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PowerPedia:Plasma

Plasma, in the contex of energy, refer to an ionized gas. It is used in a variety of applications (such as plasma displays, a flat-panel electronic visual display technology).

Introduction

In physics and chemistry, a plasma is typically an ionized gas, and is usually considered to be a distinct phase of matter in contrast to solids, liquids, and gases because of its unique properties. "Ionized" means that at least one electron has been dissociated from a proportion of the atoms or molecules. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.

This fourth state of matter was first identified in a discharge tube (or Crookes tube), and so described by Sir William Crookes in 1879 (he called it "radiant matter"). The nature of the Crookes tube "cathode ray" matter was subsequently identified by English physicist Sir J.J. Thomson in 1897, and dubbed "plasma" by Irving Langmuir in 1928, perhaps because it reminded him of a blood plasma . Langmuir wrote:

"Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons."

Plasma typically takes the form of neutral gas-like clouds or charged ion beams, but may also include dust and grains (called dusty plasmas). They are typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move freely.

Common plasmas

Plasmas are the most common phase of matter. Some estimates suggest that up to 99% of the entire visible universe is plasma[1]. Since the space between the stars is filled with a plasma, albeit a very sparse one (see interstellar medium and intergalactic space), essentially the entire volume of the universe is plasma (see astrophysical plasmas). In the solar system, the planet Jupiter accounts for most of the non-plasma, only about 0.1% of the mass and 10−15% of the volume within the orbit of Pluto. Notable plasma physicist Hannes Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of plasma (see dusty plasmas).

 Common forms of plasma include Artificially produced plasma That found in plasma displays and TVs Inside fluorescent lamps (low energy lighting), neon signs Rocket exhaust Arc flash hazards in electrical distribution equipement The area in front of a spacecraft's heat shield during reentry into the atmosphere Fusion energy research The electric arc in an arc lamp, an arc welder or plasma torch Plasma ball (sometimes called a plasma sphere or plasma globe) Plasma used to etch dielectric layers in the production of integrated circuits Terrestrial plasmas Space and astrophysical plasmas The Sun and other stars(which are plasmas heated by nuclear fusion) The solar wind The interplanetary medium(the space between the planets) The interstellar medium(the space between star systems) The Intergalactic medium(the space between galaxies) The Io-Jupiter flux-tube Accretion disks Interstellar nebulae

Plasma properties and parameters

The Earth's "plasma fountain", showing oxygen, helium, and hydrogen ions that gush into space from regions near the Earth's poles. The faint yellow gas shown above the north pole represents gas lost from Earth into space; the green gas is the aurora borealis-or plasma energy pouring back into the atmosphere. Plasma properties are strongly dependent on the bulk (or average) parameters. Some of the most important plasma parameters are the degree of ionization, the plasma temperature, the density and the magnetic field in the plasma region. We explain these parameters, and then describe how plasmas interact with electric and magnetic fields and outline the qualitative differences between plasmas and gases

Definition of a plasma

Although a plasma is loosely described as a quasineutral collection of charged particles, a more rigorous definition requires three criteria to be satisfied:

The plasma approximation
Charged particles must be close enough together that each particle influences many nearby charged particles, rather than just the interacting with the closest particle (these collective effects are a distinguishing feature of a plasma). The plasma approximation is valid when the number of electrons within the sphere of influence (called the Debye sphere whose radius is the Debye (screening) length) of a particular particle is large. The average number of particles in the Debye sphere is given by the plasma parameter, Λ.
Bulk interactions
The Debye screening length (defined above) is short compared to the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place.
Plasma frequency
The electron plasma frequency (measuring plasma oscillations of the electrons) is large compared to the electron-neutral collision frequency (measuring frequency of collisions between electrons and neutral particles). When this condition is valid, plasmas act to shield charges very rapidly (quasineutrality is another defining property of plasmas).

