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