Plasma Physics

#REDIRECT Plasma physics

Plasma physics

''This article is about plasma in the sense of an ionized gas. For other uses of the term, such as blood plasma, see plasma (disambiguation).'' Plasma_lamp_Ph:_Luc_Viatour">Image:Plasma-lamp.jpg|thumb|300px|right|Plasma lamp Ph: Luc Viatour In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase (matter). "Ionized" means that at least one electron has been removed from a significant fraction of the molecules. The free charges make the plasma electrical conductivity so that it couples strongly to electromagnetic fields. This fourth state of matter was first identified by Sir William Crookes in 1879 and dubbed "plasma" by Irving Langmuir in 1928. [[Image:Large_helical_device_shot.gif|right|thumb|200px|View of a "shot" on the Large Helical Device though a side port showing hot plasma affected by magnetic fields.]] == Common plasmas == Plasmas are the most common Phase (matter), comprising more than 99% of the visible universe. Commonly encountered forms of plasma include: * Man-made ** Fire (ie. fire) ** Inside fluorescent lamps (low energy lighting), neon signs ** Rocket exhaust ** The area in front of a spacecraft's heat shield during reentry into the earth's atmosphere ** Fusion energy research ** The electric arc in an arc lamp or an arc welding ** Plasma ball (sometimes called a plasma sphere or plasma globe) * Atmospheric ** Lightning, including sprites, jets, elves and tigers ** The ionosphere ** The Polar aurora ** The Io-Jupiter flux-tube * :Category:Space plasmas and astrophysics ** 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 solar systems) ** The Intergalactic space (the space between galaxies) ** Accretion disks ** Interstellar nebula == Characteristics == The term plasma is generally reserved for a system of charged particles large enough to behave collectively. Even a partially ionized gas in which as little as 1% of the particles are ionized, can behave as a plasma and have the characteristics of a plasma (ie. respond to magnetic fields, and be highly electrically conductive). In technical terms, the typical characteristics of a plasma are: # electric field screening lengths that are short compared to the physical size of the plasma. # Large number of particles within a sphere with a radius of the Debye length. # Mean time between collisions usually is long when compared to the period of plasma oscillations. === Plasma scaling === Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude):

	Typical Plasma Scaling Ranges: Orders of Magnitude (OOM)
Characteristic	Terrestrial Plasmas	Cosmic Plasmas
Size in metres (m)	10^-6m (lab plasmas) to: 10²m (lightning) (~8 OOM)	10^-6m (spacecraft sheath) to 10²⁵m (intergalactic nebula) (~31 OOM)
Lifetime in seconds (s)	10^-12s (laser-produced plasma) to: 10⁷s (flurescent lights) (~19 OOM)	10¹s (solar flares) to: 10¹⁷s (intergalactic plasma) (~17 OOM)
Density particles per cubic metre	10⁷ to: 10²¹ (inertial confinement plasma)	10³⁰ (stellar core) to: 10⁰(ie: 1) (intergalactic medium)
Temperature Kelvin (K)	~0K (Crystalline non-neutral plasma) to: 10⁸ (magnetic fusion plasma)	10²K (aurora) to: 10⁷K (Solar core)
Magnetic fields Tesla (T)	10^-4T (Lab plasma) to: 10³T (pulsed-power plasma)	10^-12T (intergalactic medium) to: 10⁷T (Solar core)

=== Temperatures === The defining characteristic of a plasma is ionization. Although ionization can be caused by UV radiation, energetic particles, or strong electric fields, processes that tend to result in a non-James Clerk Maxwell#Kinetic theory electron distribution function, it is most commonly caused by heating the electrons in such a way that they are close to thermal equilibrium so the electron temperature is relatively well-defined. Because the large mass of the ions relative to the electrons hinders energy transfer, it is possible for the ion temperature to be very different (usually lower). The degree of ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. If only a small fraction of the gas molecules are ionized (for example 1%), then the plasma is said to be a cold plasma, even though the electron temperature is typically several thousand degrees. The ion temperature in a cold plasma is ofter near the ambient temperature. Because the plasmas utilized in plasma technology are typically cold, they are sometimes called technological plasmas. They are often created by using a very high electric field to accelerate electrons, which then ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source, e.g. microwaves. Common applications of cold plasmas include plasma-enhanced chemical vapor deposition, plasma ion doping, and reactive ion etching. A hot plasma, on the other hand, is nearly fully ionized. This is what would commonly be known as the "fourth-state of matter". The Sun is an example of a hot plasma. The electrons and ions are more likely to have equal temperatures in a hot plasma, but there can still be significant differences. === 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

n_0

. In a hot plasma this is small, but may still determine important physics. The degree of ionization is

n_i/(n_0+n_i)

