Plasma (physics)

In physics, a plasma (Greek πλάσμα plasma " formations ") is a mixture of particles at the atomic- molecular level, the components of which are partially or completely " split " into ions and electrons. That is, a plasma contains free charge carriers. Depending on the particle densities, temperatures, and acting on the relative areas of strength (eg, electrical, magnetic or gravitatitve fields and combinations thereof) to plasmas such as gases, but also completely behave differently. If there is a neutral particle, the kinetic energy is small compared to the kinetic energy of the free charge carriers, it will be often referred to as a background gas or neutral gas. More than 99 % of the visible luminous matter in the universe is in the plasma state.

The term plasma is in this sense back to Irving Langmuir (1928 ). As the plasma condition can be generated by further supply of energy from the gaseous state, it is often referred to as the fourth state.

In certain cases, can be simply described as an electrically conducting gas with the help of MHD plasma. Generally also transport processes ( radiation transport, transport of thermal energy, particle transport, momentum transfer ) must be considered, as well as others that the plasma composition be determined processes (that is, inter alia, ionization, recombination, dissociation, molecule and / or exciton, and chemical reactions of the species present, excitation and absorption processes), so that a complete description can be far more complex.

According to the present or prevailing properties plasmas are often named something concrete- descriptive. Thus one speaks, for example, by high-pressure or low-pressure plasmas, cold or hot plasmas, non- ideal plasmas, plasmas densities, to name but a few. Likewise, the constituents of a plasma can be used to label, such as high-pressure mercury plasma. In addition, the generation mechanism also plays in the characterization of plasmas a role: one thinks, for example, inert gas low-pressure discharge a signal generated by electrical means noble gas plasma with low plasma pressure.

In particle physics, the quasi- free state of quarks and gluons in analogy is called quark-gluon plasma.

• 4.1 plasma pressure
• 4.2 Thermal equilibrium
• 4.3 ionization

• 6.1 Thermal suggestions
• 6.2 Chemical and nuclear reactions
• 6.3 Radiation suggestions
• 6.4 Suggestions by electrostatic fields 6.4.1 excitation by dc
• 6.4.2 Wire Explosion

• 6.5.1 Capacitive electrical stimulation
• 6.5.2 Inductive (magnetic) excitation
• 6.5.3 excitation by microwaves
• 6.5.4 pinch effect

Properties

A plasma is generally a mixture of neutral and charged particles (eg partially ionized plasma). In special cases, only charged particles, electrons and ions and / or charged molecules, are present (eg in fully ionized plasma). Plasmas can be characterized, among others, the following three properties:

A plasma is characterized by the existing species (electrons, positive and negative ions, neutral atoms, neutral and charged molecules ), their densities and temperatures (which need not be equal ), and three-dimensional structure, in particular load flows and electric and magnetic fields and.

Plasmas are usually quasi- neutral, that is, the net charge density is very small compared to the electron density, or the ratio of the charges of the negatively and positively charged particle species is approximately 1 exceptions are limited to regions of the size of the Debye length, for example in the outer layer.

The relationship between ion mass and electron mass is large, at least in 1836 ( at a hydrogen plasma). Many properties of plasmas can be derived.

A characteristic feature of plasmas is their typical glow that is caused by radiation emission excited gas atoms, ions or molecules. Exceptions are plasmas that are very cold ( as often in space ), or which are so hot that the atoms are fully ionized (as in the center of stars ).

Occurrence

Part of the "empty" space between the heavenly bodies located in the plasma state; also the sun and other stars.

On Earth, can be found in the ionosphere and in flashes natural plasmas. Flames have (depending on temperature) also partially properties of a plasma in spite of weak ionization.

In the biosphere there is no practically useful natural plasmas. To apply a plasma technically, you have to create it therefore. This is usually done with the help of a gas discharge.

Applications

Different running in the plasma chemical or physical processes may be utilized.

The use of plasmas can be broken down as follows:

• Gas discharge lamps: including energy saving lamps, fluorescent lamps and arc lamps contain matter in the plasma state
• Surface Treatment: plasmas in semiconductor technology for plasma etching and plasma-induced material deposition (PECVD ) were used. In coating technology functional layers such as reflective coatings or anti-adhesive layers are applied. In materials engineering plasmas for surface modification ( roughening ), for plasma induced material deposition (PECVD and plasma ) for surface cleaning and plasma oxidation are used
• Analytical technique: the digestion of sample materials (plasma ashing ) and in measuring instruments for trace detection of metals ( Inductively Coupled Plasma (ICP ), ICP -MS, English inductively coupled plasma mass spectrometry, LIBS, English laser- induced breakdown spectroscopy, see Atomic Spectroscopy )
• Materials Processing: Arc welding and plasma cutting
• Plasma screen
• Fusion Research: The fuel in a fusion experiment with magnetic confinement is in the plasma state.
• Plasma medicine: Plasma disinfection: disinfection of objects, body parts, wounds, etc.
• Coblation: The high-energy ions in the plasma can separate below 70 ° C human tissue at relatively low temperatures. This is used for surgical procedures on the intervertebral discs, tonsils or turbinates.

