Plume Splitting and Sharpening in Laser Produced Al Plasma

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Plume Splitting and Sharpening in Laser Produced
Al Plasma

• S S Harilal, B Harilal, M S Tillack F Najmabadi

Center for Energy Research
University of California San Diego, La Jolla, CA
92093
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Outline

• Expansion dynamics of Al plasma studied at
  different air pressures
• Ablation plume expansion strongly depends on
  background pressure
• The plasma plume shows plume splitting,
  sharpening, RT instability and stagnation at
  different background pressure levels.
• Through a combination of fast photography and TOF
  spectroscopy, a triple structure of the plume is
  observed at intermediate pressure regimes.

3
Background

• Laser produced plasma are very important in
  applications such as PLD, cluster production etc.
  
• ICCD photography is far sensitive than ordinary
  photography and so even fainter emission can be
  captured by ICCD even at larger distances from
  the target
• TOF distributions of plasma species provide the
  best observations for understanding plasma
  dynamics
• Laser plasma expansion in magnetic field would be
  useful to IFE chamber dynamics as the use of
  magnetic diversion would help to control the ion
  bombardment with the IFE chamber wall
• Knowledge of the RT instability in laser ablated
  plume fronts is crucial to the success of ICF

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Experimental Setup
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Plume evolution at 1 x 10-6 torr
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Plume evolution at 1 x 10-2 torr
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Plume evolution at 0.15 torr
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Plume evolution at 1.3 torr
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Plume evolution at 10 torr
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Plume evolution at 100 torr
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Plume front position vs time

• Fitting Curves
• Free expansion
• R t
• Shock
• R (Eo/ro)1/5 t2/5
• Drag
• R Ro(1exp-bt)

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Plume behavior at different pressure regimes

• 10-6 torr ltPlt5x10-2 torr Free expansion, not
  much influence of the ambient air pressure, plume
  edge maintains a constant velocity ( 107cm/s)
• 50 mtorrltPlt500mtorr the plume behavior is
  characterized by strong interpenetration of the
  laser plasma and and ambient low density gas,
  observed a plume splitting and sharpening.
  Coincidentally, this pressure range falls within
  the transition from collisional to collisionless
  interaction of the plume species with the gas
• 500mtorrltPlt5 torr the mutual penetration of the
  laser plasma species and the ambient gas
  decreases. the front part of the plume penetrates
  into the ambient gas and the instability observed
  mostly in the slower component. This could be due
  to Rayleigh Taylor (RT) sets in the plasma.
• Pgt5 torr Plasma stagnates due to resistance from
  collisions with background gas

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TOF emission profile also shows plume splitting

• TOF emission profiles show plume splitting
• At low pressure (lt50mtorr) TOF profile shows
  single peak
• At pressure gt 50mtorr, TOF profile shows twin
  peak structure
• Twin peak structure appears only after a
  particular distance from the target and this
  distance decreases with increasing pressure
• The faster peak which has a velocity of 107
  cm/s escapes collisions with background gas
• The slower peak is strongly affected by
  background gas collisions.

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Summary

• Fast photography using highly sensitive ICCD is
  extremely useful for studying hydrodynamic
  effects.
• Plume dynamics at different ambient pressure can
  be classified as follows
• a. low pressure lt50 mtorr free expansion
• b. 50 mtorrltPlt500mtorr plume splitting
• c. 500mtorrltPlt5 torr R-T instability
• d. pressuregt5 torr spatial confinement of the
  plume
• TOF profiles also show plume splitting
• Combining imaging and TOF diagnostics, a triple
  structure of the plume is deduced at intermediate
  pressure levels.