Ranges of plasma parameters

Plasma parameters can take on values varying by many orders of magnitude, but the properties of plasmas with apparently disparate parameters may be very similar (see plasma scaling). The following chart considers only conventional atomic plasmas and not exotic phenomena like quark gluon plasmas:

 Typical ranges of plasma parameters: orders of magnitude (OOM) Characteristic Terrestrial plasmas Cosmic plasmas Sizein metres 10−6 m (lab plasmas) to102 m (lightning) (~8 OOM) 10−6 m (spacecraft sheath) to1025 m (intergalactic nebula) (~31 OOM) Lifetimein seconds 10−12 s (laser-produced plasma) to107 s (fluorescent lights) (~19 OOM) 101 s (solar flares) to1017 s (intergalactic plasma) (~17 OOM) Density in particles percubic metre 107 m-3 to1032 m-3 (inertial confinement plasma) 1030 (stellar core) to100 (i.e., 1) (intergalactic medium) Temperaturein kelvins ~0 K (Crystalline non-neutral plasmaSee The Nonneutral Plasma Group at the University of California, San Diego) to108 K (magnetic fusion plasma) 102 K (aurora) to107 K (Solar core) Magnetic fieldsin teslas 10−4 T (Lab plasma) to103 T (pulsed-power plasma) 10−12 T (intergalactic medium) to1011 T (near neutron stars)

Degree of ionization

For plasma to exist, ionization is necessary. The degree of ionization of a plasma is the proportion of atoms which have lost (or gained) electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive). The degree of ionization, α is defined as α = ni/(ni + na) where ni is the number density of ions and na is the number density of neutral atoms.

Temperatures

Plasma temperature is commonly measured in Kelvin or electron volts, and is (roughly speaking) a measure of the thermal kinetic energy per particle. In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium among themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason the ion temperature may be very different from (usually lower than) the electron temperature. This is especially common in weakly ionized technological plasmas, where the ions are often near the ambient temperature.

Based on the relative temperatures of the electrons, ions and neutrals, plasmas are classified as thermal or non-thermal. Thermal plasmas have electrons and the heavy particles at the same temperature i.e. they are in thermal equilibrium with each other. Non thermal plasmas on the other hand have the ions and neutrals at a much lower temperature (normally room temperature) whereas electrons are much "hotter".

Temperature controls the degree of plasma ionization. In particular, plasma ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. A plasma is sometimes referred to as being hot if it is nearly fully ionized, or cold if only a small fraction (for example 1%) of the gas molecules are ionized (but other definitions of the terms hot plasma and cold plasma are common). Even in a "cold" plasma the electron temperature is still typically several thousand degrees. Plasmas utilized in plasma technology ("technological plasmas") are usually cold in this sense.

Densities

Next to the temperature, which is of fundamental importance for the very existence of a plasma, the most important property is the density. The word "plasma density" by itself usually refers to the electron density, that is, the number of free electrons per unit volume. The ion density is related to this by the average charge state $\langle Z\rangle$ of the ions through $n_e=\langle Z\rangle n_i$. (See quasineutrality below.) The third important quantity is the density of neutrals n0. In a hot plasma this is small, but may still determine important physics. The degree of ionization is ni / (n0 + ni).

Potentials

Since plasmas are very good conductors, electric potentials play an important role. The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the plasma potential or the space potential. If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to the development of a Debye sheath. Due to the good electrical conductivity, the electric fields in plasmas tend to be very small. This results in the important concept of quasineutrality, which says that it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges over large volumes of the plasma ($n_e=\langle Z\rangle n_i$), but on the scale of the Debye length there can be charge imbalance. In the special case that double layers are formed, the charge separation can extend some tens of Debye lengths.

The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation, $n_e \propto e^{e\Phi/k_BT_e}$. Differentiating this relation provides a means to calculate the electric field from the density: $\vec{E} = (k_BT_e/e)(\nabla n_e/n_e)$.

It is, of course, possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise it will be dissipated by the repulsive electrostatic force.

In astrophysical plasmas, Debye screening prevents electric fields from directly affecting the plasma over large distances (ie. greater than the Debye length). But the existence of charged particles causes the plasma to generate and be affected by magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magnetohydrodynamics.

