The loosest definition of a plasma is that it is an electrically conducting
gas. At normal temperatures and pressures gases are usually very good
electrical insulators. This is because the electrons in the gas are tightly
bound inside gas atoms and are not free to move in response to externally
applied electric or magnetic fields.
Under certain conditions, however, some or all of the electrons can be removed
from their parent atoms, a process called ionization. The gas then consists of
a mixture of negatively charged electrons, positively charged atoms, called
ions, and un-ionized neutrally charged atoms. Now the electrons and ions are
free to move under the action of applied electromagnetic fields and the gas can
conduct electricity. Due to their much smaller mass the electrons respond to
the applied fields much more readily than the ions and carry most of the
current. Since electrons and ions are produced in pairs and have opposite
charges most of the plasma remains electrically neutral.
There are three principal methods for ionizing a gas. The first, called field
ionization, involves applying an extremely high electrical field that acts on
the electrons in a neutral atom and essentially disrupts the atom. The second,
called thermal ionization, involves raising the temperature of the gas until
collisions knock electrons out of the atoms. Thus, a plasma does not have to be
"hot", although some are extremely so. The third method involves bombarding the
gas with high energy radiation or other sub-atomic particles. Because the
properties of a plasma are so very different from those of a neutral gas the
plasma state is sometimes called " the fourth state of matter ".
In practice, the plasma state covers an extremely large range of temperature and
pressure, from the gas in the fluorescent lamps in your home to the fusion
reactions in the center of the sun. Although you may have to search for a
plasma in your daily life, most of the visible matter in the universe is in the
plasma state.
Technological applications of plasmas include: fluorescent lights, welding arcs,
steelmaking furnaces, experimental fusion reactors, semiconductor processing,
flat panel displays, photo-voltaics, solar coatings, architectural coatings, and
hazardous waste processing.
Thermal plasma systems always contain some mechanism for inducing the flow of electric current through an ionized working gas. The current flow heats the gas to very high temperatures through the mechanism of resistive, or Joule, heating. Through electronic, atomic, and molecular collisions the gas is maintained in an ionized state and the plasma becomes self-sustaining. Typical thermal plasma temperatures are in the range 10,000 K to 30,000 K and result in heat transfer rates that are difficult to match by alternative processing techniques.
In addition to heating, the current also interacts with the magnetic fields present to produce electromagnetic stirring, or Lorentz, forces which produce high velocity (100 to 10,000 m/s) flows in the plasma.
A thermal, or collision dominated, plasma exists at temperatures and pressures where the continuum assumption is valid and where the plasma can usefully be modeled as a variable property fluid. Consequently, computer modeling of thermal plasma systems involves the solution of the Navier-Stokes equation, the continuity equation, one or more energy equations, and Maxwell's Equations.
From the modeling point of view plasma processing systems are highly non-linear. All physical properties are strongly temperature dependent and the governing equations are fully and interactively coupled.
As well as modeling the plasma itself it is also necessary to account for it's interaction with the system being processed, which may take the form of powder or gas
phases dispersed within the plasma or as a workpiece upon which the plasma impinges.
In the RF induction plasma a radio frequency coil external to the plasma region is used to produce an oscillating magnetic field inside the plasma. This, in turn, induces an electric field in the plasma which causes the flow of current.
The inductively coupled plasma has proven to be a very attractive tool both for the thermal treatment of powders and for materials synthesis. The use of inert atmospheres and the absence of electrodes produces the very clean plasmas that are essential for some processing applications. In addition, induction plasmas can be made much larger and more uniform than other thermal plasma systems.
In a series of projects sponsored by G.T.E. Laboratories, Waltham, Massachusetts, the partial differential equations of heat transfer, fluid flow and electromagnetism in the induction plasma were solved using the SIMPLER control volume finite difference algorithm.
The model developed represented a considerable advance over existing models in that it used a two-dimensional representation of the electromagnetic field, rather than a one-dimensional representation. Later extensions to the model included non-equilibrium effects in the plasma and the cooling effects on the plasma of high particle loadings.
'A Theoretical Comparison of Conventional and Hybrid r.f. Plasma Reactors', J.W. McKelliget, N. El-Kaddah, Combustion and Plasma Synthesis of High Temperature Materials , Eds. Z.A. Munir, J.B. Holt, VCH Publishers, 1990.
'Computer Aided Design of Hybrid Plasma Reactors For Use in Materials Synthesis.', J.W. McKelliget, N. El-Kaddah, Metallurgical Processes for the year 2000 and Beyond, Ed.H. Y. Sohn and E.S. Geskin, T.M.S., 169-182, (1989).
