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

2. Reduce pellet mass while increasing implosion velocity (v 400 km/sec)
Goal: Reduce the total laser energy required to achieve significant gain for direct-drive ICF implosions
NRL Laser Fusion
ablator
Hot
fuel
Cold fuel
Burn
DT ice
(fuel) D
Pellet shell imploded by laser ablation
to v  300 km/sec for >MJ designs
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 

3. NRL target physics experiments provide data relevant to pellet designs
NRL Laser Fusion
Hydrodynamic Instabilities
Core experiments explore strategies for mitigation of pertubation growth:
High-Z coatings
Spike prepulses
Exploratory hydrodynamics experiments:
Richtmyer-Meshkov instability in colliding foils
Convergence 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

4. Nike laser optimized for laser-driven hydrodynamics
NRL Laser Fusion
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
Intensity y x
103
104
5×104
(Averaging time)/(Coherence time)
RMS Nonuniformity (%)
40 overlapping Nike beams
Single-beam ISI theory
Single-beam measurements
0.3 1 3
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 Dn = 1 THz. Nike operates at bandwidths up to 3 THz.

5. Y. Aglitskiy, et al. , Phys. Rev. Letters, 86, 265001 (2001)
X-ray radiography is major tool to study hydrodynamic evolution of laser-accelerated planar targets
NRL Laser Fusion
BACKLIGHTER
LASER BEAMS
1.86 keV
imaging
QUARTZ
CRYSTAL
MAIN
LASER BEAMS
RIPPLED
TARGET
BACKLIGHTER
TARGET Si
0 to 100 km/sec in <4 ns
Sample RT Data
Time
2D IMAGE
STREAK CAMERA
Y. Aglitskiy, et al. , Phys. Rev. Letters, 86, (2001)

6. Laser imprint is effectively smoothed by early time “indirect-drive”
NRL Laser Fusion
Plastic
Plastic + Au layer
Laser
0.4 mm
Time
High Intensity
Acceleration phase
Low Intensity
compression phase
Thin high-Z layer
DT-loaded
CH foam
Au layer X-rays
High-Z layers may also help mitigate RM and RT due to increased mass ablation rates & softer ablation profiles
Side views of X-ray emission

7. We need to verify that fuel preheat
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)
NRL Laser Fusion
Flat CH:
strong imprint
growth
Flat CH + 450Å Au:
imprint is suppressed
We need to verify that fuel preheat
remains small.
Time (ns)
Space (µm)
Space (µm)
Laser pulse

8. Spike prepulse can help mitigate perturbation growth
no spike
NRL Laser Fusion
Strong reduction of growth rates due to increased ablation velocity, particularly for high modes.
Goncharov et al. PoP 10, 1906 (2003).
Relaxation spike used
for present Nike experiments
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 r
Shock front
Laser beam g a
Target with pre-formed density gradient
Ablation front
spike
main
foot
J. P. Knauer et al., PoP 12, (2005).
Theory: K. Anderson and R. Betti, PoP 10, 4448 (2003);
R. Betti et al., PoP 12, (2005).
N. Metzler et al., PoP 6, 3283 (1999).

9. Spike pulse in Nike front end Pulse shape after final amplifier
Well characterized spike prepulse capability installed on Nike
Spike pulse in Nike front end
Pulse shape after final amplifier
NRL Laser Fusion
Normalized Signal
Signal (arb. units)
Time (ns)
Time (ns)
y (mm) -
0.5
Time (
nsec ) I
spike
= 5.1 × 10 12
W/cm 2 =
3.5
8.3
Time (ns)
Velocity (km/s)
VISAR Streak Image
Theory matches Observation
Jaechul Oh, Andrew Mostovych, et al.

10. Low-amplitude spike prepulse suppresses ablative RM growth triggered by target surface roughness
NRL Laser Fusion
Early
Late

11. Double–foil experiment, first results
NRL Laser Fusion
New capability: orthogonal simultaneous imaging
Promising technique to study perturbation growth in
decelerating systems
Applications to studies related to impact ignition
70 μm
30 μm
30 μm
p-to-v 5 μm
Plastic
Plastic

12. Convergent geometry, planned experiment
NRL Laser Fusion t1 t2 t3 t4
Target thickness 2.47 mg/cc - max shim thickness 1.81 mg/cc

