什么是等离子

发布网友 发布时间：2022-04-20 09:10

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热心网友 时间：2022-04-12 10:38

等离子体
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等离子体（等离子态，电浆，英文：Plasma）是一种电离的气体，由于存在电离出来的自由电子和带电离子，等离子体具有很高的电导率，与电磁场存在极强的耦合作用。等离子态在宇宙中广泛存在，常被看作物质的第四态（有人也称之为“超气态”）。等离子体由克鲁克斯在1879年发现，“Plasma”这个词，由朗廖尔在1928年最早采用。
目录
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*
*
o 2.1 电离
o
o 2.3 速率分布
* 3 参见

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常见的等离子体

等离子体是存在最广泛的一种物态，目前观测到的宇宙物质中，99%都是等离子体。

* 人造的等离子体
o 荧光灯，霓虹灯灯管中的电离气体
o 核聚变实验中的高温电离气体
o 电焊时产生的高温电弧
* 地球上的等离子体
o 火焰（上部的高温部分）
o 闪电
o 大气层中的电离层
o 极光
* 宇宙空间中的等离子体
o 恒星
o 太阳风
o 行星际物质
o 恒星际物质
o 星云
* 其它等离子体

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等离子体的性质

等离子态常被称为“超气态”，它和气体有很多相似之处，比如：没有确定形状和体积，具有流动性，但等离子也有很多独特的性质。
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电离

等离子体和普通气体的最大区别是它是一种电离气体。由于存在带负电的自由电子和带正电的离子，有很高的电导率，和电磁场的耦合作用也极强：带电粒子可以同电场耦合，带电粒子流可以和磁场耦合。描述等离子体要用到电动力学，并因此发展起来一门叫做磁流体动力学的理论。
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组成粒子

和一般气体不同的是，等离子体包含两到三种不同组成粒子：自由电子，带正电的离子和未电离的原子。这使得我们针对不同的组分定义不同的温度：电子温度和离子温度。轻度电离的等离子体，离子温度一般远低于电子温度，称之为“低温等离子体”。高度电离的等离子体，离子温度和电子温度都很高，称为“高温等离子体”。

相比于一般气体，等离子体组成粒子间的相互作用也大很多。
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速率分布

一般气体的速率分布满足麦克斯韦分布，但等离子体由于与电场的耦合，可能偏离麦克斯韦分布。

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参见

* 等离子体物理学

取自"http://zh.wikipedia.org/wiki/%E7%AD%%E7%A6%BB%E5%AD%90%E4%BD%93"

Category: 等离子体物理学

Plasma (physics)
From Wikipedia, the free encyclopedia.
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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).

A Plasma lamp, illustrating some of the more complex 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 conctive so that it couples strongly to electromagnetic fields. This fourth state of matter was first identified by Sir William Crookes in 1879 and bbed "plasma" by Irving Langmuir in 1928, because it reminded him of a blood plasma Ref.
Contents
[hide]

* 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. http://antwrp.gsfc.nasa.gov/apod/ap020516.html Ref & Credit
Enlarge
A solar coronal mass ejection blasts plasma throughout the Solar System. http://antwrp.gsfc.nasa.gov/apod/ap020516.html Ref & Credit

Plasmas are the most common 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 e to their electric charge, very small grains also behave as ions and form part of a plasma (see sty plasmas).

Commonly encountered forms of plasma include:

* Artificially proced
o Inside fluorescent lamps (low energy lighting), neon signs
o Rocket exhaust
o The area in front of a spacecraft's heat shield ring 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 conctive).

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-proced 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 inctively 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.
Enlarge
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 conctors, 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 e to the development of a Debye sheath. Due to the good electrical conctivity, 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 proce 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 proce 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 Conctivity 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
Enlarge
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 proce 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 inctance, and the behaviour of the plasma depends on the entire circuit. Such circuits also store inctive energy, and should the circuit be disrupted, for example, by a plasma instability, the inctive 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 concted 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 conctive 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

热心网友 时间：2022-04-12 11:56

PDP（Plasma Display Panel，等离子显示）是一种利用气体放电的显示技术，其工作原理与日光灯很相似。它采用了等离子管作为发光元件，屏幕上每一个等离子管对应一个像素，屏幕以玻璃作为基板，基板间隔一定距离，四周经气密性封接形成一个个放电空间。放电空间内充入氖、氙等混合惰性气体作为工作媒质。在两块玻璃基板的内侧面上涂有金属氧化物导电薄膜作激励电极。 当向电极上加入电压，放电空间内的混合气体便发生等离子体放电现象。气体等离子体放电产生紫外线，紫外线激发荧光屏，荧光屏发射出可见光，显现出图像。当使用涂有三原色（也称三基色）荧光粉的荧光屏时，紫外线激发荧光屏，荧光屏发出的光则呈红、绿、蓝三原色。当每一原色单元实现256级灰度后再进行混色，便实现彩色显示。等离子体显示器技术按其工作方式可分为电极与气体直接接触的直流型PDP和电极上覆盖介质层的交流型PDP两大类。目前研究开发的彩色PDP的类型主要有三种：单基板式（又称表面放电式）交流PDP、双基板式（又称对向放电式）交流PDP和脉冲存储直流PDP。
PDP最先进显示技术之一。它固有的优势决定了其生命力。 从技术原理上看，由于PDP屏幕中发光的等离子管在平面中均匀分布，这样显示图像的中心和边缘完全一致，不会出现扭曲现象，实现了真正意义上的纯平面。由于其显示过程中没有电子束运动，不需要借助于电磁场，因此外界的电磁场也不会对其产生干扰，具有较好的环境适应性。 PDP是一种自发光显示技术，不需要背景光源，因此没有LCD显示器的视角和亮度均匀性问题，而且实现了较高的亮度和对比度。而三基色共用同一个等离子管的设计也使其避免了聚焦和汇聚问题，可以实现非常清晰的图像。与CRT和LCD显示技术相比，PDP的屏幕越大，图像的景深和保真度越高。 除了亮度、对比度和可视角度优势外，PDP技术也避免了LCD技术中的响应时间问题，而这些特点正是动态视频显示中至关重要的因素。因此从目前的技术水平看，PDP显示技术在动态视频显示领域的优势更加明显，更加适合作为电视机或家庭影院显示终端使用。 PDP显示器无扫描线扫描，因此图像清晰稳定无闪烁，不会导致眼睛疲劳。PDP也无X射线辐射。由于这两个特点，PDP堪称真正意义上的绿色环保显示产品，是替代传统CRT彩电的理想产品。