NRL Target Physics Experiments

J. Weavera, M. Karasika, V. Serlina, J. Ohb, Y. Aglitskiyc ,S. Obenschaina, J. Sethiana, L-Y. Chana, D. Kehnea , A. N. Mostovychd , J. Seelye, U. Feldmanf, C. Browne, G. Hollandg, A. Fieldingh, C. Mankab, B. Afeyani, R. H. Lehmberga,

J. Batesa, A. J. Schmitta, L. Phillipsj, A. Velikovicha, N. Metzlerf

a. Plasma Physics Division, Naval Research Laboratory, b. Research Support Instruments, c. Science Applications International Corporation, d. Enterprise Sciences Inc.,

e. Space Sciences Division, Naval Research Laboratory, f. ARTEP Inc., g. SFA Inc., h. Commonwealth Technologies Inc., i. Polymath Research, j. Lab. for Comp. Physics & Fluid

Dynamics, Naval Research Laboratory

Presented at 15th High Average Power Lasers Workshop, San Diego, CA August 8, 2006

Goal: Reduce the total laser energy required to achieve significant gain for direct-drive ICF implosions

NRL Laser Fusion

DT ice(fuel)

ablator

D

Pellet shell imploded by laser ablationto v 300 km/sec for >MJ designs

Hotfuel

Cold fuel

• Reduce pellet mass while increasing implosion velocity (v 400 km/sec)

• Increase peak drive irradiance and concomitant ablation pressure (~2x)

• Use advanced pellet designs that are resistant to hydro-instability

• Use the KrF laser’s deep UV light and large

Burn

NRL Laser Fusion

Hydrodynamic Instabilities

• Core experiments explore strategies for mitigation of pertubation growth:High-Z coatingsSpike prepulses

• Exploratory hydrodynamics experiments:Richtmyer-Meshkov instability in colliding foilsConvergence effects in hemispherical targets

Laser Plasma Instabilities

• Establish high intensity laser pulse operation in the range of actual implosions

• Study instability thresholds with enhanced diagnostic capabilities

• Study hot electron generation and possible threat to target conditions

NRL target physics experiments provide data relevant to pellet designs

Intensity

yx

Overlapping Nike beams produce the smoothest laser irradiation in ICF, <0.3% variation in a 2-3 kJ, 4 ns long pulse at 248 nm

avt

1

Induced Spatial Incoherence beam smoothing technique: time-averaged focal distribution with residual speckle non-uniformities of 1% rms in a single beam and <0.3% in a 37 beam overlap at = 1 THz. Nike operates at bandwidths up to 3 THz.

103 104 5×104

(Averaging time)/(Coherence time)

RM

S N

on

un

ifo

rmit

y (

%)

40 overlapping Nike beams

Single-beam ISI theory

Single-beam measurements

0.3

1

3

NRL Laser Fusion

Nike laser optimized for laser-driven hydrodynamics

X-ray radiography is major tool to study hydrodynamic evolution of laser-accelerated planar targets

Y. Aglitskiy, et al. , Phys. Rev. Letters, 86, 265001 (2001)

MAIN LASER BEAMSQUARTZ

CRYSTAL

1.86 keVimaging

2D IMAGE

STREAK CAMERA

Tim

e

Sample RT Data RT Data

BACKLIGHTERLASER BEAMSRIPPLED

TARGET

BACKLIGHTERTARGET Si

0 to 100 km/sec in <4 ns

NRL Laser Fusion

Laser imprint is effectively smoothed by early time “indirect-drive”

Au layer X-rays

Plastic

Plastic + Au layer Laser

0.4 mm

Tim

e

High IntensityAcceleration phase

Low Intensitycompression phase

Side views of X-ray emission

Thin high-Z layer

DT-loaded CH foam

High-Z layers may also help mitigate RM and RT due to increased mass ablation rates & softer ablation profiles

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Flat CH:strong imprint

growth

Flat CH + 450Å Au:imprint is

suppressed

Laser imprint suppression with high-Z layers is working at higher foot intensities (8 TW/cm2 - within a factor of 2 of the pellet designs)

Laser pulse

Tim

e (n

s)

Space (µm) Space (µm)

NRL Laser Fusion

We need to verify that fuel preheat remains small.

Spike prepulse can help mitigate perturbation growth

J. P. Knauer et al., PoP 12, 056306 (2005). Theory: K. Anderson and R. Betti, PoP 10, 4448 (2003); R. Betti et al., PoP 12, 042703 (2005).