Magnetization

A plasma in which the magnetic field is strong enough to influence the motion of the charged particles is said to be magnetized. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic field before making a collision (ie. ωce / νcoll > 1 where ωce is the "electron gyrofrequency" and νcoll is the "electron collision rate"). It is often the case that the electrons are magnetized while the ions are not. Magnetized plasmas are anisotropic, meaning that their properties in the direction parallel to the magnetic field are different from those perpendicular to it. While electric fields in plasmas are usually small due to the high conductivity, the electric field associated with a plasma moving in a magnetic field is given by E = -V x B (where E is the electric field, V is the velocity, and B is the magnetic field), and is not affected by Debye shielding. [2]

Comparison of plasma and gas phases

Plasma is often called the fourth state of matter. It is distinct from the three lower-energy phases of matter; solid, liquid, and gas, although it is closely related to the gas phase in that it also has no definite form or volume. There is still some disagreement as to whether a plasma is a distinct state of matter or simply a type of gas. Most physicists consider a plasma to be more than a gas because of a number of distinct properties including the following:

Property Gas Plasma
Electrical Conductivity Very low
The air is quite a good insulator, as demonstrated by high voltage electric power transmission where wires typically carry 110,000 Volts. High voltages may lead to electrical breakdown, as can lower pressures in fluorescent lights and neon signs
Very high
1. For many purposes the electric field in a plasma may be treated as zero, although when current flows the voltage drop, though small, is finite, and density gradients are usually associated with an electric field according to the Boltzmann relation.
2. Any electric currents in the plasma "couple" (ie., connect and influence) strongly to magnetic fields, resulting in a large variety of structures such as filaments, sheets, and jets.
3. Collective phenomena are common because the electric and magnetic forces are both long-range and potentially many orders of magnitude stronger than gravitational forces.
Independently acting species One
All gas particles behave in a similar way, influenced by gravity, and collisions with one another
Two or three
Electrons, ions, and neutrals can be distinguished by the sign of their charge so that they behave independently in many circumstances, having different velocities or even different temperatures, leading to phenomenon such as new types of waves and instabilities
Velocity distribution
 Maxwellian The velocity distributes of all gas particles has a characteristic shape:

</td> <td>May be non-Maxwellian
Whereas collisional interactions always lead to a Maxwellian velocity distribution, electric fields influence the particle velocities differently. The velocity dependence of the Coulomb collision cross section can amplify these differences, resulting in phenomena like two-temperature distributions and run-away electrons. </td> </tr>

<tr valign=top> <td bgcolor=#eeeeee>Interactions</td> <td>Binary
Two-particle collisions are the rule, three-body collisions extremely rare.</td> <td>Collective
Each particle interacts simultaneously with many others. These collective interactions are about ten times more important than binary collisions. </td> </tr>

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Complex plasma phenomena

Although the underlying equations governing plasmas are relatively simple, plasma behaviour is extraordinarily varied and subtle: the emergence of unexpected behaviour from a simple model is a typical feature of a complex system. Such systems lie in some sense on the boundary between ordered and disordered behaviour, and cannot typically be described either by simple, smooth, mathematical functions, or by pure randomness. The spontaneous formation of interesting spatial features on a wide range of length scales is one manifestation of plasma complexity. The features are interesting, for example, because they are very sharp, spatially intermittent (the distance between features is much larger than the features themselves), or have a fractal form. Many of these features were first studied in the laboratory, and have subsequently been recognised throughout the universe. Examples of complexity and complex structures in plasmas include:

Filamentation

The striations or "stringy" things,[3] seen in many plasmas, like the aurora,[4] lightning,[5] electric arcs, solar flares[6], and supernova remnants[7]. They are sometimes associated with larger current densities, and are also called magnetic ropes, [8]. (See also Plasma pinch)

Shocks or double layers

Narrow sheets with sharp gradients, such as shocks or double layers which support rapid changes in plasma properties. Double layers involve localised charge separation, which causes a large potential difference across the layer, but does not generate an electric field outside the layer. Double layers separate adjacent plasma regions with different physical characteristics, and are often found in current carrying plasmas. They accelerate both ions and electrons.

Electric fields and circuits

Quasineutrality of a plasma requires that plasma currents close on themselves in electric circuits. Such circuits follow Kirchhoff's circuit laws, and possess a resistance and inductance. These circuits must generally be treated as a strongly coupled system, with the behaviour in each plasma region dependent on the entire circuit. It is this strong coupling between system elements, together with nonlinearity, which may lead to complex behaviour. Electrical circuits in plasmas store inductive (magnetic) energy, and should the circuit be disrupted, for example, by a plasma instability, the inductive energy will be released as plasma heating and acceleration. This is a common explanation for the heating which takes place in the solar corona. Electric currents, and in particular, magnetic-field-aligned electric currents (which are sometimes generically referred to as Birkeland currents), are also observed in the Earth's aurora, and in plasma filaments.