Lighting technology

The typical plasmas lights is exploited. In the plasma collision processes lead fast electrons with gas atoms or molecules to the fact that electrons supplied from the sheath of the measures particle energy. This energy is then released at a later time than light emitted. The resulting spectrum is strongly dependent on the gases present, the pressure and the average energy of the electrons.

In some cases, the emitted light can be used directly, such as in some metal -vapor high -pressure lamps (eg sodium - to acknowledge the strong yellow light), the spread in the street lights were used and come or certain noble gas high pressure discharges (for example, xenon). In other cases, the emission is more in the UV range (mainly mercury vapor lamps ), to electromagnetic radiation in the human visible light needs to be converted. This is achieved by phosphors which are coated on the wall of the discharge vessels. The ultraviolet radiation is absorbed in the phosphor and emitted as radiation in the visible again. Examples are used in the interior lighting fluorescent and energy saving lamps and high-pressure mercury lamps used in projectors and outdoors.

Plasma-chemical applications

The use of plasmas for chemical reactions based on the supplied through them high concentrations of chemically reactive molecular fragments. In the past there have been attempts to use an industrial method for the plasma-chemical synthesis. The complex plasma composition makes such reactions, however, very time-consuming and inefficient. Plasma-chemical processes are therefore today rarely used in chemical synthesis, but only at the disposal of toxic gases.

An example of the successful application is the synthesis of diamond. In this case, a diamond from the plasma is deposited on a surface. These diamond layer is polycrystalline and has not the quality of gem diamonds. The growth of this layer are very small (about 1 micron / h). Therefore, thicker layers are very expensive.

On a large scale plasma chemistry continues to be operated in the semiconductor industry. Here plasmas to (dry ) etching ( plasma etching ) and for layer deposition PECVD be used. In etching processes in contrast to the lighting equipment, the direct contact of the plasma with the surface is used to achieve selective removal of material. Play a key role in this case the dominant near the wall electric fields, which are characteristic of boundary layers. Another significant proportion to form the etching removal contained in plasma free radicals ( ions). This can be accelerated with the aid of magnetic fields and thus achieve additional, directional etching removal. The plasma must not be associated with chemically reactive processes and is therefore a physical application.

Physical applications

Plasmas are used for cutting, welding and brazing with plasma torches. The arc welding uses a burning between the workpiece and an electrode arc.

The magnetosphere plasma dynamics (see MGD ) flowing describes the behavior of plasmas in a magnetic field. It can be recovered electric energy ( MHD generator ) or it is used to propel spacecraft (magneto plasma dynamic drive ).

High density hot plasmas - produced by laser pulse irradiation or by electrical discharges - serve as EUV radiation source. Potential users is the EUV lithography.

Classification

A classification of very different forms of plasma may be based on several criteria. One of these is the plasma density. Naturally occurring plasmas vary in density by more than ten orders of magnitude. Extremely high density has the plasma in the solar interior, extremely low density prevailing in interstellar nebulae. Accordingly extremely, the differences in the physical properties of plasma.

Other parameters for the differentiation of plasmas, plasma pressure and plasma temperature.

Plasma pressure

It can be distinguished between

• Low-pressure plasmas
• Plasmas at atmospheric pressure or atmospheric pressure plasmas
• High-pressure plasmas

Low pressure plasmas are generated in the diluted gas, the pressure significantly lower than the atmospheric pressure. Examples are neon bulbs, the northern lights or fluorescent lamps.

In high-pressure plasmas, the plasma pressure is significantly higher than the atmospheric pressure. A typical example is high and very high pressure gas discharge lamps. Also in thunderstorm lightning and sparks there momentarily very high pressure.

Atmospheric pressure plasmas are generated at approximately atmospheric pressure. A typical application is the dielectric-barrier discharges, which are used for example in the processing of plastic materials. Another example are arcs, such as from electric welding.

Thermal equilibrium

An important feature of a plasma is the extent to which it is in thermal equilibrium ( TG):

• The complete thermal equilibrium, the heavy species (molecules, atoms, ions) have the same temperature as the electrons and the plasma is also in radiative equilibrium with the environment, that is, it emits blackbody radiation.

• In local thermal equilibrium (LTE or engl. LTE) all particles have approximately the same local temperature, which may vary from place to place. However, there is no equilibrium with the radiation field. There are emitted characteristic spectral lines and therefore also different from the cavity radiation continua. The state of the LTG can always be accepted if collision processes in relation to the radiation processes clearly dominate. This is the case for example in many technical operations plasmas with temperature gradients, such as in lighting technology with medium and high pressure discharges. In plasmas must be provided for LTG unnecessary high plasma pressure and high plasma density. The dominance of collision processes can also by large turbulence sufficiently strong collective effects - are reached or internal magnetic fields - that is, by strong interaction between the particles.

