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等離子燈
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等離子燈
等離子體（等離子態，電漿，英文：Plasma）是壹種電離的氣體，由於存在電離出來的自由電子和帶電離子，等離子體具有很高的電導率，與電磁場存在極強的耦合作用。等離子態在宇宙中廣泛存在，常被看作物質的第四態（有人也稱之為“超氣態”）。等離子體由克魯克斯在1879年發現，“Plasma”這個詞，由朗廖爾在1928年最早采用。
目錄
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*
*
o 2.1 電離
o
o 2.3 速率分布
* 3 參見
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常見的等離子體
等離子體是存在最廣泛的壹種物態，目前觀測到的宇宙物質中，99%都是等離子體。
* 人造的等離子體
o 熒光燈，霓虹燈燈管中的電離氣體
o 核聚變實驗中的高溫電離氣體
o 電焊時產生的高溫電弧
* 地球上的等離子體
o 火焰（上部的高溫部分）
o 閃電
o 大氣層中的電離層
o 極光
* 宇宙空間中的等離子體
o 恒星
o 太陽風
o 行星際物質
o 恒星際物質
o 星雲
* 其它等離子體
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等離子體的性質
等離子態常被稱為“超氣態”，它和氣體有很多相似之處，比如：沒有確定形狀和體積，具有流動性，但等離子也有很多獨特的性質。
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電離
等離子體和普通氣體的最大區別是它是壹種電離氣體。由於存在帶負電的自由電子和帶正電的離子，有很高的電導率，和電磁場的耦合作用也極強：帶電粒子可以同電場耦合，帶電粒子流可以和磁場耦合。描述等離子體要用到電動力學，並因此發展起來壹門叫做磁流體動力學的理論。
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組成粒子
和壹般氣體不同的是，等離子體包含兩到三種不同組成粒子：自由電子，帶正電的離子和未電離的原子。這使得我們針對不同的組分定義不同的溫度：電子溫度和離子溫度。輕度電離的等離子體，離子溫度壹般遠低於電子溫度，稱之為“低溫等離子體”。高度電離的等離子體，離子溫度和電子溫度都很高，稱為“高溫等離子體”。
相比於壹般氣體，等離子體組成粒子間的相互作用也大很多。
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速率分布
壹般氣體的速率分布滿足麥克斯韋分布，但等離子體由於與電場的耦合，可能偏離麥克斯韋分布。
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參見
* 等離子體物理學
取自"plex phenomena of a plasma, including filamentation
Enlarge
A Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation
In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. "Ionized" in this case means that at least one electron has been removed from a significant fraction of the molecules. The free electric charges make the plasma electrically conductive 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, because it reminded him of a blood plasma Ref.
Contents
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* 1 Common plasmas
* 2 Characteristics
o 2.1 Plasma scaling
o 2.2 Temperatures
o 2.3 Densities
o 2.4 Potentials
* 3 In contrast to the gas phase
* 4 Complex plasma phenomena
* 5 Ultracold Plasmas
* 6 Mathematical descriptions
o 6.1 Fluid
o 6.2 Kinetic
o 6.3 Particle-in-cell
* 7 Fundamental plasma parameters
o 7.1 Frequencies
o 7.2 Lengths
o 7.3 Velocities
o 7.4 Dimensionless
o 7.5 Miscellaneous
* 8 Fields of active research
* 9 See also
* 10 External links
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Common plasmas
A solar coronal mass ejection blasts plasma throughout the Solar System. mon phase of matter. The entire visible universe outside the Solar System is plasma, since all we can see are stars. Since the space between the stars is filled with a plasma, although a very sparse one (see interstellar- and intergalactic medium), essentially the entire volume of the universe is plasma. 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. Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of a plasma (see dusty plasmas).
Commonly encountered forms of plasma include:
* Artificially produced
o Inside fluorescent lamps (low energy lighting), neon signs
o Rocket exhaust
o The area in front of a spacecraft's heat shield during reentry into the atmosphere
o Fusion energy research
o The electric arc in an arc lamp or an arc welder
o Plasma ball (sometimes called a plasma sphere or plasma globe)
* Earth plasmas
o Flames (ie. fire)
o Lightning
o The ionosphere
o The polar aurorae
* Space and astrophysical
o The Sun and other stars (which are plasmas heated by nuclear fusion)
o The solar wind
o The Interplanetary medium (the space between the planets)
o The Interstellar medium (the space between star systems)
o The Intergalactic medium (the space between galaxies)
o The Io-Jupiter flux-tube
o Accretion disks
o Interstellar nebulae
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Characteristics
The term plasma is generally reserved for a system of charged particles large enough to behave as one. 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).
