6E5 ? Dispersion relation of dust acoustic waves in a DC glow discharge plasma

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6E5 ? Dispersion relation of dust acoustic waves
in a DC glow discharge plasma
2007 Pulsed Power and Plasma Science
Conference Albuquerque, NM

• Bob Merlino, Ross Fisher, Univ. Iowa
• Ed Thomas, Jr. Auburn Univ.
• Work supported by NSF and DOE

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Dust acoustic waves(Rao, Shukla, Yu, Planet.
Space. Sci. 38, 543, 1990)

• very low frequency (ltlt wp, phase speed Csd ltlt
  v,th) longitudinal compressional disturbances in
  a fluid-like dusty plasma
• the dust particles participate in the wave
  dynamics it is a dust wave
• phase speed (dust acoustic speed)

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Dust acoustic waves fluid theory
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Combining the dust momentum equation with the
plasma equations we see that (for the case of
cold dust, Td 0).
where Pe P is the total pressure due
to electrons and ions.
In the dust acoustic wave the inertia is provided
by the massive dust particles and the electrons
and ions provide the restoring force
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excitation of dust acoustic waves

• dust acoustic waves can be driven by an ion-dust
  streaming instability (Rosenberg, JVST A 14, 631,
  1996)
• a relatively modest drift uo vith of the ions
  through the dust is sufficient for instability
• DAWs are spontaneously excited in dusty plasmas
  produced in gas discharges
• are observed visually by laser light scattering

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dust acoustic dispersion relationship (Td 0)
acoustic modes (long wavelength)
w
K
Finite KlD effects
Effect of dust-gas collisions b is the
dust-neutral collision frequency
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Dust acoustic waves
Measurement of the dispersion relationship of the
dust acoustic wave
cm
Original Experiments A. Barkan, N. DAngelo, and
R. L. Merlino, Phys. Plasmas, 2, 3563 (1995). C.
Thompson, A. Barkan, N. DAngelo, and R. L.
Merlino, Phys. Plasmas, 4, 2331 (1997).
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New measurements of dust acoustic waves

• Over the past decade, numerous experimental and
  theoretical studies of DAWs have been made.
• These studies have all been in the linear
  regime of the DAW dispersion.
• Goal of new studies extend dispersion relation
  measurements to finite k?D regime.

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UI dusty plasma device
Ar, 50 -150 mtorr
ANODE GLOW
ANODE
B
LASER
DUST
300 400 V 1 mA
Parameters plasma
density 108 109 cm-3 Te 2 3 eV, T
0.025 eV Dust kaolin powder -average dust
radius 1 micron average dust charge Z
(1000 2000) dust density nd 105 cm-3,
wpd 300 - 500 s-1
For current modulation
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single frame video images of DAW
Natural
65 kHz
39 kHz
130 kHz
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video analysis
About 100 images were analyzed for each frequency
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dispersion relation(long wavelength)
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dispersion relation ? short wavelengths
Td 40 eV
Td 10 eV
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Hot dust?

• J. Williams and E. Thomas, Jr. (IEEE TPS 35, 303,
  2007) measured dust temperatures for dust
  embedded in a dc glow discharge
• Method- stereoscopic particle image velocimetry,
  determined Td in 3 directions
• Found Tds tens of eV at comparable pressures
  as those used here

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Weakly vs. strongly coupled?
If dust were cold, G would be large and
dust would be in strongly coupled state,
which it isnt.
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summary

• DAWs were investigated in a dc glow discharge
  plasma
• DAWs appear spontaneously, but their frequency
  can be controlled by modulating the discharge
  current
• Frequencies up to 200 Hz were studied to
  measure the DAW dispersion relation
• interference of spontaneously excited DAW and
  high frequency, imposed DAW was observed
• Comparison with theory requires that the dust is
  hot, with Td 10s of eV.