13. Side-on streak images show variation depending on laser spot size
NRL Laser Fusion
Hemispherical
targets made
by GA mounted
at ILE
Shell specs:
Inner diameter ≈ 940 µm
Thickness ≈ 20 µm
Composition: CH1.3O5
CH shell
Be plate
Spot size ~ hemisphere radius
Spot size < hemisphere radius
Space (µm)
Time (ns)
Laser
Space (µm)
500 µm spot (with KPP)
300 µm spot (no KPP)

14. Laser Plasma Instabilities
NRL Laser Fusion
Plasma Mode
Or EM Wave
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
Laser
Long history of research, still many unanswered questions
– KrF lasers relatively unexplored territory
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 preheat
but will look for all evidence of LPI in initial stages

15. EMW --> EPW + EPW EMW --> EMW + IAW EMW --> EMW + EPW
Thresholds for the 3 wave parametric instabilities in inhomogeneous plasmas for m light
NRL Laser Fusion
EMW --> EPW + EPW
EMW --> EMW + IAW
EMW --> EMW + EPW

16. LPI threat to sub-MJ targets: 2p could be problematic SRS & SBS do not appear dangerous
NRL Laser Fusion
Estimates of LPI risk near peak intensities for FTF implosions show 2wp is
most highly over threshold
There is a lack of experimental data for LPI physics for ~0.25 mm lasers with broad bandwidth, and ISI smoothing

17. Initial geometry for LPI experiments
NRL Laser Fusion
2 Redirected
Main Beams
Nike Target Facility
10-12 Backlighter
Beams
Target Vacuum Vessel
X-ray
Pinhole
Camera
F/20
Lens array
44 Main
Beams
135o
Target
Crystal
Imager
Main
Target
F/40
Lens array
Use of backlighter array allows
smaller focal distribution
X-ray Streak
Camera
Main beams with independently controlled spot size,
energy, and pulse shape can be introduced into backlighter beam path
Can vary plasma conditions with main beams and vary LPI interaction by controlling
backlighter beams
Focal spots data at full power used face-on imaging with streak and pinhole camera

18. Amplification of short pulse through final amplifiers
increases intensity
NRL Laser Fusion
Standard Backlighter Pulse
Spike-only Backlighter Pulse
Energy: 36 J
Energy: 18 J
Diode Signal (V)
Diode Signal (V)
0.4 ns
5 ns
Time (ns)
Time (ns)
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

19. Low-energy, time integrated focal distributions
NRL Laser Fusion
Beam 4
Beam 32
Beam 1
100 mm
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
Measurements show FWHM of 70 – 110 mm
Relative shot to shot overlap error is estimated to be less than spot diameter (s<50 mm)

20. Time-resolved, single beam focal distributions at high intensity
NRL Laser Fusion
Single beam spot on Si target
Time (ns)
Position (mm)
Counts
115 mm
Position (mm)
375 ps
Counts
Spot diameter ~ 115 mm, pulse width ~ 375 ps
Working on time-resolved multibeam overlap image
for small spot, spike pulse
Time (ns)

21. Estimated range of focal intensities for LPI experiments
NRL Laser Fusion
Spot Size (mm)
Total Energy (J)
Intensity (1014 W/cm2)
120 68 75
160 91
200
113
120 38
100
160 51
Range most
consistent
with current
observations
200 63
120 24
125
160 33
200 41
120 17
150
160 23
200 28
Assumes 400 ps pulse duration

22. LPI diagnostics are being fielded at Nike laser for next stage
NRL Laser Fusion
Detector plane of
165 nm spectrometer
165 nm Tandem Wadsworth Spectrometer
Diode
array
Dual grating mount
Telescope
Time resolution ~ 300 ps
Spectral resolution ~2.5 Ang/mm
Spectrometer developed in
collaboration with Space Science
Division at NRL
Bandpass hard x-ray
photodetectors
Visible time-resolved spectrometers
X-ray pinhole cameras
X-ray spectrometers
Absolute calibrations for 165 nm spectrometer have been performed at Brookhaven National Laboratory

23. LPI experimental program is still in preliminary stages
NRL Laser Fusion
Preliminary experiments will determine instability thresholds as a functions of
Total intensity (energy per beam, spot size)
Pulse shapes
Target 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
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 mechanisms
Hot spot effects (size of hot spot, beam overlap, bandwidth)
Advanced diagnostics – Thomson scattering

24. Summary: Near term goals for NRL target physics experiments
NRL Laser Fusion
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