Decaying shock (DS)Strong spike, target adiabat is shaped by the decaying shock from the spike

Relaxation (RX)Weak spike shapes a graded density profile, target adiabat is shaped by the decelerating shock from the foot

spike

spike

main

mainfoot

Goncharov et al. PoP 10, 1906 (2003).

Strong reduction of growth rates due to increased ablation velocity, particularly for high modes.

spike

no spike

NRL Laser Fusion

r

Shock front

Laser beam

g

a

Target with pre-formed density gradient

Ablation front

N. Metzler et al., PoP 6, 3283 (1999).

Relaxation spike usedfor present Nike experiments

y (m

m)

- 0.5

0

0.

5

Time (nsec)

-2 0 2 4

Ispike = 5.1×1012 W/cm2

Ispike = 3.5×1012 W/cm2

Ispike = 8.3×1012 W/cm2

Time (ns)

Vel

ocity

(km

/s)

Well characterized spike prepulse capability installed on Nike

Jaechul Oh, Andrew Mostovych, et al.

NRL Laser Fusion

Time (ns)

Sig

nal

(ar

b.

un

its)

Spike pulse in Nike front end

Time (ns)

No

rmal

ized

Sig

nal

Pulse shape after final amplifier

VISAR Streak Image Theory matches Observation

Low-amplitude spike prepulse suppresses ablative RM growth triggered by target surface roughness

Early

Late

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Plasti

Plasti

cc Plasti

Plasti

cc

Double–foil experiment, first results

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70 μm

30 μm

30 μm

p-to-v 5 μm

• New capability: orthogonal simultaneous imaging

• Promising technique to study perturbation growth in decelerating systems

• Applications to studies related to impact ignition

0 100

-100

0

100

col

row

2.5 5.0 7.5 10.0 12.5dxy08_d

50 150

-200

-100

0

100

200

col

row

2.5 5.0 7.5 10.0dxy07_d

50 150

-200

-100

0

100

200

col

row

1.25 2.50 3.75 5.00 6.25 7.50dxy06_d

50 150

-200

-100

0

100

200

col

row

1.25 2.50 3.75 5.00dxy05_d

0 100 200

-200

-100

0

100

200

col

row

0.5 1.0 1.5 2.0 2.5 3.0 3.5dxy03_d

0 100 200

-200

-100

0

100

200

col

row

0.5 1.0 1.5 2.0 2.5 3.0dxy01_d

0 100

-100

0

100

col

row

2.5 5.0 7.5 10.0 12.5dxy08_d

Target thickness 2.47 mg/cc - max shim thickness 1.81 mg/cc

t1

t2

t4

t3

Convergent geometry, planned experiment

NRL Laser Fusion

Side-on streak images show variation depending on laser spot size

300 µm spot (no KPP)

Space (µm)500 µm spot (with KPP)

Space (µm)

Tim

e (n

s)

Spot size ~ hemisphere radius Spot size < hemisphere radius

NRL Laser Fusion

Laser

CH shell Be plate

Shell specs:● Inner diameter ≈ 940 µm● Thickness ≈ 20 µm● Composition: CH

1.3O

5

Hemisphericaltargets made by GA mountedat ILE

Laser Plasma Instabilities

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Laser plasma instabilities:

Three wave parametric processes in which laser light couples to natural modes in the coronal plasma thereby generating new radiation and altering target conditions

Plasma ModeOr EM Wave

Laser

Plasma Mode

Two primary plasma modes: • Electron plasma waves – Stimulated Raman scattering, Two-plasmon decay• Ion acoustic waves – Stimulated Brillouin scattering, filamentation

Primarily interested in generation of hot electrons that could lead to target preheatbut will look for all evidence of LPI in initial stages

Long history of research, still many unanswered questions – KrF lasers relatively unexplored territory

Thresholds for the 3 wave parametric instabilities in inhomogeneous plasmas for 0.248 m light

I142 p 2.16

Te,keVLN ,100m

I14SBS 6.8

Te,keVn nc Lv,100m

I14SRS 160

1

LN ,100m

EMW --> EPW + EPW

EMW --> EMW + IAW

EMW --> EMW + EPW

Polymath Research Inc.

pe2 4 ne e

2

me

e2

c

1

137

NRL Laser Fusion

LPI threat to sub-MJ targets: 2p could be problematicSRS & SBS do not appear dangerous

Polymath Research Inc.

pe2 4 ne e

2

me

e2

c

1

137

Estimates of LPI risk near peak intensities for FTF implosions show 2wp ismost highly over threshold