Cellular structure

Narrow sheets with sharp gradients may separate regions with different properties such as magnetization, density, and temperature, resulting in cell-like regions. Examples include the magnetosphere, heliosphere, and heliospheric current sheet. Hannes Alfvén wrote: ""From the cosmological point of view, the most important new space research discovery is probably the cellular structure of space. As has been seen, in every region of space which is accessible to in situ measurements, there are a number of `cell walls', sheets of electric currents, which divide space into compartments with different magnetization, temperature, density, etc ."[9]

Critical ionization velocity

The Critical ionization velocity is the relative velocity between an ionized plasma and a neutral gas. It is sufficient to substantially energise any neutrals which lose an electron. This energisation feeds back to cause yet more ionization, and the process can run away, to almost completely ionize the gas. Critical phemonema in general are typical of complex systems, and may lead to sharp spatial or temporal features.

Ultracold plasma

It is possible to create ultracold plasmas, by using lasers to trap and cool neutral atoms to temperatures of 1 mK lower. Another laser then ionizes the atoms by giving each of the outermost electrons just enough energy to escape the electrical attraction of its parent ion.

The key point about ultracold plasmas is that by manipulating the atoms with lasers, the kinetic energy of the liberated electrons can be controlled. Using standard pulsed lasers, the electron energy can be made to correspond to a temperature of as low as 0.1 K ­ a limit set by the frequency bandwidth of the laser pulse. The ions, however, retain the millikelvin temperatures of the neutral atoms. This type of non-equilibrium ultracold plasma evolves rapidly, and many fundamental questions about its behaviour remain unanswered. Experiments conducted so far have revealed surprising dynamics and recombination behaviour that are pushing the limits of our knowledge of plasma physics.

Non-neutral plasma

The strength and range of the electric force and the good conductivity of plasmas usually ensure that the density of positive and negative charges in any sizeable region are equal ("quasineutrality"). A plasma that has a significant excess of charge density or that is, in the extreme case, composed of only a single species, a called a non-neutral plasma. In such a plasma, electric fields play a dominant role. Examples are charged particle beams, an electron cloud in a Penning trap, and positron plasmas[10]

Dusty plasma and grain plasma

A dusty plasma is one containing tiny charged particles of dust (typically found in space) that also behaves like a plasma. A plasma containing larger particles is called a grain plasma.

Mathematical descriptions

To completely describe the state of a plasma, we would need to write down all the particle locations and velocities, and describe the electromagnetic field in the plasma region. However, it is generally not practical or necessary to keep track of all the particles in a plasma. Therefore, plasma physicists commonly use less detailed descriptions known as models, of which there are two main types:

Fluid

Fluid models describe plasmas in terms of smoothed quantities like density and averaged velocity around each position (see Plasma parameters). One simple fluid model, magnetohydrodynamics, treats the plasma as a single fluid governed by a combination of Maxwell's Equations and the Navier Stokes Equations. A more general description is the two-fluid picture, where the ions and electrons are described separately. Fluid models are often accurate when collisionality is sufficiently high to keep the plasma velocity distribution close to a Maxwell-Boltzmann distribution. Because fluid models usually describe the plasma in terms of a single flow at a certain temperature at each spatial location, they cannot capture velocity space structures like beams, double layers, and resolve wave-particle effects.

Kinetic

Kinetic models describe the particle velocity distribution function at each point in the plasma, and therefore do not need to assume a Maxwell-Boltzmann distribution. A kinetic description is often necessary for collisionless plasmas. There are two common approaches to kinetic description of a plasma. One is based on representing the smoothed distribution function on a grid in velocity and position. The other, known as the particle-in-cell (PIC) technique, includes kinetic information by following the trajectories of a large number of individual particles. Kinetic models are generally more computationally intensive than fluid models.

