Upload
phungnga
View
219
Download
3
Embed Size (px)
Citation preview
Complex Plasma Summer School
Goree
Dusty Plasmas
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
Dusty plasma
• absorb electrons & ions, emit photoelectrons
dust = micron-size particles of solid matter:
• become charged
Dusty plasma = dust + electrons + ions + gas
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
DUSTY PLASMAS
• Solar nebula • planetary rings • interstellar medium • comet tails • noctilucent clouds • lightning
• Combustion • Microelectronic
processing • rocket exhaust • fusion devices
Natural Man-made
An early temperature measurement in a dusty plasma.
A flame is a very weakly ionized plasma that contains soot particles.
Semiconductor Manufacturing
dust Si
Semiconductor Processing System
dust
silane (SiH4) + Ar + O2 → SiO2 particles
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
Rosette Nebula
ASTRONOMY
Interstellar medium is partially ionized gas + dust
Comet: Ion tail (ionized by UV) & dust tail
Image: Richard Wainscoat HST Image: NASA
Star-forming region: Gas (ionized by UV) & dust
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
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
Dust Streams from Jupiter
Io
volcano
Dusty Plasma
DUST
• 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
Book chapter discussion
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
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
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
Forces acting on dust particles
∝ volume Gravity ∝ area Drag Forces, Radiation pressure ∝ radius Electric, Lorentz
side port window in vacuum chamber
side port window in vacuum chamber
lower electrode
dust particle suspension
QE
mg
Forces & levitation
Microgravity conditions Equipotential Contours
electrode
electrode
positive
potential
electrode
electrode
Without gravity: Many particles would fill a 3D volume
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
Microgravity
Cross-sectional view, parabolic-flight experiment Arp, Goree & Piel, Phys. Rev. E 2012
electrostatic trapping of particles
particles sediment to 2D layer
QE
mg
forces
despite gravity… 3D dust clouds
particles fill a 3D volume
QE
mg
Glass box – enhances horizontal E field
3D dust cloud
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
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
Acceleration of particle by radiation pressure
reflection
transmission } contribute to the force
Ashkin, PRL 1970
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
Without laser manipulation
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
With laser manipulation
Book chapter discussion
Experimental methods
RF glow discharge plasma
RF glow discharge plasma
Radio-frequency (RF)
high voltage applied to
lower electrode.
13.6 MHz
100 Vpp
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
Experimental setup
Sheath above lower electrode has a vertical dc electric field
lower electrode
Edc
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
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
Top-view image of suspension
The circular boundary is due to the sheath’s curvature.
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
Experiment
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
Dust acoustic wave
onset of self-excited DDW (412-405 mTorr)
ramp down the gas pressure (~ 1 mTorr / sec)
saturated self-excited DDW (382 mTorr)