In technical terms, the typical characteristics of a plasma are:
1. Debye screening lengths that are short compared to the physical size of the plasma.
2. Large number of particles within a sphere with a radius of the Debye length.
3. Mean time between collisions usually is long when compared to the period of plasma oscillations.
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Plasma scaling
Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude). The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, quark gluon plasmas:
Typical plasma scaling ranges: orders of magnitude (OOM)
Characteristic Terrestrial plasmas Cosmic plasmas
Size
in metres (m) 10-6 m (lab plasmas) to:
102 m (lightning) (~8 OOM) 10-6 m (spacecraft sheath) to
1025 m (intergalactic nebula) (~31 OOM)
Lifetime
in seconds (s) 10-12 s (laser-produced plasma) to:
107 s (fluorescent lights) (~19 OOM) 101 s (solar flares) to:
1017 s (intergalactic plasma) (~17 OOM)
Density
in particles per
cubic metre 107 to:
1021 (inertial confinement plasma) 1030 (stellar core) to:
100 (i.e., 1) (intergalactic medium)
Temperature
in kelvins (K) ~0 K (Crystalline non-neutral plasma[2]) to:
108 K (magnetic fusion plasma) 102 K (aurora) to:
107 K (Solar core)
Magnetic fields
in teslas (T) 10-4 T (Lab plasma) to:
103 T (pulsed-power plasma) 10-12 T (intergalactic medium) to:
107 T (Solar core)
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Temperatures
The central electrode of a plasma lamp, showing a glowing blue plasma streaming upwards. The colors are a result of the radiative recombination of electrons and ions and the relaxation of electrons in excited states back to lower energy states. These processes emit light in a spectrum characteristic of the gas being excited.
Enlarge
The central electrode of a plasma lamp, showing a glowing blue plasma streaming upwards. The colors are a result of the radiative recombination of electrons and ions and the relaxation of electrons in excited states back to lower energy states. These processes emit light in a spectrum characteristic of the gas being excited.
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-Maxwellian electron distribution function), it is more 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 from (usually lower than) the electron temperature.
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 often 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.
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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).
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Potentials
Lightning is an example of plasma present at Earth's surface. Typically, lightning discharges 30 thousand amps, at up to 100 million volts, and emits light, radio waves, x-rays and even gamma rays [1]. Plasma temperatures in lightning can approach 28,000 kelvins and electron densities may exceed 1024/m3.
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Lightning is an example of plasma present at Earth's surface. Typically, lightning discharges 30 thousand amps, at up to 100 million volts, and emits light, radio waves, x-rays and even gamma rays [1]. Plasma temperatures in lightning can approach 28,000 kelvins and electron densities may exceed 1024/m3.
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, 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.
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In contrast to the gas phase
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
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. 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.
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 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.
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Complex plasma phenomena
Tycho's Supernova remnant, a huge ball of expanding plasma. Langmuir coined the name plasma because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting high-speed electrons
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Tycho's Supernova remnant, a huge ball of expanding plasma. Langmuir coined the name plasma because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting high-speed electrons
Plasma may exhibit complex behaviour. And just as plasma properties scale over many orders of magnitude (see table above), so do these complex features. Many of these features were first studied in the laboratory, and in more recent years, have been applied to, and recognised throughout the universe. Some of these features include:
* Filamentation, the striations or "stringy things" seen in a "plasma ball", the aurora, lightning, and nebulae. They are caused by larger current densities, and are also called magnetic ropes or plasma cables.
* Double layers, localised charge separation regions that have a large potential difference across the layer, and a vanishing electric field on either side. Double layers are found between adjacent plasmas regions with different physical characteristics, and can accelerate ions and produce synchrotron radiation (such as x-rays and gamma rays).
* Birkeland currents, a magnetic-field-aligned electric current, first observed in the Earth's aurora, and also found in plasma filaments.
* Circuits. Birkeland currents imply electric circuits, that follow Kirchhoff's circuit laws. Circuits have a resistance and inductance, and the behaviour of the plasma depends on the entire circuit. Such circuits also store inductive energy, and should the circuit be disrupted, for example, by a plasma instability, the inductive energy will be released in the plasma.
* Cellular structure. Plasma double layers 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.
* Critical ionization velocity in which the relative velocity between an ionized plasma and a neutral gas, may cause further ionization of the gas, resulting in a greater influence of electomagnetic forces.
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Ultracold Plasmas
It is also possible to create ultracold plasmas, by using lasers to trap and cool neutral atoms to temperatures of 1 mK or 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.
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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 Maxwell's equations will be needed to describe them. The coupling of the description of a conductive fluid to electromagnetic fields is known generally as magnetohydrodynamics, or simply MHD.
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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.
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Kinetic
For some cases the fluid description is not sufficient. Kinetic models inc