Plasma parameters

Plasma parameters define various characteristics of a plasma, an electrically-conductive collection of charged particles that responds collectively to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or charged ion beams, but may also include dust and grains. <ref>Peratt, Anthony, Physics of the Plasma Universe (1992); </ref> The behaviour of such particle systems can be studies statistically. <ref>Parks, George K., Physics of Space Plasmas (2004, 2nd Ed.)</ref>

Fundamental plasma parameters

All quantities are in Gaussian cgs units except temperature expressed in eV and ion mass expressed in units of the proton mass μ = mi / mp; Z is charge state; k is Boltzmann's constant; K is wavelength; γ is the adiabatic index; ln Λ is the Coulomb logarithm.

Frequencies

• electron gyrofrequency, the angular frequency of the circular motion of an electron in the plane perpendicular to the magnetic field:
$\omega_{ce} = eB/m_ec = 1.76 \times 10^7 B \mbox{rad/s}$
• ion gyrofrequency, the angular frequency of the circular motion of an ion in the plane perpendicular to the magnetic field:
$\omega_{ci} = eB/m_ic = 9.58 \times 10^3 Z \mu^{-1} B \mbox{rad/s}$
• electron plasma frequency, the frequency with which electrons oscillate when their charge density is not equal to the ion charge density (plasma oscillation):
$\omega_{pe} = (4\pi n_ee^2/m_e)^{1/2} = 5.64 \times 10^4 n_e^{1/2} \mbox{rad/s}$
• ion plasma frequency:
$\omega_{pe} = (4\pi n_iZ^2e^2/m_i)^{1/2} = 1.32 \times 10^3 Z \mu^{-1/2} n_i^{1/2} \mbox{rad/s}$
• electron trapping rate
$\nu_{Te} = (eKE/m_e)^{1/2} = 7.26 \times 10^8 K^{1/2} E^{1/2} \mbox{s}^{-1}$
• ion trapping rate
$\nu_{Ti} = (ZeKE/m_i)^{1/2} = 1.69 \times 10^7 Z^{1/2} K^{1/2} E^{1/2} \mu^{-1/2} \mbox{s}^{-1}$
• electron collision rate
$\nu_e = 2.91 \times 10^{-6} n_e\,\ln\Lambda\,T_e^{-3/2} \mbox{s}^{-1}$
• ion collision rate
$\nu_i = 4.80 \times 10^{-8} Z^4 \mu^{-1/2} n_i\,\ln\Lambda\,T_i^{-3/2} \mbox{s}^{-1}$

Lengths

$\Lambda_e= \sqrt{\frac{h^2}{2\pi m_ekT_e}}= 6.919\times 10^{-8}\,T_e^{-1/2}\,\mbox{cm}$
• classical distance of closest approach, the closest that two particles with the elementary charge come to each other if they approach head-on and each have a velocity typical of the temperature, ignoring quantum-mechanical effects:
$e^2/kT=1.44\times10^{-7}\,T^{-1}\,\mbox{cm}$
• electron gyroradius, the radius of the circular motion of an electron in the plane perpendicular to the magnetic field:
$r_e = v_{Te}/\omega_{ce} = 2.38\,T_e^{1/2}B^{-1}\,\mbox{cm}$
• ion gyroradius, the radius of the circular motion of an ion in the plane perpendicular to the magnetic field:
$r_i = v_{Ti}/\omega_{ci} = 1.02\times10^2\,\mu^{1/2}Z^{-1}T_i^{1/2}B^{-1}\,\mbox{cm}$
• plasma skin depth, the depth in a plasma to which electromagnetic radiation can penetrate:
$c/\omega_{pe} = 5.31\times10^5\,n_e^{-1/2}\,\mbox{cm}$
• Debye length, the scale over which electric fields are screened out by a redistribution of the electrons:
$\lambda_D = (kT/4\pi ne^2)^{1/2} = 7.43\times10^2\,T^{1/2}n^{-1/2}\,\mbox{cm}$

