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Queens University of Belfast
Definition of Laser Ablation

• Laser ablation is the removal of material after
  heating by laser radiation.It is a very
  complicated process that involves many
• physical phenomena
• Absorption of laser beam by the target surface.
• Electronic excitation heating the surface to
  melting then evaporation.
• Evaporated material interacts with laser beam
  resulting in
• excitation, ionisation and plasma formation.

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Definition, applications and Schematic diagram
of laser ablation

• Laser ablation is the removal of material after
• heating by laser radiation. It is a very
• complicated process that involves many
• physical phenomena
• Absorption of laser beam by the target surface.
• Electronic excitation heating the surface to
• melting then evaporation.
• Evaporated material interacts with laser
• beam resulting in excitation, ionisation and
• plasma formation.

KrF Excimer Laser (248nm), 30ns fwhm
Laser ablations has been applied in many
areas such as Medical technology,
Metallurgy, Electronic Industry and Material
science. In material science laser ablation
plays an important role in growing thin film of
various materials.
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Applications
Laser ablations has been applied in many areas
Medical technology Metallurgy Electronic
Industry Material science In material
science, laser ablation plays an important role
in growing thin films of various materials.
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Plasma Parameters and Diagnostic techniques
Electron, Ion and Neutral densities, Temperature
and Velocities
Ion Probe (Ne, Te, Velocity)
Absorption Spectroscopy
Map 3-D Ion and Neutrals
Laser Induced Fluorescence
And Interferometric Techniques (Ne)
Analytical and Numerical Modelling which led to
the development of computer simulations.
Hydro codes has been created to deal with high
intensity laser plasma interaction. Recent
additional modification had enabled the Hydro
code Pollux to simulate the low intensity
regime.
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The axial velocity of the plasma leading edge
Due to the high pressure near the surface
target the evaporated material travel outward
with a high acceleration. Further acceleration
will take place due to the interaction of the
plasma with the laser pulse. The axial velocity
of the plume is actually the leading edge of the
plume. Similar results have been obtained
experimentally under the same conditions for
electron and ions of magnesium low-temperature plu
mes. The dashed line in the figure is the
shape of the laser pulse which has a rise time at
5ns and a 30ns fwhm.
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Phase Transitions for Magnesium
Temperature (eV)
Temperature (eV)
Density (g/cc)
Density (g/cc)
The two figures show the different phase
transition boundaries which are defined by the
Chart-D (EOS) for magnesium. The critical point
is determined where the critical temperature is
0.33 e.V and density of 0.4 gm cm-3.
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Experimental set up for Absorption Spectroscopy
and Laser Induced Fluorescence
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Mach Zehnder Interferometric Technique
Plume
ICCD
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Queens University of Belfast
CHART-D EOS
Helmholtz free energy superposition of three
physical phenomena
F(r,T)Ec(r) Fn(r,T) Fe(r,T)
Cold component Nuclear component
Electronic component
THOMAS-FERMI EOS
CHART-D EOS
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CHART-D Cold Component
Compression state
Expansion state
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POLLUX Geometry and Mesh Configuration
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THOMAS-FERMI EOS
Average Ionisation
Temp (eV)
Density(g/cc)
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Queens University of Belfast
CHART-D Nuclear Component
Pressure (dynes/cm2)
Temp(eV)
Density(g/cc)
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Temporal Profile for Electron Density _at_ 200mm
from target for different Energy Densities
This figure shows the temporal profile of
electron density, for 3, 6 and 10 Jcm-2, The
solid lines are fits using an analytic model of a
power-driven expansion into vacuum in cylindrical
symmetry. This model had been derived by
Farnsworth in 1980. The results show
reasonable agreement. Experimental data
from the interfero- metric measurements for a
fluence of 9.2 Jcm-2, also shown in the figure
