The Abdus Salam International Center for Theoretical Physics Summer College on Plasma Physics 30 July — 24 August 2007 DUSTY PLASMA PHYSICSBasic Theory and Experiments Professor Robert L. Merlino Department of Physics and Astronomy The University of Iowa Iowa City, IA
CONTENTS • Part I.— Introduction (what is a dusty plasma and where are they found) • Part II.— Basic processes in dusty plasmas • Part III.— Waves and instabilities in dusty plasmas
General references • Shukla & Mamun, Introduction to Dusty Plasma Physics, IOP, Bristol, 2002. (SM) • Goertz, Rev. Geophys. 27, 271, 1989. (G) • Fortov et. al., Physics Reports 421, Dec. 2005. (F) • Mendis and Rosenberg, Cosmic Dusty Plasma, Annu. Rev. Astron. Astrophys. 32, 419, 1994. (MR) • Bouchoule, ed., Dusty Plasmas: Physics, Chemistry and Technical Impacts in Plasma Processing, John Wiley, New York, 1999. (B) • Plasma Sources Science & Technology, Vol. 3 Number 3, August 1994. (PSST) • references to above sources
Part I.—Introduction • what is a dusty plasma • were are dusty plasmas • in space, astrophysics, and the lab • comets • noctilucent clouds, PMSEs • Saturn’s rings • plasma processing reactors • fusion devices
plasma = electrons + ions dusty plasma = plasma + small particles of solid matter • absorbs electrons and ions • becomes negatively charged • Debye shielding What is a dusty plasma?
Cosmic dusty plasmas (MR) • Solar nebulae • Planetary nebulae • Supernova shells • Interplanetary medium • Molecular clouds • Circumsolar rings • Asteroids
Dusty plasmas in the solar system (G) • Cometary tails and comae • Planetary ring systems – Saturn’s rings • Dust streams ejected from Jupiter • Zodiacal light Dusty plasmas on the earth • Ordinaryflames • Atmospheric aerosols • charged snow • lightning on volcanoes
Rocket exhaust Dust on surfaces in space (space station) Dust in fusion devices Thermonuclear fireballs Dust precipitators used to remove pollution from smoke stacks Plasmas used for microelectronic fabrication, e.g. semiconductor chips, solar cells and flat panel displays Plasma Enhanced Chemical Vapor Deposition (PECVD) technologies Dusty plasma devices (DPDs) used to produce and study dusty plasmas in the laboratory Man-made dusty plasmas
A flame is a very weakly ionized plasma that contains soot particles the high degree of ionization in ordinary hydrocarbon flames (five orders of magnitude higher than that predicted by the Saha equation) is due to thermionic electron emission from 10 nm particles of unburnt carbon (soot) D. A. Mendis, New Vistas in Dusty Plasmas, AIP Conf. Proc. 799, American Inst. Physics, Melville, N. Y. 2005, p 583. An early temperature measurement in a dusty plasma.
ASTRONOMY Rosette Nebula • Gravity was the focus of 20th Century Astronomy • For the 21st Century, it will be electromagnetismand plasmas in addition • astrophysicists now realize that the dust may be charged and that must be taken into account Our solar system accumulated out of a dense cloud of gas and dust forming everything that is now part of our world.
ion tail dust tail CometHale-Bopp(MR)
Noctilucent Clouds • an unusual atmospheric phenomenon occurring in the high latitude region of the earth’s summer –(–140 C) mesosphere (50 – 85 km); • glowing, silvery white clouds of ice crystals (50 nm) at about 80 km • usually seen just after sunset • associated with PMSE -polar mesospheric summer echoes – unusually strong radar echoes and electron “bite-outs’
average height of PMSEs and number of displays of NLCs M. Rapp & F. –J. Lübken, Atmos. Chem. Phys. 4, 2601, 2004.
Apollo astronauts see “moon clouds” • dust acquires a positivecharge due to solar UV • some grains are lifted off of the moon’s surface by the electrostatic force electrostatically levitated dust http://www.space.com/scienceastronomy/061007_moon_dust.html
Spokes in Saturn’s B ring (G, MR) • discovered by Voyager 2 in 1980 • nearly radial spokes rotating around the outer portion of the dense B ring • spokes seen in forward scattered light – fine dust • spokes exhibit dynamical behavior on timescales of minutes.
SiO2 particles silane (SiH4) + Ar + O2 Semiconductor Processing (B, PSST)
dust Semiconductor Manufacturing(B, PSST) Si The formation of dust during the processing of semiconductor electronics is a serious problem for the industry. It has been estimated that up to one-half of all semiconductor chips were contaminated during processing.
Rocket Exhaust is a Dusty Plasma Columbia Oct. 20, 1995 • 0.01-10 mm Al2O3 particles • Charged dust may be trappedin earth’s B field • Particles may reach highaltitudes and contribute toseed population for NLC • Occurrence of NLC hasincreased over past 30 years!
dust in fusion devices • the “dust” is a result of the strong interaction between the material walls and energetic plasma which causes flaking, blistering, arching and erosion of the carbon limiters or beryllium surfaces. • one problem is that the dust may retain a large inventory of tritium • studies indicate that dust can be transported deep into the plasma causing a serious contamination problem. • dust poses a serious concern for ITER
snowy plasma • blowing snow can get charged by triboelectric charging (friction) • both positive and negative snow has been found: –200 mC/kg and +72 mC/kg. • The transport of snow along a surface is a process called saltation. The particles hop along the surface rebounding to heights of about 10 cm. The bouncing particles are usually negative, while the particles on the surface are positive. • the electrostatic forces on the snow may be an important consideration in avalanches.
