Ultrashort Pulse LaserMatter Interaction at Relativistic Laser Intensities

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Ultrashort Pulse Laser-Matter Interaction at
Relativistic Laser Intensities
Ronnie Shepherd1, Hui Chen1 , Anatoly Faenov2,
Tania Pikuz2, Klaus Widmann1, Gilliss Dyer3, Yuan
Ping1, Scott Wilks1,Andreas Kemp4, Stephanie
Hansen1, Hyun-Kyung Chung1, Kevin Fournier1,
Angie Niles1, James Hunter1, Todd Ditmire3, Tom
Cowan4 1Physics and Advanced Technologies,
Lawrence Livermore National Laboratory 2VNIIFTRI,
Moscow region, Russia 3 University of Texas 4
University of Nevada, Reno
B
E B
04-ERD-023
April 11, 2006
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Outline

• High intensity laser-matter absorption
• Motivation
• Two experiments
• 1) Absorbed energy and formation of non-thermal
  electrons
• 2) Partitioning of energy between thermal and
  non-thermal electrons
• Conclusions

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Motivation Energetic electron are formed in the
interaction of high intensity lasers with matter

• Determine the creation method of relativistic
  electrons
• Study the spatial extent of the relativistic
  electrons
• Measure the transfer of energy to solid targets
  (transfer to thermal electrons)

Understanding the fundamental interaction physics
at relativistic laser intensities is important
for all HEPW physics
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Experiment 1Energy balance is used to infer the
absorbed energy
JanUSP absorption setup
I1017-1020 W/cm2
Several key measurements were made along with the
laser absorption
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Experiment 1Measurements with S P-polarized
light indicate increasing absorption with laser
intensity
New results with S-polarized light show good
agreement with previous measurements and extend
measurement to relativistic regime. P-polarized
absorption shows unexpected high values.
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Scale length measurements
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Experiment 1Three major absorption mechanisms
contribute to the absorbed laser light
Absorption mechanisms depend on laser intensity
and angle of incidence
Target
JXB

• Resonant absorption (RA)
• Laser pulse preplasma plasma
• wave at critical density
• - hot electrons at front side
• JXB heating (JXB)
• JXB Ecos(wt) x Bcos(wt) k1cos (2wt)
• hot electrons in the laser direction
• Vacuum heating (VH)
• Electrons are accelerated by laser electric
  field perpendicular to target surface (only works
  at oblique incidence)
• hot electrons at target normal

RA
VH
Laser pulse
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Experiment 1High absorption is observed as the
laser intensity becomes relativistic
Thermoluminescence dosimeters (TLD)
Target
JXB
Resonant absorption
Heating beam
Target
Vacuum heating
Laser pulse
1018 W/cm2
1019 W/cm2
1020 W/cm2
The directionality of the hot electrons suggests
vacuum heating is the dominant process as
electrons become relativistic
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Experiment 2 The relative thermal/non-thermal
population is inferred from relative x-ray
intensities
Ti-Ka
Al K-shell emission
t
Al-Hea
l
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Experiment 2 X-ray intensity-scaling data
suggest an intensity dependent energy-coupling
transition
weak, short Ti-Ka moderate, long time
history Al Hea energy shift of Ti-Ka! No
Li-like satellites
weak, short Ti-Ka (?) bright, long time
history Al Hea (?) energy shift of Ti-Ka! No
Li-like satellites
intense, short Ti-Ka weak, long time
history Al Hea energy shift of Ti-Ka! No
Li-like satellites
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The measurements suggests an increase in the
thermal electron population as laser intensity
become relativistic
For the first time, an experimental determination
of the energy partition between thermal and
non-thermal electrons has been made.
Data suggest Increase in Hea intensity as
electrons become relativistic Ka risetime
shortens with intensity, ratio peaks earlier as
laser intensity increases Kashift of 14 eV
in 10 ps!
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Experiment 2 Relativistic electrons
collisionally damp their energy in the background
Ti
Collisional coupling between two species
(A.Kemp,et. al, in preparation)
Thermal/Hot electron Equilibration time
NRL plasma formulary
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Experiment 2 Electron spectroscopy data suggest
similar Thot until vos/c gt 90

• A sizable increase in the population of hots is
  noticed at the highest laser intensity. Slope
  suggest Thot100-300 keV

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Experiment 2 Collisionally damped electrons
increase the thermal population, leading to
greater He? emission
The K-shell aluminum spectra suggest a thermal
temperature high enough to burn through the
Li-like charge state, or, Tegt 400 eV
100 keV(333eV)
200 keV(667eV)
200 keV(667eV)
The modeled K-shell spectra based on the simple
model shows good agreement with the time history
and spectral content of the observed aluminum
spectra.
As electrons become relativistic, a rapid
collisional relaxation leads to a large increase
in the temperature and number of thermal electrons
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Strategy

• Plasma conditions change with time
• Bulk e- temperature and density
• Hot e- temperature and density
• K? Shift is most sensitive to Bulk e- temperature
• Fix both bulk and hot e- densities and hot e-
  temperature
• Vary Bulk e- temperature as a function of time
• Evolution of K? spectra as a function of time for
  a fixed Te
• Steady-state spectra in 2 ps if Te lt 200 eV
  see case (A)
• The lower Te is the faster Ka reaches its
  steady-state at the given Te
• Evolution of K? spectra for a linearly increasing
  Te

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case A K? reflects the bulk Te at the given
timeif Te lt 200 eV and slowly changes ( gt2ps)
Te50 eV
Te100 eV
Te200 eV
Te200 eV
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Case B K? spectra for linearly increasing Te

• Calculations done as a function time as Te
  increases from 10 eV to 200 eV in 20 ps
• K? shifts 20 eV as Te changes from 10 eV to 100
  eV
• At 100 eV, distinct shifts by L-shell ions are
  slightly noticeable

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Case B K? shifts due to L-shell ions are shown
at higher Te than 100 eV
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Conclusions

• Absorption measurements at 45? incidence show
  extremely high absorption (80-90 absorption at
  Igt1020 W/cm2)!
• For the first time, the dominant absorption
  mechanism has been determined (using
  space-dependent dosimeters). The measurements
  show RA at 1018 W/cm2, JxB at 1019 W/cm2, and
  VH at 1020 W/cm2.
• WDM is formed by refluxing electrons in solid Ti
• The time-dependent heating from relaxing
  non-thermal electrons can be inferred from the
  time-resolved Ti data.