Velocities

$v_{Te} = (kT_e/m_e)^{1/2} = 4.19\times10^7\,T_e^{1/2}\,\mbox{cm/s}$
$v_{Ti} = (kT_i/m_i)^{1/2} = 9.79\times10^5\,\mu^{-1/2}T_i^{1/2}\,\mbox{cm/s}$
• ion sound velocity, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons:
$c_s = (\gamma ZkT_e/m_i)^{1/2} = 9.79\times10^5\,(\gamma ZT_e/\mu)^{1/2}\,\mbox{cm/s}$
• Alfven velocity, the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field:
$v_A = B/(4\pi n_im_i)^{1/2} = 2.18\times10^{11}\,\mu^{-1/2}n_i^{-1/2}B\,\mbox{cm/s}$

Dimensionless

• square root of electron/proton mass ratio
$(m_e/m_p)^{1/2} = 2.33\times10^{-2} = 1/42.9$
• number of particles in a Debye sphere
$(4\pi/3)n\lambda_D^3 = 1.72\times10^9\,T^{3/2}n^{-1/2}$
• Alven velocity/speed of light
$v_A/c = 7.28\,\mu^{-1/2}n_i^{-1/2}B$
• electron plasma/gyrofrequency ratio
$\omega_{pe}/\omega_{ce} = 3.21\times10^{-3}\,n_e^{1/2}B^{-1}$
• ion plasma/gyrofrequency ratio
$\omega_{pi}/\omega_{ci} = 0.137\,\mu^{1/2}n_i^{1/2}B^{-1}$
• thermal/magnetic energy ratio ("beta")
$\beta = 8\pi nkT/B^2 = 4.03\times10^{-11}\,nTB^{-2}$
• magnetic/ion rest energy ratio
$B^2/8\pi n_im_ic^2 = 26.5\,\mu^{-1}n_i^{-1}B^2$

Research and utilization

This is just a partial list of topics. A more complete and organised list can be found on the Web site for Plasma science and technology[11].

 Plasma theory Plasma equilibria and stability Plasma interactions with waves and beams Guiding center Adiabatic invariant Debye sheath Coulomb collision Plasmas in nature The Earth's ionosphere Space plasmas, e.g. Earth's plasmasphere (an inner portion of the magnetosphere dense with plasma) Plasma cosmology Plasma Astronomy Industrial plasmas Plasma sources Dusty Plasmas Plasma diagnostics Plasma applications Fusion power Magnetic fusion energy (MFE) — tokamak, stellarator, reversed field pinch, magnetic mirror, dense plasma focus Inertial fusion energy (IFE) (also Inertial confinement fusion — ICF) Plasma-based weaponry Food processing (Nonthermal plasma)

Plasma displays

A plasma display panel (PDP) is an emissive flat panel display where visible light is created by phosphors excited by a plasma discharge between two flat panels of glass. The gas discharge contains no mercury (contrary to the backlights of an AMLCD). An inert mixture of noble gases (neon and xenon) is used instead.

The plasma display panel was invented at the University of Illinois at Urbana-Champaign by Donald L. Bitzer and H. Gene Slottow in 1964 for the PLATO Computer System. The original monochrome (usually orange or green, sometimes yellow) panels enjoyed a surge of popularity in the early 1970s because the displays were rugged and needed neither memory nor circuitry to refresh the images. A long period of sales decline followed in the late 1970s as semiconductor memory made CRT displays cheaper than plasma displays. Nonetheless, plasma's relatively large screen size and thin profile made the displays attractive for high-profile placement such as lobbies and stock exchanges.

In 1983, IBM introduced a 19" orange on black monochrome display (model 3290 'information panel') which was able to show four simultaneous IBM 3270 virtual machine (VM) terminal sessions. That factory was transferred in 1987 to startup Company, Plasmaco that one of Dr. Bitzer's students, Dr. Larry F. Weber founded with Stephen Globus, and James Kehoe, who was the IBM plant manager. In 1992, Fujitsu introduced the world's first 21-inch full color display. It was a hybrid based on the plasma display created at the University of Illinois at Urbana-Champaign and NHK STRL, achieving superior brightness. In 1996, Matsushita purchased Plasmaco and its color AC technology and American facility. In 1997 Pioneer started selling the first plasma television to the public.

