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Complex Plasma Summer School Goree Dusty Plasmas What is dust? - - PowerPoint PPT Presentation
Complex Plasma Summer School Goree Dusty Plasmas What is dust? - - PowerPoint PPT Presentation
Complex Plasma Summer School Goree Dusty Plasmas What is dust? Small particles of solid matter a Size 10 nm to 100 microns Material: dielectric or conductor Where you get dust: Grow it. Buy it. Any shape.
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What is dust?
- Small particles of solid matter
- Size 10 nm to 100 microns
- Material: dielectric or conductor
- Where you get dust:
– Grow it. – Buy it.
- Any shape.
– Theorists often assume spheres. – Experimenters can buy spheres
a
image: microParticles GmbH
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Dusty plasma
- absorb electrons & ions, emit photoelectrons
dust = micron-size particles of solid matter:
- become charged
Dusty plasma = dust + electrons + ions + gas
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Dust particle charging
- Particles immersed in plasma acquire a charge.
- Charge is negative due to higher thermal velocity of electrons
a
- 103 e is a typical charge for sphere a = 1 µm
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DUSTY PLASMAS
- Solar nebula
- planetary rings
- interstellar medium
- comet tails
- noctilucent clouds
- lightning
- Combustion
- Microelectronic
processing
- rocket exhaust
- fusion devices
Natural Man-made
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An early temperature measurement in a dusty plasma.
A flame is a very weakly ionized plasma that contains soot particles.
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Semiconductor Manufacturing
dust
Si
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Semiconductor Processing System
dust silane (SiH4) + Ar + O2 → SiO2 particles
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Rocket Exhaust is a Dusty Plasma
- 0.01-10 µm Al2O3 particles
- Charged dust may be trapped
in earth’s B field
- Particles may reach high
altitudes and contribute to seed population for NLC (noctilucent clouds)
- Occurrence of NLC has
increased over past 30 years!
Columbia Oct. 20, 1995
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Rosette Nebula ASTRONOMY Interstellar medium is partially ionized gas + dust
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Comet: Ion tail (ionized by UV) & dust tail
Image: Richard Wainscoat HST Image: NASA
Star-forming region: Gas (ionized by UV) & dust
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Noctilucent Clouds
- Occur in the summer polar mesosphere (~ 82 km)
- 50 nm ice crystals
- Associated with unusual radar echoes and reductions
in the local ionospheric density
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Apollo astronauts see “moon clouds”
- dust acquires a positive
charge due to solar UV
- some grains are lifted
the moon’s surface
electrostatically levitated dust
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Dust Streams from Jupiter Io
volcano
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Dusty Plasma
DUST
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- electrons move about 100 times faster than the positive ions
- initially, electrons hit the grain first, giving it a negative charge
- eventually some + ions are attracted to the grain and some electrons are turned away
- in equilibrium, the dust ends up with a negative charge
Dust Grain Charging
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Book chapter discussion
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The Charge on a Dust Grain
- Grain is floating →
- Currents depend on VS, surface potential
- Floating condition determines VS
- Charge Q = Ze = 4πεoa VS , a = grain radius
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The Charge on a Dust Grain
In typical lab plasmas there is no electron emission Electron thermal speed >> ion thermal speed so the grains charge to a negative potential VS relative to the plasma, until the condition Ie = Ii is achieved.
