complex plasma summer school - sektion physikbonitz/si14/download/slides/si14... · rf microspheres...
TRANSCRIPT
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Complex Plasma Summer School
Goree
Dusty Plasmas
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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
akTeV
mkTenI
akTeV
mkTenI
i
S
i
iii
e
S
e
eee
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
VrcmNf pgas2
34πδ=
δ Millikan coefficient N number density of gas m mass of gas molecule mean velocity of molecule microsphere radius V the speed of microsphere
cpr Epstein, Phys. Rev. 1924
Define drag coefficient:
VfR 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
laserp Irc
nqF 21 π=
2prπ
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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 laserbeam 1
Ar laserbeam 2
scanningmirrors
scanningmirrors
532 nm laser beam 2
532 nm laser
beam 1
xy
video camera(top view)
lower electrode
RF
microspheres
Ar laserbeam 1
Ar laserbeam 2
scanningmirrors
scanningmirrors
532 nm laser beam 2
532 nm laser
beam 1
video camera(top view)
lower electrode
RFAr laserbeam 1
Ar laserbeam 2
scanningmirrors
scanningmirrors
532 nm laser beam 2
532 nm laser
beam 1
xy
xy
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
TkrQ
B
02 4/
energy kinetic particleenergy potential cleinterparti 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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onset of self-excited DDW (412-405 mTorr)
ramp down the gas pressure (~ 1 mTorr / sec)
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saturated self-excited DDW (382 mTorr)