. === 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, although where ''double layers'' are formed, the potential drop can be large enough to accelerate ions to relativistic velocities and produce synchrotron radiation such as x-rays and gamma rays. This results in the important concept of quasineutrality, which says that, on the one hand, it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges (

n_e=\langle Z\rangle n_i

), but that, on the other hand, electric fields can be assumed to exist as needed for the physics at hand. 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, electric field screening prevents electric field 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. == In contrast to the gas phase == Plasma is often called the ''fourth state of matter''. It is distinct from the three lower-energy Phase (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	Very high 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. The possibility of currents couples the plasma strongly to magnetic fields, which are responsible for a large variety of structures such as filaments, sheets, and jets. 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	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 new types of waves and instabilities, among other things
Velocity distribution	Maxwellian	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.
Interactions	Binary Two-particle collisions are the rule, three-body collisions extremely rare.	Collective Each particle interacts simultaneously with many others. These collective interactions are about ten times more important than binary collisions.

== Mathematical descriptions == Plasmas may be usefully described with various levels of detail. However the plasma itself is described, if electric or magnetic fields are present, then Maxwells equations will be needed to describe them. The coupling of the description of a conductive fluid dynamics to electromagnetic fields is known generally as magnetohydrodynamics, or simply MHD. === Fluid === The simplest possibility is to treat the plasma as a single fluid governed by the Navier Stokes Equations. A more general description is the two-fluid picture, where the ions and electrons are considered to be distinct. === Kinetic === For some cases the fluid description is not sufficient. Kinetic models include information on distortions of the velocity distribution functions with respect to a Maxwell-Boltzmann distribution. This may be important when currents flow, when Waves in plasmas are involved, or when gradients are very steep. === Particle-In-Cell === Particle-In-Cell models include kinetic information by following the trajectories of a large number of individual particles. Charge and current densities are determined by summing the particles in cells which are small compared to the problem at hand but still contain many particles. The electric and magnetic fields are found from the charge and current densities with appropriate boundary conditions. == Fundamental plasma parameters == All quantities are in Gaussian Centimetre gram second system of units units except temperature expressed in eV and ion mass expressed in units of the proton mass

\mu = m_i/m_p

; ''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 === *electron deBroglie length, the minimum extension of an electron due to quantum mechanics: :

\lambda\!\!\!\!- = \hbar/(m_ekT_e)^{1/2} = 2.76\times10^{-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 === *electron thermal velocity, typical velocity of an electron in a Maxwell-Boltzmann distribution: :

v_{Te} = (kT_e/m_e)^{1/2} = 4.19\times10^7\,T_e^{1/2}\,\mbox{cm/s}

*ion thermal velocity, typical velocity of an ion in a Maxwell-Boltzmann distribution: :

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}

*Hannes Alfven velocity, the speed of the Alfv�n wave 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 = 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

=== Miscellaneous === * Bohm diffusion coefficient :

D_B = (ckT/16eB) = 6.25\times10^6\,TB^{-1}\,\mbox{cm}^2/\mbox{s}

* transverse Spitzer resistivity :

\eta_\perp = 1.15\times10^{-14}\,Z\,\ln\Lambda\,T^{-3/2}\,\mbox{s} = 1.03\times10^{-2}\,Z\,\ln\Lambda\,T^{-3/2}\,\Omega\,\mbox{cm}