2 2
1 exp a kT eV m kT en I a kT eV m kT en I
i S i i i i e S e e e e
π π − = =
a
Q = (4πεoa) VS
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Typical Lab Plasma
For T e = Ti = T in a hydrogen plasma VS = − 2.5 (kT/e) If T ≈ 1 eV and a = 1 µm, Q ≈ − 2000 e Charge/Mass ratio is small because m ≈ 5 × 1012 mproton
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Forces acting on dust particles
∝ volume Gravity ∝ area Drag Forces, Radiation pressure ∝ radius Electric, Lorentz
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side port window in vacuum chamber side port window in vacuum chamber lower electrode dust particle suspension
QE mg
Forces & levitation
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Microgravity conditions
Equipotential Contours
electrode electrode positive potential electrode electrode
Without gravity: Many particles would fill a 3D volume
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Need for Microgravity: Sedimentation
Equipotential Contours (parallel- plate plasma)
electrode electrode positive potential electrode electrode
With gravity: particles sediment to high-field region ⇒ 2-D layer
QE mg
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Microgravity
Cross-sectional view, parabolic-flight experiment
Arp, Goree & Piel, Phys. Rev. E 2012
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electrostatic trapping of particles
particles sediment to 2D layer QE mg
forces
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despite gravity… 3D dust clouds
particles fill a 3D volume QE mg
Glass box – enhances horizontal E field
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3D dust cloud
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forces
Forces acting on a particle Ion drag
∝ a2
← big for high-density plasmas Radiation pressure
∝ a2
← if a laser beam hits particle Gas drag
∝ a2
← requires gas Thermophoretic force
∝ a2
← requires gas Coulomb
QE ∝ a
← provides levitation Lorentz
Q v× B ∝ a
← usually tiny in the lab Gravity
∝ a3
← tiny unless a > 0.1 µm a
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Gas drag (molecular flow regime
V r c m N f
p gas 2
3 4π δ =
δ Millikan coefficient N number density of gas m mass of gas molecule mean velocity of molecule microsphere radius V the speed of microsphere
c
p
r Epstein, Phys. Rev. 1924
Define drag coefficient:
V f R
gas
≡ 1 ≤ δ ≤ 1.444
Depending on how gas atoms interact with the particle surface
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Acceleration of particle by radiation pressure
reflection transmission } contribute to the force
Ashkin, PRL 1970
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Laser radiation pressure force
n1 index of refraction of medium c light speed in vacuum Ilaser incident laser intensity cross-section area of sphere
laser p I
r c n q F
2 1 π
=
2 p
r π
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Without laser manipulation
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Laser manipulation
Two laser beams:
- Give particles
random kicks in ±x direction
- Move about,
drawing Lissajous figures on the suspension
Nosenko et al., Phys. Plasmas (2006).
To melt the crystalline lattice & maintain a liquid, we use laser heating
video camera (top view) lower electrode RF microspheres Ar laser beam 1 Ar laser beam 2 scanning mirrors scanning mirrors
532 nm laser beam 2 532 nm laser beam 1
x y
video camera (top view) lower electrode RF microspheres Ar laser beam 1 Ar laser beam 2 scanning mirrors scanning mirrors
532 nm laser beam 2 532 nm laser beam 1
video camera (top view) lower electrode RF Ar laser beam 1 Ar laser beam 2 scanning mirrors scanning mirrors
532 nm laser beam 2 532 nm laser beam 1
x y x y
dust particles
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With laser manipulation
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Book chapter discussion
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Experimental methods
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RF glow discharge plasma
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RF glow discharge plasma
Radio-frequency (RF) high voltage applied to lower electrode. 13.6 MHz 100 Vpp
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RF glow discharge plasma
- Low-pressure argon gas in a vacuum chamber.
- Plasma sustained by electron-impact ionization.
- Electrons are accelerated by the RF electric fields.
Radio-frequency (RF) high voltage applied to lower electrode. 13.6 MHz 100 Vpp
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Experimental setup
Sheath above lower electrode has a vertical dc electric field
lower electrode Edc
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2D dusty plasma suspension
Electric levitation: the suspension of dust particles does not contact any surface.
side port window in vacuum chamber side port window in vacuum chamber lower electrode dust particle suspension
QE mg
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Dusty plasma parameters
Polymer microspheres: diameter 8.1 µm suspension size >5500 particles interparticle distance 0.67 mm Argon RF plasma: gas 14 mTorr Argon RF low power, 13.6 MHz
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Top-view image of suspension
The circular boundary is due to the sheath’s curvature.
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Strongly coupled plasmas
T k r Q
B 2 4
/ energy kinetic particle energy potential cle interparti πε = = Γ
Our experiment:
- start with a solid Γ ≈ 1700
- then heat it, to maintain a liquid, Γ ≈ 68
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Experiment
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Dusty plasma
Dusty plasma = dust + electrons + ions + gas
in an electron microscope in plasma
Polymer microspheres:
(image: microParticles GmbH)
- absorb electrons & ions, emit photoelectrons
dust = micron-size particles of solid matter:
- become charged
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Dust acoustic wave
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- nset of self-excited DDW (412-405 mTorr)
ramp down the gas pressure (~ 1 mTorr / sec)
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