'The Effect of Coil Design on Materials Synthesis in an Inductively Coupled Plasma Torch', J.W. McKelliget, N. El-Kaddah, Journal of Applied Physics, 64(6), 2948-2954, (1988).
'Theoretical Prediction of the Effect of Coil Configuration on the Mixing of Gases in an Inductively Coupled Plasma Torch', J.W. McKelliget, N. El-Kaddah, Materials Research Society Symposia Proceedings, 98, 21-27, (1987).
'A Mathematical Model of an Inductively Coupled RF Plasma Torch Used for Particle Melting and Spheroidization', J.W. McKelliget, C. Brecher. TR 0098-05-90-055, GTE Laboratories Waltham MA 02254. December 1990.
'A Mathematical Model of an Inductively Coupled Plasma Torch', J.W. Mckelliget, TR 86-112.1, G.T.E. Laboratories, Waltham MA 02254, September 1986.
In the transferred arc an electric current passes between a cathode electrode and an anode work piece. The plasma is heated to high temperature and the Lorentz forces propel it down towards the anode in the form of a high velocity jet.
Transferred arcs encompass a wide range of operating conditions, from welding arcs of a few hundred amps to steelmaking furnace arcs in the 50,000 amp range. Both of these systems have been modeled and much light has been shed on the underlying transport processes.
Past sponsors have included D.O.E., and Ishikawajima-Harima Heavy Industries Co., Yokohama, Japan
'A Mathematical Model of the Electric Arc Furnace', J. Szekely, J.W. McKelliget, Contract # EX-76-A-01-2295, U.S. Dept. of Energy, Office of Industrial Programs, Washington DC, July 1982.
'Heat Transfer and Fluid Flow in the Welding Arc', J. W. McKelliget, J. Szekely, Met. Trans. A, 17A, 1139-1148, (1986).
'Heat Transfer, Fluid Flow, and Bath Circulation in Electric Arc Furnaces', J. Szekely, J. W. McKelliget, M. Choudhary, Ironmaking and Steelmaking, 10, 169-179, (1983).
'A Mathematical Model of the Cathode Region of a High Intensity Carbon Arc', J. W. McKelliget, J. Szekely, J. Phys. D: Appl. Phys., 16, 1007-1022, (1983).
'Heat and Fluid Flow Phenomena in Arc Welding Operations', with J. Szekely, G. Oreper, J.W. McKelliget, Proceedings of the Engineering Foundation 3rd Conference on the Modeling of Casting and Welding Operations, January 12-17 1986, Santa Barbara, CA, p. 247, Eds. Sindo Kou, R. Mehrabian, T.M.S, (1986).
'A Mathematical Model of Electric Furnace Operations and its Use for Control and Process Optimization.', J. Szekely, J.W. McKelliget, Control '84: Mineral/Metallurgical Processing, Ed. J.A. Herbst, A.I.M.E., pp. 329-36, (1984).
'A Mathematical Model of Heat and Fluid Flow Phenomena in the Electric Arc Furnace', J. Szekely, J.W. McKelliget, Proc. 3rd Int. Arc Furnace Conf.,
Miskolc, Hungary, (1981).
One of the most widespread uses of thermal plasma technology involves the thermal spraying of particles to produce protective coatings. The plasma generator usually involves a transferred arc with a hollow cylindrical anode that forms the nozzle of a plasma torch. The plasma emerges as a high velocity, high temperature flame into which fine particles can be introduced. The crucial step in modeling these systems is to accurately represent the interaction between the plasma and the injected particles.
'Heat Transfer and Fluid Flow in Plasma Spraying', N. El-Kaddah, J.W. McKelliget, J. Szekely, Met. Trans. B, 15B, 59-70, (1984).
'The Temperature and Velocity Fields in a Gas Stream Exiting a Plasma Torch', J.W. McKelliget, J. Szekely, Plasma Chemistry and Plasma Processing, 2(3), 317-332, (1982).
'A Comprehensive Model of a Plasma Spraying Process', J.W. McKelliget, N. El-Kaddah and J. Szekely, Chemistry and Physics of Rapidly Solidified
Materials, ed. B.J. Berkowitz & R.O. Scattergood, A.I.M.E., pp. 243-59, (1983).
ibid. Proc. 7th Conf. on Vacuum Metallurgy, 26 Nov-2 Dec 1982, Tokyo, Iron and Steel Inst. of Japan.
© 2000, J. McKelliget