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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)