Trampoline effect and the force field inside the void in complex plasma under microgravity condition

1
Trampoline effect and the force field inside
the void in complex plasma under microgravity
conditions

• M. Kretschmer, S. A. Khrapak, S. K. Zhdanov,
• H. M. Thomas, and G. E. Morfill
• Max-Planck-Institut fuer extraterrestrische
  Physik, Garching, Germany
• V. E. Fortov, A. M. Lipaev, and V. I. Molotkov
• Institute for High Energy Densities, RAS, Moscow,
  Russia
• I. Ivanov and M. V. Turin
• RSC Energia'', Korolev, Russia

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Motivation

• Observation of void formation in ground-based
  (SamsonovGoree, 1999) and microgravity
  conditions (Morfill et al., 1999)
• Qualitative explanation of void formation in
  terms of the ion drag force (SamsonovGoree,
  1999, Goree et al., 1999)
• The existing expressions for the ion drag force
  yielded, however, values too small to explain
  void formation (AkdimGoedheer, 2001)
• This motivated theoretical study of the ion drag
  force in complex plasmas (Khrapak et al.,
  2002,2003,2005 Ivlev et al., 2004,2005).
• At the same time different explanations of voids
  were proposed (Morfill et al., 1999 Avinash,
  2001 Mamun et al., 2002)
• In this talk we report the first experimental
  confirmation of the ion drag mechanism as being
  responsible for the void formation

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Experiment
PKE-Nefedov facility
Typical structure of grain cloud
Ar gas at 12 Pa, rf voltage of 21 V, two grain
sizes a 1.7 and a3.4 ?m
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Trampoline effect

• Weak instability of the void-complex plasma
  interface
• The origin of the instability is under
  investigation
• Similarity to the heart beat instability
• However, two important distinctive features (i)
  the perturbations are confined to the surface
  layer (ii) a small number of grains is injected
  into the void

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Instability stages
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Grain trajectories
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Trajectory analysis
k1? 7800 eV/cm2
k2? 1660 eV/cm2
L ? 0.4 cm
Equation of motion
Potential energy
U0 ? 110 eV
x ? 0.33 cm
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Void qualitative analysis

• In the central part of the discharge electric
  field is weak, FE E and Fi viE . The ratio
  Fi/FE is independent of the electric field.
  Necessary condition for void formation is
  Fi/FE?constgt1.
• The electric field increases from the center to
  the periphery, and so does the ion drift
  velocity. In suprathermal drift regime viE1/2
  and Fi vi-2 E-1. The ratio Fi/FEdecreases
  fast.
• The point characterized by Fi/FE 1 is the
  stable equilibrium for individual grain. It
  characterizes the position of the void boundary.
  

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Void quantitative analysis

• Model for the ion drag force
• collisionless ions
• arbitrary drift velocity
• nonlinear ion scattering for subthertmal ion
  drifts
• Hybrid approach to the ion drag force
• binary collision approach to get a general
  expression accounting for weakly nonlinear ion
  scattereing and finite grain size
• linear kinetic approach to obtain the dependence
  of the effective screening length on the ion flow
  speed

Khrapak, Ivlev, Zhdanov, and Morfill, PoP 12,
042308 (2005)
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Comparison
THEORY
EXPERIMENT

• Plasma parameters Te 2 eV, Ti Tn 0.025 eV,
  ne ni 109 cm-3, z 0.6 (Z 103)
• Additional assumption electric field grows
  linearly in trhe vicinity of the center
• (E ? 10 V/cm2 )

• Void characteristics
• Potential energy barrier U0 110 eV
• Half-width (in the vertical direction) 0.4 cm
• Electric field and dimensionless ion drift
  velocity close to the void boundary are E 4
  V/cm and u 2.5

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Conclusions

• Trampoline effect is observed and used to
  determine experimentally the distribution of
  grain potential energy inside the void region
• Comparison between experiment and theory
  demonstrates good agreement
• Thus, the first experimental confirmation of the
  ion drag mechanism of the void formation is
  reported

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PKE-Nefedov

• Acknowledgements
• MPE DLR/BMBF
• IHED RosaviaKosmos, RKK-Energia
• Parabolic flights ESA/DLR

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THANK YOU FOR YOUR ATTENTION!