Screen sizes have increased since the 21 inch display in 1992. The largest plasma display in the world was shown at the CES (Consumer Electronics Show) in Las Vegas in 2006. It measured 103" and was made by Matsushita Electrical Industries (Panasonic). Until quite recently, the superior brightness, wider color range, and wider viewing angle of color plasma displays, when compared to LCD televisions, made them one of the most popular forms of display for HDTV. However since that time improvements in LCD technology have closed the gap dramatically. The lower weight, price, and power consumption of LCDs have seen them make large inroads into the former plasma market.

Nonthermal plasma

A nonthermal plasma is in general any plasma which is not in thermodynamic equilibrium, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution.

In the context of food processing, a nonthermal plasma (NTP) is specifically an antimicrobial treatment being investigated for application to fruits, vegetables and other foods with fragile surfaces. These foods are either not adequately sanitized or are otherwise unsuitable for treatment with chemicals, heat or other conventional food processing tools. The term cold plasma has been recently used as a convenient descriptor to distinguish the one-atmosphere, near room temperature plasma discharges from other plasmas, operating at hundreds or thousands of degrees above ambient (see here for more on plasma temperatures). In practical use, however, within the context of food processing the term "cold" can engender misleading images of refrigeration requirements as a part of the plasma treatment.

The nomenclature for nonthermal plasma found in the scientific literature is varied. In some cases, the plasma is referred to by the specific technology used to generate it ("gliding arc", "plasma needle", "plasma jet", "resistive barrier discharge", etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated ("one atmosphere uniform glow discharge plasma", "atmospheric plasma", "ambient pressure nonthermal discharges", "non-equilibrium atmospheric pressure plasmas", etc.). The two features which distinguish NTP from other mature, industrially applied plasma technologies, is that they are 1) nonthermal and 2) operate at or near atmospheric pressure.

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 NTP Technology Class I. Remote treatment II. Direct treatment III. Electrode contact Nature of NTP applied Decaying plasma (afterglow) - longer lived chemical species Active plasma - short and long-lived species Active plasma - all chemical species, including shortest lived and ion bombardment NTP density and energy Moderate density - target remote from electrodes. However, a larger volume of NTP can be generated using multiple electrodes Higher density - target in the direct path of a flow of active NTP Highest density - target within NTP generation field Spacing of target from NTP-generating electrode Approx. 5 - 20 cm; arcing (filamentous discharge) unlikely to contact target at any power setting Approx. 1 - 5 cm; arcing can occur at higher power settings, can contact target Approx. ≤ 1 cm; arcing can occur between electrodes and target at higher power settings Electrical conduction through target No Not under normal operation, but possible during arcing Yes, if target is used as an electrode OR if target between mounted electrodes is electrically conductive Suitability for irregular surfaces High - remote nature of NTP generation means maximum flexibility of application of NTP afterglow stream Moderately high - NTP is conveyed to target in a directional manner, requiring either rotation of target or multiple NTP emitters Moderately low - close spacing is required to maintain NTP uniformity. However, electrodes can be shaped to fit a defined, consistent surface. Examples of technologies Remote exposure reactor, plasma pencil Gliding arc; plasma needle; microwave-induced plasma tube Parallel plate reactor; needle-plate reactor; resistive barrier discharge; dielectric barrier discharge References Gadri et al., 2000. Surface Coatings Technol 131:528-542 Laroussi and Lu, 2005. Appl. Phys. Lett. 87:113902 Montie et al., 2000. IEEE Trans Plasma Sci 28:41-50 Lee et al., 2005. Surface Coatings Technol 193:35-38 Niemira et al., 2005. P2. IFT NPD Mtg., Wyndmoor, PA NIemira et al., 2005. P2-40. IAFP Mtg., Baltimore, MD Sladek and Stofffels, 2005. J Phys D: Appl Phys 38:1716-1721 Stoffels et al., 2002. Plasma Sources Sci. Technol. 11:383-388 Deng et al., 2005. Paper #056149, ASAE Ann. Mtg., Tampa, FL Kelly-Wintenberg et al., 1999. J. Vac. Sci. Technol. A 17(4):1539-44 Laroussi et al., 2003. New J Phys 5:41.1-41.10 Montenegro et al., 2002. J Food Sci 67:646-648 Niemira et al., 2005. P2. IFT NPD Mtg., Wyndmoor, PA NIemira et al., 2005. P2-40. IAFP Mtg., Baltimore, MD

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