There is a lack of experimental data for LPI physics for ~0.25 mm lasers with broad bandwidth, and ISI smoothing

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10-12 Backlighter Beams

44 MainBeams

MainTarget

Target Vacuum Vessel

Initial geometry for LPI experiments

F/20Lens array

F/40 Lens array

• Use of backlighter array allows smaller focal distribution

• Main beams with independently controlled spot size, energy, and pulse shape can be introduced into backlighter beam path

2 Redirected Main Beams

• Can vary plasma conditions with main beams and vary LPI interaction by controlling backlighter beams

Nike Target Facility

NRL Laser Fusion

Target

X-ray Streak Camera

CrystalImager

X-rayPinholeCamera

135o

• Focal spots data at full power used face-on imaging with streak and pinhole camera

Amplification of short pulse through final amplifiers increases intensity

Time (ns) Time (ns)D

iode

Sig

nal (

V)

5 ns

0.4 ns

Energy: 36 J Energy: 18 J

Standard Backlighter Pulse Spike-only Backlighter Pulse

Dio

de S

igna

l (V

)

Spike-only pulse through time-multiplexed KrF amplifier generates higher intensity pulses

Pulse length decreased by factor 10-12, energy only down by ½ Power increase 4-5x

Studies of spike propagation incomplete, but above result appears robust over many shots

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Low-energy, time integrated focal distributions

• Measurements show FWHM of 70 – 110 m

• Spot size at target chamber center measured with thin UV fluorescent glass, microscope, and CCD; only oscillator and first stage of amplification used (low energy laser pulses)

• Spot size controlled by selection of initial apertures for ISI beam optics

Beam 4 Beam 32Beam 1

100 m

• Relative shot to shot overlap error is estimated to be less than spot diameter (<50 m)

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Time (ns)

Po

siti

on

(m

)

Position (m)

Co

un

tsC

ou

nts

Time (ns)

115 m

Time-resolved, single beam focal distributions at high intensity

Single beam spot on Si target

Spot diameter ~ 115 m, pulse width ~ 375 ps

375 ps

Working on time-resolved multibeam overlap imagefor small spot, spike pulse

NRL Laser Fusion

Estimated range of focal intensities for LPI experiments

Spot Size (m) Total Energy (J) Intensity (1014 W/cm2)

75

100

125

150

120

160

200

120

160

200

120

160

200

120

160

200

68

91

113

38

51

63

24

33

41

17

23

28

Assumes 400 ps pulse duration

Range mostconsistentwith currentobservations

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Detector plane of165 nm spectrometer

LPI diagnostics are being fielded at Nike laser for next stage

Time resolution ~ 300 psSpectral resolution ~2.5 Ang/mm

Spectrometer developed in collaboration with Space Science

Division at NRL

165 nm Tandem Wadsworth Spectrometer

Absolute calibrations for 165 nm spectrometer have been performed at Brookhaven National Laboratory

Bandpass hard x-ray photodetectors

Telescope

Dual grating mount

Diodearray

Visible time-resolved spectrometersX-ray pinhole camerasX-ray spectrometers

NRL Laser Fusion

LPI experimental program is still in preliminary stages

• Preliminary experiments will determine instability thresholds as a functions of Total intensity (energy per beam, spot size)

Pulse shapesTarget type (CH, BN, Si, Au, foam – either CH or Si aerogel, cryo D2)Geometry (target tilt, angle of beam overlap, instrumental line of sight)Laser bandwidth

NRL Laser Fusion

• First physics experiments will focus on hot electron generation and target heating Hard x-ray monitors (1-100 keV) will serve as first diagnostics X-ray spectrometers Specialized target designs

• Second stage physics experiments will take more detailed exploration of LPI physics to enhance predictive capabilities

Saturation mechanismsHot spot effects (size of hot spot, beam overlap, bandwidth)Advanced diagnostics – Thomson scattering

Summary: Near term goals for NRL target physics experiments

Target physics program will evaluate hydrodynamic instabilities Relevant to pellet designs and restrictions on laser intensity due to

laser-plasma instabilities

Essential data to support physics for pellet designs:

• Continued examination of high-Z layers and spike prepulses as mitigation techniques for early time perturbations

• Develop techniques with double foils for RM physics and target diagnostics

• Study convergence effects in hemispherical targets

• Characterization of relevant thresholds for parametric instabilities

• Generation of hot electron and target heating by hot electrons

NRL Laser Fusion