== Fields of active research == * 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 sources * Plasma diagnostics ** Thomson scattering ** Langmuir probe ** Spectroscopy ** Interferometry ** Ionospheric heater ** Incoherent scatter radar * Plasma applications ** Fusion power *** Magnetic fusion energy (MFE) -- tokamak, stellarator, reversed field pinch, magnetic mirror *** Inertial fusion energy (IFE) (also Inertial confinement fusion - ICF) ** Industrial plasmas *** plasma chemistry *** plasma processing *** plasma display == See also == * MHD generator * Electric field screening * List of plasma physicists * list of publications in physics#Plasma physics * Plasma Aerodynamics == External links == * [http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasma4StateMatter.html Plasmas: the Fourth State of Matter] * [http://www.plasmas.org/ Plasma Science and Technology] * [http://plasma-gate.weizmann.ac.il/PlasmaI.html Plasma on the Internet] comprehensive list of plasma related links. * [http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html Introduction to Plasma Physics: a graduate level lecture course given by Richard Fitzpatrick] * [http://plasmas.org/ An overview of plasma links and applications] * [http://wwwppd.nrl.navy.mil/nrlformulary/index.html NRL Plasma Formulary online] (or an [http://w3.pppl.gov/~dcoster/nrl/ html version]) * [http://www.plasmacoalition.org/ Plasma Coalition page] * [http://starfire.ne.uiuc.edu/ Plasma Material Interaction] * [http://jnaudin.free.fr/html/oa_plasmoid.htm How to build a Stable Plasmoid at One Atmosphere] (requires pre-ignition) * [http://jnaudin.free.fr/html/oa_plsm4.htm How to build a Stable Plasmoid with this Enhanced Generator] (self-igniting) Plasma physics Phases of matter Academic disciplines

Plasma physics

==Plasma Screen TVs== Do these types of TVs produce real plasma? how do they stop it from blowing up like a fusion bomb? :It would have to undergo the process of either nuclear fission or nuclear fusion, which would be impossible for a plasma TV, as plasma TVs don't actually use or produce any type of plasma. _JarlaxleArtemis">User:JarlaxleArtemis 05:58, Jun 19, 2005 (UTC) ==(??should these.......??)== (??should these broad fields be broken down here or in separate links - and if in separate links, ho so because the naming is broad and fairly arbitrary??) (?? I'd say leave the broad picture here then tree out from here on subpages. e.g. as I did Plasma Sources. RBYII ??) --Anon == units == The equation shows 'cm' and very large powers of 10. Can somebody revise the equations to show 'm'? User:Bobblewik User talk:Bobblewik 11:23, 28 Apr 2005 (UTC) :It's not necessary, as 1 cm = 100 m. _JarlaxleArtemis">User:JarlaxleArtemis 06:00, Jun 19, 2005 (UTC) == Table == I think Iantresman's contribution (Characteristics table, plasma vs gas) is a good idea because it is not always clear why a plasma is special enough to be called a distinct phase of matter, but I am not entirely happy with the content. To begin with, it should be emphasized that all the differences mentioned (except "complex properties") are the result of a single difference, the electrical conductivity. Most of the "complex properties" arise from the presence of multiple species that are distinguished by their electrical charge (and in some cases mass). Finally, the electrical charge provides a means to influence the particle velocities directly, not just through collisions, leading to non-Maxwellian distributions and associated phenomena. Some specific comments:

Electrical Conductivity

Very low
Acts as a dielectric or insulator

Near perfect
Supports electric currents that can spark or arc; in space plasmas, they are often called Kristian Birkeland currents. Such electric currents enable energy to be transferred 'invisibly' from one plasma region to another.

What is a Birkeland current? Currents do not necessarily transfer energy and energy transfer does not require currents.

Duality

None

Yes: passive and active plasmas
As soon as an electric current passes through a passive plasma, it becomes active and exhibits more complicated features. In magnetohydrodynamics (MHD), it is practical to ignore the current (representing it as curl Magnetic field); but as Nobel prize-winner for his work in MHD, Hannes Alfven, points out: "this method is unacceptable in the treatment of a number of [plasma] phenomena .. which require that the electric current is taken account of explicitly" (his emphasis).

Is this standard terminology (in astrophysics)? "Duality" is misleading. What, exactly, was Alfv�n talking about?

