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. – Theorists often assume spheres. – Experimenters can buy spheres image: microParticles GmbH

Dusty plasma Dusty plasma = dust + electrons + ions + gas dust = micron-size particles of solid matter: • absorb electrons & ions, emit photoelectrons • become charged

Dust particle charging • Particles immersed in plasma acquire a charge. • Charge is negative due to higher thermal velocity of electrons a - 10 3 e is a typical charge for sphere a = 1 µ m

DUSTY PLASMAS Natural Man-made • Solar nebula • Combustion • planetary rings • Microelectronic processing • interstellar medium • rocket exhaust • comet tails • fusion devices • noctilucent clouds • lightning

A flame is a very weakly ionized plasma that contains soot particles. An early temperature measurement in a dusty plasma.

Semiconductor Manufacturing Si dust

Semiconductor Processing System dust silane (SiH 4 ) + Ar + O 2 → SiO 2 particles

Rocket Exhaust is a Dusty Plasma Columbia Oct. 20, 1995 • 0.01-10 µ m Al 2 O 3 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!

ASTRONOMY Interstellar medium is partially ionized gas + dust Rosette Nebula

Image: Richard Wainscoat HST Image: NASA Star-forming region: Comet: Gas (ionized by UV) Ion tail (ionized by UV) & dust & dust tail

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

Dust Grain Charging • 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

Book chapter discussion

The Charge on a Dust Grain • Grain is floating → • Currents depend on V S , surface potential • Floating condition determines V S • Charge Q = Ze = 4 πε o a V S , 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 V S relative to the plasma, until the condition I e = I i is achieved.   a kT eV   = π 2 e S I en exp a   e e   m kT e e Q = (4 πε o a) V S  −  kT eV   = π 2 i S I en 1 a   i i   m kT i i

Typical Lab Plasma For T e = T i = T in a hydrogen plasma V S = − 2.5 (kT/e) If T ≈ 1 eV and a = 1 µ m, Q ≈ − 2000 e Charge/Mass ratio is small because m ≈ 5 × 10 12 m proton

Forces acting on dust particles ∝ volume Gravity ∝ area Drag Forces, Radiation pressure ∝ radius Electric, Lorentz

Forces & levitation side port window in vacuum chamber dust particle suspension QE lower electrode mg side port window in vacuum chamber

Microgravity conditions Equipotential electrode electrode Contours Without gravity: Many particles would positive fill a 3D volume potential electrode electrode

Need for Microgravity: Sedimentation Equipotential electrode electrode Contours (parallel- plate plasma) positive potential QE With gravity: particles sediment to mg high-field region ⇒ 2-D layer electrode electrode

Microgravity Cross-sectional view, parabolic-flight experiment Arp, Goree & Piel, Phys. Rev. E 2012

electrostatic trapping of particles forces QE particles sediment to mg 2D layer

despite gravity… 3D dust clouds Glass box – enhances horizontal E field QE particles fill a mg 3D volume

3D dust cloud

forces a Forces acting on a particle ← provides levitation ∝ a Coulomb QE Q v × B ← usually tiny in the lab ∝ a Lorentz ← big for high-density plasmas ∝ a 2 Ion drag ← if a laser beam hits particle ∝ a 2 Radiation pressure ← requires gas ∝ a 2 Gas drag ← requires gas ∝ a 2 Thermophoretic force ← tiny unless a > 0.1 µ m ∝ a 3 Gravity

Gas drag (molecular flow regime 4 π = δ 2 f N m c r V 1 ≤ δ ≤ 1.444 gas p 3 Depending on how gas atoms interact with the particle surface δ Millikan coefficient N number density of gas m mass of gas molecule c mean velocity of molecule r microsphere radius Epstein, Phys. Rev. 1924 p V the speed of microsphere Define drag coefficient: f ≡ gas R V

Acceleration of particle by radiation pressure Ashkin, PRL 1970 reflection transmission } contribute to the force

Laser radiation pressure force 1 π n = 2 F q r p I laser c n 1 index of refraction of medium c light speed in vacuum I laser incident laser intensity π 2 r cross-section area of sphere p

Without laser manipulation

Laser manipulation To melt the crystalline lattice & maintain a liquid, we use laser heating video camera video camera video camera (top view) (top view) (top view) Two laser beams: y y y scanning scanning scanning scanning scanning scanning dust particles x x x • Give particles microspheres microspheres mirrors mirrors mirrors mirrors mirrors mirrors random kicks in ± x direction • Move about, drawing Lissajous lower electrode lower electrode lower electrode figures on the suspension RF RF RF 532 nm 532 nm 532 nm 532 nm 532 nm 532 nm Ar laser Ar laser Ar laser Ar laser Ar laser Ar laser laser laser laser laser laser laser beam 2 beam 2 beam 2 beam 1 beam 1 beam 1 beam 2 beam 2 beam 2 beam 1 beam 1 beam 1 Nosenko et al., Phys. Plasmas (2006).

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 V pp

RF glow discharge plasma Radio-frequency (RF) high voltage applied to lower electrode. 13.6 MHz 100 V pp Low-pressure argon gas in a vacuum chamber. • Plasma sustained by electron-impact ionization. • • Electrons are accelerated by the RF electric fields.

Experimental setup E dc lower electrode Sheath above lower electrode has a vertical dc electric field

2D dusty plasma suspension side port window in vacuum chamber dust particle suspension QE lower electrode mg side port window in vacuum chamber Electric levitation: the suspension of dust particles does not contact any surface.

Dusty plasma parameters Argon RF plasma: Polymer microspheres: 8.1 µ m gas 14 mTorr Argon diameter RF low power, 13.6 MHz suspension size > 5500 particles interparticle distance 0.67 mm

Top-view image of suspension The circular boundary is due to the sheath’s curvature.

Strongly coupled plasmas 2 4 πε Q / r interparti cle potential energy Γ = = 0 particle kinetic energy k T B 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 dust = micron-size particles of solid matter: • absorb electrons & ions, emit photoelectrons • become charged Polymer microspheres: in an electron microscope in plasma (image: microParticles GmbH)

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)