Post on 29-Jan-2016
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
NRL Laser Fusion
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
NRL Laser Fusion
Plasti
Plasti
cc Plasti
Plasti
cc
Double–foil experiment, first results
NRL Laser Fusion
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
NRL Laser Fusion
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
NRL Laser Fusion
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
NRL Laser Fusion
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)
NRL Laser Fusion
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
NRL Laser Fusion
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