Milestones in dusty plasma research • the discovery of the spokes in Saturn’s B ring in 1980 and • the realization of the dust contamination problem in the semiconductor processing industry at about the same time • provided the impetus that allowed the field of dusty plasma physics to flourish. • the discovery of the dust problem in fusion devices in1998 is another factor that continues to drive dusty plasma research
Part II.—Basic processes in dusty plasmas (SM, MR, F) • dust charging theory • the electrostatic potential around a dust particle • forces on dust particles in a plasma • strongly and weakly coupled dusty • Dusty plasmas under microgravity • particle growth in plasmas
A. Dust Charging Processes • electron, positive andnegative ion collection • secondary emission • UV induced photoelectron emission Total current to an electrically floating grain = 0 I = Ie + I+ + I–+ Isec + Ipe = 0 The dust particle floats to a potential at which I =0
a The Charge on a Dust Grain • In typical lab plasmas Isec = Ipe = 0, also take I– = 0 • Electron thermal speed >> ion thermal speed so the grains • charge to a negative potential jS (= Vs-Vp) relative to the • plasma, until the condition Ie = Ii is achieved. • Take spherical grains of radius a • electron current • orbital motion limited • (OML) ion current
Isolated vs.closely-packed dust (G, MR) • when computing the charge Qd =eZd on dust particles we must first consider whether or not the particles can be considered as “isolated” or not. • a single dust particle in a plasma is isolated • when many dust particles are present with a number density nd, the dust charge will be a function of the ratio of the interparticle spacing, to the plasma Debye length, lD.
Isolated dust particles (F) • eni = eni + Qdnd, nd 0, so ni ni • charging equation • define:
Typical Laboratory Plasma For T e = Ti = T in a hydrogen plasma, jS = 2.5 (kT/e) If T 1 eV and a = 1 m, Qd 2000 e Mass, md 5 1012 mp
Evolution of grain charge H+ ions, Te= 2.5 eV, Ti = 0.025 eV, ne,i = 1015 m3
2 1.5 Graphite 1 Glass 0.5 0 0 20 40 60 80 100 120 140 160 Electron Energy (eV) Dust Charge Measurements Walch, Horanyi, & Robertson, Phys. Rev. Lett. 75, 838 (1995)
Langmuir probe measurements Ie Probe plasma dust ON dust off dust off probe current, Ie time
Dusty Plasma Device (F61) S O L E N O I D C O I L R O T A T I N G D U S T D I S P E N S E R T a H O T P L A T E dusty plasma P L A S M A C O L U M N Probe C O L D E N D P L A T E K O V E N Q machine T O P U M P
Ie Ieo Langmuir probe measurements • When the dust is turned ON, the • electrons become attached to the dust grains and as a result • the • electron current to the probe • decreases. • The quantity Zdnd and • estimated by measuring the • reduction in the electron probe • current and using the neutrality • condition ni = ne + Zdnd, using • Zdnd = no(1 – Ie,dust/Ieo) • where nois the initial plasma density. Dust OFF Dust ON
close-packing effect (G) • if many dust grains are present the charge on a single grain is reduced, usually by a large value • this reduction is due to the fact that the grain surface potential is not only due to the charge on this grain but to all other dust grains in its vicinity • this effect is important when the intergrain spacing D ~ lD
ej/kT ej/kT x/lD x/lD close-packing effect (G) Equally spaced infinite plane sheets of dust grains when the electrons are shared by many particles, each particle gets a smaller portion.
P = ndakT/eni close-packing effect: experiment fixed nd (see, F61)
a r B. the electrostatic potential around a dust particle-isotropic plasma Yukawa Potential
electrostatic PE between particles (F18) experiments
ion density enhancement the electrostatic potential around a dust particle-with ion flow F157-159
C. Forces on dust particles (F, p 40) • gravity: Fg = mdg = (4p/3)a3rd g where rd is the density of the dust material, typically ~ 1000 – 2000 kg/m3 for thin-walled hollow microspheres of wall thickness t, md 4pa2t
QdE E md g -V 2) electric force: Fe = QdE can be used to levitate negative particles: for a 1 micron particle md ~ 8x10-15 kg Q ~ -2000e mdg = QdE E = mdg/Qd ~ 2.5 V/cm
3) neutral drag force: Fnd (F, p. 46) ud = dust velocity, vT,n = gas thermal speed, N = gas density • ud >> vT,n • ud << vT,n dust-neutral collision frequency, ndn where 1<d<2 is a factor that depends on how the atoms are scattered on the dust surface.
4) thermophoresis force: FTh (F, p 46) This force arises if there is a temperature gradient in the neutral gas. It is due to the asymmetry in the momentum transfer to dust from neutrals and is directed toward lower gas temperatures. This force can be used to levitate particles against gravity.
5) Ion drag force, Fid (F, p41) • This force arises from the momentum transfer from flowing ions to charged microparticles in a plasma. • This can be one of the most important forces that gives rise to particle transport in dusty plasmas. • Ion flows are usually produced by large scale electric fields that typically exist in plasmas.