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Experiments and Simulations of
Ablatively Driven Shock Waves in
Gadolinium
Richard Kraus, Eric Loomis, Shengnian Luo,
Dennis Paisley, Achim Seifter, and Damian Swift
P-24, Shock Physics and Materials Dynamics Team
APS SCCM 2007
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LA-UR 06-5507
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Abstract
 Lanthanides are fascinating metals to study because they exhibit physical
properties that vary with 4f occupancy. Another important reason to study
lanthanides is that they may provide basic level information about actinides
while being safer and easier to handle. Specifically Gadolinium is interesting
because there are multiple structural phase transitions accessible below 100
GPa.
 Experiments were performed on Gadolinium metal in which shock waves were
driven in Gadolinium foils through direct laser ablation. The velocity at the
opposite surface of the drive beam was measured with line-imaging laser
Doppler velocimetry of the Velocity Interferometer System for Any Reflector
(VISAR) type.
 Simulations of the experiment were done using a radiation hydrodynamic
model which takes the measured irradiance history of the laser and predicts
the pressure history at the ablation surface; this pressure history is then used
as a time-dependant boundary condition for a continuum mechanics
simulation. From this we obtain a simulated surface velocity profile, which we
then compare with the velocity profile obtained by the line VISAR diagnostic
technique to validate the simulations. With this experimental series we
achieved shock pressures under ten gigapascals; specific experimental and
simulated results to be presented.
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Shocks Driven by Laser Ablation
 2.4 ns pulse length at 527 nm.
 High irradiance causes the back surface of the sample to
ionize.
 Plasma pressure causes sudden increase in pressure
and supports shock wave through sample.
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Trident Laser Facility and Experimental Setup
 Facility used for shock studies
 Variable pulse length (ms to ps)
and pulse shape.
 Energies up to 500 Joules
 1054 nm, 527 nm, 351 nm, andU N C L A S S I F I E D
264 nm
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Velocimetry
VISAR System for this Experiment
Etalon chosen so as to obtain an 800 m/s fringe
constant (i.e. one full fringe shift means an 800 m/s
surface velocity)
Position
Line imaging, so as to obtain the velocity history of
the surface along a 1 mm line.
Time
Gd disc
1 mm
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6 mm
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4
Gadolinium
Hugoniot
 4f electron metal
3
bcc
 multiple phase changes below 100
hex(9)
2
GPa
 Oxidizes relatively slowly
1
 Safe to handle
 Polycrystalline, 25 mm thick foils from
Goodfellows.
Liquid
hcp
500
1000
T(K)
1500
Low pressure phase
Gadolinium Hugoniot
High pressure phase
us  ci  si  u p
 From S.P. Marsh, 1980
 Low Pressure Phase:
 C0= 2.21 km/s , S0= 0.92
41 GPa
 High Pressure Phase:
 C1= 1.77 km/s, S1= 1.29
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2000
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Shot 18809 and 18813
These 4 shots were taken with a 2 mm Lithium Fluoride window attached to the
VISAR side of the 25 mm Gadolinium sample.
A window is used to prevent pressure release into the sample.
18809, Laser Energy= 16 J
18813, Laser Energy= 26 J
Peak Pressure= 4.1 GPa
Peak Pressure= 4.9 GPa
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Shot 18814 and 18817
18814, Laser Energy= 44 J
18817, Laser Energy= 75 J
Peak Pressure= 6.0 GPa
Peak Pressure= 5.4 GPa
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Simulating the Shock
 Radiation Hydrodynamics in 1D (HYADES program)
 Solves continuum mechanical equations in numerical form
governing conservation of mass, momentum, and energy explicitly
for each mesh
 Includes Thomas Fermi ionization model to simulate laser induced
ionization.
 Continuum Mechanics in 1D (LagC program)
 Solves same continuum mechanical equations as HYADES
 Can include strength model such as Steinberg-Guinan
 These simulations did not include strength model.
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Simulating the Shock
Pressure (1 mm)
Laser irradiance used
as input for HYADES
Pressure @ 1 mm from
ablated surface is used as
a boundary condition for
LagC
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Simulating 18809 and 18813
Experimental parameters were determined and applied as accurately as
possible to the simulations for each shot.
18809, Laser Energy= 16 J
Shot 18809
18813, Laser Energy= 26 J
Shot 18813
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Simulating 18814 and 18817
18814, Laser Energy= 44 J
Shot 18814
18817, Laser Energy= 75 J
Shot 18817
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Experiment vs. Simulations
Experiment and
Simulations agree to
within the error bars on 3
out of 4 shots.
Where it does not agree
I believe the VISAR trace
to lose signal before the
surface is done
accelerating.
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Problems and Difficulties
The surface of the Gadolinium
sample was not polished initially.
The reflectivity of the
Gadolinium surface decreases
significantly when the shock front
reaches it; making it difficult to
trace the fringes a few ns after
shock breakout.
The drive energy used in these
shots was not enough to reach
more than one phase change
boundary.
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20 ns
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Solutions and Future Plans
Samples will be polished to obtain a strong specular reflection.
Or, could coat the back surface of the LiF window with Aluminum
Implement additional diagnostics, such as ellipsometry, to detect phase
changes.
Use more robust velocimetry system to obtain better VISAR fringes.
Example of better VISAR
trace obtained recently
(Data not yet analzed)
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Acknowledgements
 I would like to express my thanks to the following individuals.
 Damian Swift
 Achim Seifter
 Shengnian Luo
 Dennis Paisley
 Eric Loomis
 Trident Laser Facility Staff
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