U N C L A S S I F I E D Experiments and Simulations of Ablatively Driven Shock Waves in Gadolinium...
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Transcript of U N C L A S S I F I E D Experiments and Simulations of Ablatively Driven Shock Waves in Gadolinium...
U N C L A S S I F I E D
U N C L A S S I F I E D
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
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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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.
Shocks Driven by Laser Ablation
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Trident Laser Facility and Experimental Setup
Facility used for shock studies
Variable pulse length (s to ps) and pulse shape.
Energies up to 500 Joules
1054 nm, 527 nm, 351 nm, and 264 nm
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Velocimetry
VISAR System for this Experiment
Line imaging, so as to obtain the velocity history of the surface along a 1 mm line.
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)
Pos
ition
Gd disc
6 mm
1 mm
Time
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4f electron metal
multiple phase changes below 100 GPa
Oxidizes relatively slowly
Safe to handle
Polycrystalline, 25 m thick foils from Goodfellows.
Gadolinium
41 GPa
s i i pu c s u= + ×Gadolinium Hugoniot
From S.P. Marsh, 1980
Low Pressure Phase: C0= 2.21 km/s , S0= 0.92
High Pressure Phase: C1= 1.77 km/s, S1= 1.29
Low pressure phase
High pressure phase
Hugoniot
Liquid
hex(9)
hcp
bcc
500 1000 1500 2000
4
3
2
1
T(K)
P(G
Pa)
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18809, Laser Energy= 16 J
Peak Pressure= 4.1 GPa
Shot 18809 and 18813
18813, Laser Energy= 26 J
Peak Pressure= 4.9 GPa
These 4 shots were taken with a 2 mm Lithium Fluoride window attached to the VISAR side of the 25 m Gadolinium sample.
A window is used to prevent pressure release into the sample.
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18814, Laser Energy= 44 J
Peak Pressure= 6.0 GPa
Shot 18814 and 18817
18817, Laser Energy= 75 J
Peak Pressure= 5.4 GPa
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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.
Simulating the Shock
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
Laser irradiance used as input for HYADES
Pressure @ 1 m from ablated surface is used as a boundary condition for
LagC
Pressure (1 m)
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Simulating 18809 and 18813
Experimental parameters were determined and applied as accurately as possible to the simulations for each shot.
Shot 18809 Shot 18813
18809, Laser Energy= 16 J 18813, Laser Energy= 26 J
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Simulating 18814 and 18817
Shot 18814 Shot 18817
18814, Laser Energy= 44 J 18817, Laser Energy= 75 J
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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.
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)