Complex properties

None

Active plasmas may also exhibit:

Noise (i.e., oscillations within a large frequency band)
Non-Maxwellian energy distribution (i.e., not smooth)
An electron temperature orders of magnitude greater than the ion temperature, which may be much higher than the neutral gas temperature
At large current densities, may contract into filaments, sometimes called magnetic ropes or plasma cables, (as seen in lightning, aurora and nebulae)
Can separate component gas mixtures in partially ionized gases
Can produce double layers, the boundary between adjacent plasmas regions with different physical characteristics, that can explode, facilitate charge separation, and accelerate ions and produce synchotron radiation (such as x-rays and gamma rays)
Can produce instabilities, which are identified by their distinct morphology (shape)

A smooth velocity distribution can also be non-Maxwellian. What does "separate component gas mixtures" mean? How can a double layer explode? A double layer does not ''facilitate'' charge separation, it ''is'' charge separation. What is meant by the "shape" of an instability? The Rayleigh-Taylor instability also exists in neutral gases. There are a number of minor problems with the content and formulation as well. This table is a good start, but I think it needs a lot of work. Before I start it would help to have some answers to the questions above. User:Art Carlson 07:59, 2005 Jun 9 (UTC) Ian Tresman writes: I was a little surprised to find that someone had removed the table in its entirety, rather than simply moving it to another location. You have some good questions, and thanks for pointing out my lack of understanding of non-Maxwellian, etc. I wonder whether some of these definitions should be answered with their own separate entries? I would certainly like to expand on Double Layers, for example. I'll add some clarification too. User:iantresman 14:40, 2005 Jun 10 (GMT) :I didn't remove the table, I moved it to its own section (while editing it substantially). :*''Duality:'' In the wave-particle duality there are two equal aspects of quantum behavior that must both be taken into account to understand some phenomena. In contrast, if you have a model that describes a plasma with current, then you can use also use it to describe a plasma without current simply by setting ''j'' = 0. Besides, current is not the real issue. There are many phenomena involving currents that can be described perfectly well within MHD. :*''Non-Maxwellian:'' "Uneven" isn't any more helpful than "not smooth". Why not just "non-Maxwellian" with a link to Maxwell-Boltzmann distribution? :*''Double layers:'' Do you really need a current to produce a double layer? Won't you get a double layer at any boundary between two plasmas with different temperatures? A separate entry for this topic is probably a good idea. :*''Instabilities:'' Like I said, neutral gases also show a number of instabilities (Rayleigh-Taylor instability, Kelvin-Helmholtz instability, Baroclinic Instability), so this is not a distinguishing characteristic. :User:Art Carlson 13:02, 2005 Jun 11 (UTC) Thanks for your input: * I don't want to compare plasmas to wave-particle duality, but to the fact that plasmas exhibit a wave aspect (that can be modelled with MHD), and a particle aspect (when an electric current passes through it, and MHD fails.) Sure, the electric current can be neglected in many models (especially when dealing with MHD and waves). But as soon as you get for example filamentation and double layers, then the electric current must be taken explicitly into account. ::That sounds likes what I would call a fluid description as opposed to a kinetic description. Let's leave quantum mechanics out of it. For the third time, MHD does not automatically fail when current is present. User:Art Carlson 12:05, 2005 Jun 12 (UTC) * We could link 'non-Maxwellian' to the Maxwell-Boltzmann distribution page, but it would be nice to express in a word or two, what this really means. In other words, how is it significant and what does it mean. * True, double layers do not require a current, and are produced between any adjacent plasma regions with different properites. ::Then what makes them explode? User:Art Carlson 12:05, 2005 Jun 12 (UTC) * Plasma instabilities formed when an electric current flows through them can have their own characteristics. For example, fluids can produce a Kelvin-Helmholtz instability including plasmas; but pass an electic current through a plasma, and it can produce a diocotron instability which looks remarkably like a Kelvin-Helmholtz instability, but is produced in a different way and has unique characteristics. ::Of course a plasma has different and more instabilities than a gas. But that fact would not normally be enough to classify it as a new phase of matter. User:Art Carlson 12:05, 2005 Jun 12 (UTC) * If you want to have a go at tidying up the table/page, please go ahead. I can provide you with some addition information if you want to contact me by email, details at User:iantresman 22:20, 2005 Jun 11 (GMT)

Plasma physics

Plasma physics is the study of ionized gases. Phases of matter Physics Astrophysics

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