
10–8
10–2
10–6
1
–10 –5 0 5 10
Time (ps)
Delayed pumpDelayed signal
Eout = 50 nJ
C. Dorrer et al., Opt. Lett. 32, 2143 (2007).
OPCPA amplification degrades the extreme contrast delivered by the UOPA seed source
E17221
• Temporalcontrastappearstobedominatedbyphenomenalike – OPCPA pump-induced noise – spectral phase noise?
-40
-40
-60
-80
-120
-100
-100 -50 500 100
0
Inte
nsi
ty (
dB
)
Time (ps)
Noise floor
MTW output (after OPCPA)
UOPA seed pulse to MTW laser
Regenerative filtering shows promise for limiting OPCPA pump laser noise transferred to the signal
E15841a
Extremely narrowband filtering is achieved by filtering ASE on each regenerative amplifier round trip with a volume Bragg grating (VBG).
C. Dorrer et al., Opt. Lett. 32, 2378 (2007).
Fast-electron refluxing in small-mass targets allows access to high-energy-density phenomena
E16142b
• RefluxingiscausedbyDebye sheath field effects1,2
• Majorityoffastelectrons are stopped in the target
• Providesasimplegeometryfor testing laser-coupling, electron-generation, and target-heating models3,4
• Hightemporalcontrastisrequired to achieve refluxing
1019 W/cm2
2 to 20 nm
20 to500 nm
Bremsstrahlung K-photonproduction
Debye sheathsuE q á 1012 V/m
Fastestelectrons escape
KaPBR
Kb
1S. P. Hatchett et al., Phys. Plasmas 7, 2076 (2000). 2R. A. Snavely et al., Phys. Rev. Lett. 85, 2945 (2000). 3W. Theobald et al., Phys. Plasmas 13, 043102 (2006). 4J. Myatt et al., Phys. Plasmas 14, 056301 (2007).
N
Copperenergy levels
M
L
K
Ka(L8.05 keV)
Ionization
Depletedpopulation
Kb(L8.91 keV)
K-photon radiation reveals hot electron production and bulk heating of small-mass targets*
E16143c
• Intenselaser-plasmainteractionproduces energetic electrons that leave K-shell vacancies
• Ka yield indicates hot electron conversion efficiency
• Inelasticcollisionsheatthetargetand ionize outer shell electrons
• Collisionalionizationwiththermal background plasma occurs
• Te > 100 eV causes significant M-shell depletion, which affects Kb yield
• Targetheatingisinferred from Kb/Ka
*J. Myatt et al., Phys. Plasmas 14, 056301 (2007).*G. Gregori et al., Contrib. Plasma Phys. 45, 284 (2005).
Normalized Ka yield is approximately constant for 1-10 ps pulses with the same peak intensity
E17222
• Laser-to-electronenergy-conversionefficiency (hL-e) is inferred using a Ka production model
• PlanarCutargets(500 # 500 # 20 nm3)
• 1to10pslaserpulsesareenergyscaled(constant at ~ 1018 W/cm2)
Ka yields are consistent with a refluxing electron model assuming hL"e = (23!8)%withnoobservedpulsewidthdependence.
No
rmal
ized
Kα
en
erg
y
0.01
0.001ηL-e=40%
ηL-e=40%
ηL-e=30%
ηL-e=30%ηL-e=20%ηL-e=10%
ηL-e=20%ηL-e=10%ηL-e=5%
ηL-e=5%
0.0001
1e-05
1e-061 2 3 4 5
Pulse duration / ps6 7 8 9 10
Refluxing
Non-Refluxing
Comparison of Kb/Ka ratios to simulations are consistent with the absolute Ka yields
E17191
• Providesaself-consistencycheck on hL"e
• Providesadetaileddata set for comparison to future OMEGA EP experiments at higher energy densities
• Highpointingstability enables the smallest targets (V = 10–6 mm3)
• MeasuredKb/Ka for the smallest targets is consistent with Te . 200 eV
102 103 104
Laser energy (J)/target volume (mm3)
20 # 20 # 20 µm3
targets
105
ηL→e = 30%
ηLe = 10%
106 107
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Kb
/Ka
400
100
150
200
Tem
per
atu
re (
eV)
0
P. M. Nilson et al., Phys. Plasmas 15, 056308 (2008).
“Bookend” targets are used to study the fast-electron-conversion efficiency for cone-like target geometries
E17223
• One-pieceCutargets (~100 # 100 # 20-nm3 plates joined at 30º, 45º, and 60º)
• Radiusofcurvature(~1 nm) smaller than the focal-spot diameter
• Bookendorientationsetslaserpolarization
Laser beamspot
E-field
Front view
Side view
30º, s-pol
30º, s-pol 45º, s-pol 60º, s-pol
105 nm
105 nm
22 nm
Laser beamspot 110 nm
18 nm
110 nm
19 nm
Ka emission was recorded using a narrowband spherical crystal imager and two spectrometers
E17224
s2125 s2118
P, 60º20 µm
CCD
Single hit CCD spectrometer
KafluorHOPG spectrometer
Crystal imager
Bookendtarget
Braggcrystal
Flat foil target Bookend target
Laser
Refluxing electronscreate x-ray emission
Homogenous emission indicateshot electron refluxing in the targets.
Bookend target experiments show similar trends as OSIRIS (2-D PIC) simulations
E17225
• Fast-electronconversionefficiencyincreasesforthenarrowbookendtargets compared to flat foils.
• Resonanceabsorptionincreasesp-polarization absorption compared to s-polarization (but strong s-pol angle dependence is not observed).
A large discrepancy is observed between flat foil targets in this campaign with earlier refluxing targets that may be explained by contrast variations.
40
30
Co
nver
sio
n e
ffici
ency
into
fas
t el
ectr
on
s (%
)
Experiment
Flat foil
20
10
20 40 60 160 180 2000
0
Simulation
Cavity wedge opening angle (º)Cavity wedge opening angle (º)20 40 60 160 180 2000
p-polarizations-polarization
Flat foil, 0º, 10 n
Flat foil, 0ºFlat foil, 5º
Low-density plasma generation from laser prepulses plays an important role in fast-electron generation
E17226
• 1-DLILAC simulations estimate the low density plasma:
– measured prepulse contrast – does not account for 3-D
effects of bookend targets
• 1-DOSIRIS simulations calculated absorption for different density gradients:
– longer density ramps (more low density plasma) yield higher conversion efficiency
– angle of incidence affects the plasma profile between critical and tenth critical density
Pre-plasma plays an important role in fast-electron generation, so measuring and controlling temporal contrast is important.
25
Simulated conversion efficiency versus density ramp
Co
nver
sio
n e
ffici
ency
(%
)
Density ramp (nm)
20
15
10
2 4 6 8 10 12
5
00
The MTW laser provides a unique facility to study ultrahigh-intensity laser and target physics
E17217
• TheMTWlaserenablesimportantexperimentalcampaigns:
– high-temporal-contrast laser enhancements and diagnostic development
– high temporal contrast enables solid-target experiments
– high pointing stability makes it possible to use small, low-mass targets (Vtarget ≈ 10-6 mm3)
– high availability facilitates parametric studies with good statistics required to study important physics issues
• Laser-to-fast-electronconversionefficiencyis23%±8%whenintensepulses are energy scaled with no observed pulse width dependence
• Low-densityplasmagenerationfromlaserprepulsesplaysanimportantrole in fast electron generation, so measuring and controlling temporal contrast is important
Summary/Conclusions