Dusty Plasmas in the Laboratory and Space

Bob Merlino

April 2003 APS Meeting Philadelphia, PA

Outline

1) Introduction – what is a dusty plasma and where are they found

2) the charging of dust in a plasma

3) devices for producing dusty plasmas

4) waves in dusty plasmas

plasma = electrons + ions Plasma

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What is a dusty plasma?

D

• Debye shielding

small particle of solid matter

• becomes negatively charged

• absorbs electrons and ions

Dusty Plasma in the Universe

Dust represents much of the solid matter in the universe and this component often coexists with the ionized matter forming a dusty plasma.

Importance of Charged Dust

the dust acquires an electrical charge and thus is subject to electromagnetic as well as gravitational forces

the charged dust particles participate in the collective plasma processes

DUSTY PLASMAS

• Solar nebula• planetary rings• interstellar medium• comet tails• noctilucent clouds• lightning

• Microelectronic processing

• rocket exhaust• fusion devices

Natural Man-made

Our solar system accumulated out

of a dense cloud of gas and dust, forming everything that is now part of our world.

Rosette Nebula

Noctilucent Clouds (NLC)

• Occur in the summer polar mesosphere (~ 82 km)• 50 nm ice crystals• Associated with unusual radar echoes and reductions in the local ionospheric density

An early temperature measurement in a dusty plasma.

A flame is a very weakly ionized plasma that contains soot particles.

Comet Hale-Bopp

Spokes in Saturn’s B Ring

Voyager 2Nov. 1980

Cassini-HuygensJuly 2004

Semiconductor Processing System

PUMP

gas

13.56 MHz

substrate

EqE

mg

dust

silane (SiH4) + Ar + O2 SiO2 particles

Semiconductor Manufacturing

dustSi

Physics TodayAugust 1994

Dust Charging Processes

• electron and ion collection• secondary emission• UV induced photoelectron emission

Total current to a grain = 0

I = Ie + Ii + Isec + Ipe = 0

electronrepulsion

ion enhancement

The Charge on a Dust Grain

In typical lab plasmas Isec = Ipe = 0

Electron thermal speed >> ion thermal speed so the grainscharge to a negative potential VS relative to the plasma, untilthe condition Ie = Ii is achieved.

a

2

2

1

exp

akT

eV

m

kTenI

akT

eV

m

kTenI

i

S

i

iii

e

S

e

eee

Q = (4oa) 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 m,

Q 2000 e

Mass m 5 1012 mp

Dust Charge Measurements

0

0.5

1

1.5

2

0 20 40 60 80 100 120

Diameter (micron)2

0

0.5

1

1.5

0 20 40 60 80 100 120 140 160

Electron Energy (eV)

Glass

Graphite

Walch, Horanyi, & Robertson,Phys. Rev. Lett. 75, 838 (1995)

Devices for producing dusty plasmas

RF Dusty Plasma Devices

PUMP

ANODE DISK

FIREROD

ROTATING DUST DISPENSER

PLASMA COLUMN

HOT PLATE

B

VIDEO CAMERA

CONFINED DUST GRAINS

GAS IN

GAS IN

Equipotential profiles of an anode double layer

anode

Device for studyingthe trapping of dust

in a dc glow discharge

DUST IN A GLOW DISCHARGE

N2

Vacuumvessel

PS

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BAnode

Dust Tray

Anode Glow Plasma

Dust: kaolin (aluminum silicate)

E

mg

QE

wavefronts

Dust Acoustic Wave Image

dust mass

DA Dispersion relation

Monochromatic plane wave solutions for Te = Ti = T

1

1d

dDA Z

m

kTCf

where = ndo/n+o

0

4

8

12

16

0 50 100 150 200 (s-1)

Dust Acoustic WaveDispersion Relation

theory

Shocks in Dusty Plasma

Shocks in Dusty Plasma- results

Shocks in dusty plasmas Conclusions

• Ion acoustic compress-ional pulses are observed to steepen as they travel through a dusty plasma

• Relevant to astrophysical contexts where density disturbances travel through dust clouds

Conclusions and Outlook• Only recently have we begun to explore the

behavior of dusty plasmas in the laboratory-charging mechanisms, waves

• Technological applications of dusty plasmas are now being exploited

-ceramic deposition – composites

-growth of nanosize particles -diamond growth and deposition on metals

• Dusty plasmas in space are usually embedded in magnetic fields. This aspect of dusty plasmas has yet to be studied in the lab

R = Mv/QB >> typical lab plasmas

• Dusty plasmas may form strongly coupled systems known as Coulomb CrystalsCoulomb Crystals – a new area for plasma physics research

Dusty Plasma

DUST

RF Dusty Plasma Device