by a dashed line. These data are a factor of
2.5 lower than the simulation.
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Electron Density versus Energy Density at 35ns
for several distances from target
100um
150um
200um
250um
300um
Exp100um
Electron Density
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Number of Atoms removed per pulse for different
energy density
2.8e16
2.6e16
2.4e16
2.2e16
2.0e16
1.8e16
Atoms
1.6e16
1.4e16
1.2e16
1.0e16
8.0e15
6.0e15
2
4
6
8
10
12
Energy density(Jcm-2)
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Problems and Future Work
Problems
More cells more resolution but longer time
The ratio between the ambient and the target cell
thickness
Future work
Run the code for longer time delay and further
distance from target to compare with Ion probe
data. Run Ehybrid code which includes atomic
physics
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POLLUX 2-D Eulerian Hydrodynamic code
Pollux is an Eulerian 2-D hydrodynamic code
that uses the Flux Corrected Transport (FCT)
algorithm in order to solve the hydrodynamic flow
first order quasi-linear partial differential
equations. It treats the energy transport by the
flux limited thermal conduction for the plasma
case and semi-empirical experimental thermal
conductivity for the low temperature case.
The Chart-D model and Thomas-Fermi model has
been incorporated into the code in order to
simulate the low intensity regime of laser
produced plasma plumes. These models describe
the thermodynamic properties of the nuclear and
electronic components, respectively.
Absorption mechanism is by inverse bremsstrahlung
in the plasma phase and by dumping a fraction of
energy at critical density.
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THOMAS-FERMI equation of state (EOS)
The Thomas-Fermi equation of state is used
with recent additional approximation to
determine the electron contribution
to thermodynamic properties and calculate the
average ionisation. In Figure (a) the
electron pressure (dyne/cm2) is shown for a wide
range of density and temperature. Figure (b)
shows the average ionisation values which
determine the electron density.
Temperature (eV)
Density (gm cm-3)
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CHART-D equation of state (EOS)
Temperature (eV)
The Chart-D equation of state in its analytical,
numerical and tabular form generates full
information, which is thermodynamically
complete, and self-consistent. It is valid for
a wide range of density and temperature and
accurately calculates the energy transport
properties and treats all types of phase
mixtures. The figure shows a contour plot of
the pressure (dyne/cm2) as a function of
temperature and density. A region of
discontinuity occurs at high density and low
temperature. This region is the boundary between
the gas phase and the solid phase or solid-liquid
mixture.
Density (g/cc)
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Simulation and experimental electron density data
Figure (a) shows a typical example of the code
output as a contour plot of the electron density
50 ns after the start of an incident laser
pulse having an energy density of 9Jcm-2. These
results are compared with the experimental data
in figure (b). These results were obtained by
Mach-Zehnder interferometry. The plume expansion
in the code simulation in a good agreement with
the experiment. The electron density values
close to the target are higher in the code by a
factor of 2.5-3 with this factor
decreasing further from the target.
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Conclusion and Future Work
The laser-interaction hydrodynamic code,
which now include a realistic description of the
equation of state, has been successfully used to
model the early (lt100 ns) expansion of the
low-fluence laser ablation magnesium target.
Good qualitative agreement with the experimental
interferometric measurements of the evolution of
the electron density has been obtained.
In future these studies will be extended to later
stages of the expansion where much more extensive
data is available for comparison.
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Thomas Fermi EOS Electronic Component
Electron Specific heat(ergs/gm eV)
1.4
1.2
2e11
2e11
3e11
1.0
3e11
3e11
0.8
2e11
8e10
1e11
Temp(eV)
1e11
0.6
0.4
1e11
4e10
8e10
1e11
8e10
0.2
6e10
4e10
6e10
2e10
2e10
2e10
0.0
0.001
0.01
0.1
1
Density(g/cc)
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Queens University of Belfast
Thomas Fermi EOS Electronic component
Electron Pressure (dynes/cm2)
Temp(eV)
Density(g/cc)
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PULSE SHAPE
Normalised Intensity
Time