• In non- thermal plasmas, however, the electrons have a much higher temperature than the heavier species. Low-pressure plasmas typically have this property.

The plasma ions and electrons and thus the plasma itself is generally produced and maintained by supplying energy, usually electrical energy, neutral atoms from a solid or gas. It can reach temperatures of more than 10,000 Kelvin the electrons, the temperature of the ions and the neutral gas can at the same time much lower, for example at room temperature, lie.

With such plasmas workpieces can be machined ( coating, plasma ) without heating it excessively. So that the low-temperature plasmas are particularly suitable for example for the surface modification of temperature-sensitive polymers.

Ionization

The degree of ionization of the plasma, is a further characteristic feature. The degree of ionization is the proportion of species that have submitted by ionization electrons. If TG or at least LTG present, the Saha 's equation describes the degree of ionization of the plasma as a function of this temperature, the density and the ionization energy of the atoms.

• Thermal plasmas with high temperature ( for example, solar corona or fusion plasmas ) are almost completely ionized.
• When engineered low-pressure plasmas, however, the degree of ionization is a maximum of a few thousand, and outside of a thermal equilibrium they are beyond the description with the Saha equation.
• If the ion density of a plasma known or can be determined by appropriate methods, the degree of ionization of the plasma is simply the ratio of the ion density and the sum of neutral and ion density.
• At a low degree of ionization, many effects are determined in plasma by collisions of the ions and electrons to the existing dominant neutral gas atoms.

Which is determined by the degree of ionization and the gas pressure charge carrier density of a plasma determines the spreading ability of electromagnetic waves in the plasma, see ionosphere.

Generation

A plasma can be obtained both energy supply by internal ( eg sun) or external ( such as technical gas discharges ). Remains of the energy coupling or exceed the energy losses - for example, by thermal conduction and / or radiation emission - the energy input, the plasma state is lost. Positive and negative charge carriers recombine to form neutral atoms, molecules or radicals.

The charge carriers can be lost on the walls of vessels or discharge into the vacuum of space by ambipolar diffusion, for example. Ambipolar diffusion can take place even if the plasma state is stable.

To compensate for the loss of charged particles, such must be generated, which is done by, for example, impact ionization. Electrons with sufficiently large kinetic energy under certain circumstances ( when corresponding cross sections for the specific processes) in a position in the collision with atoms, ions or molecules to knock electrons out of their interconnectivity. This process can take as avalanche effect under appropriate conditions, provided that ( a positive ion plus) after the collision from an existing electron two. For technical plasmas the spatial confinement of the plasma can be problematic. The high-energy particles of the plasma are able in certain circumstances walls, workpieces or electrodes to damage by intense radiation or high-energy particles, the latter process is known as sputtering. Especially in lighting technology, the removal of electrode material due to the concomitant reduction in the service life is undesirable.

Methods of energy supply

Thermal suggestions

In case of thermal excitation, the charge carriers are generated by impact ionization due to thermal motion. It is necessary at normal pressure about 15,000 K, to achieve an almost complete ionization. With increasing pressure, the required temperature is rising. One possibility for this is irradiation with a focused laser radiation. Meets the collimated laser beam to a solids arise temperatures of several thousand Kelvin, thermal ionization takes place, which propagates in the gas space above the surface. The resulting plasma absorbs the laser radiation and in turn more amplifies the process. In particularly short laser pulses can occur through the plasma to the phenomenon of self-focusing or shielding of the beam.

Chemical and nuclear reactions

An exothermic reaction results in a strong heating of the gas, so that caused by the rapid movement of the molecule Stoßionisationsprozesse effect the transition to the plasma state. In response, chemical combustion, nuclear fission and nuclear fusion come into question.

Radiation suggestions

For plasma excitation by radiation, the charge carriers generated by ionizing radiation. For this purpose, the quantum energy or particle energy must exceed the ionization energy of the irradiated matter. This is already possible with ultraviolet in gases. X-ray and soft gamma radiation is absorbed little in gases it. Above a certain energy, however, pair formation takes place and the ionization is effective. A high ionization potential have beta and alpha rays.

Suggestions by electrostatic fields

Electrostatic fields lead to discharges or predischarges. Other ions are produced by electron impact ionization. Examples are the thunderstorm lightning electrostatic discharges.

Excitation by dc

Between two electrodes a sufficiently high direct electric voltage is applied. With a suitable combination of voltage, electrode spacing and gas pressure leads to a rollover and the ignition of a discharge between the electrodes. A distinction is made between gas discharges, spark discharge and vacuum spark.

In all cases, a plasma is formed, which also allows the flow of current of the discharge. If the current flow is sufficiently high, the electrodes and in the electron heating is facilitated, it creates an arc. Arcs are used in electric welding and arc lamps ( arc lamps). They can also be operated at alternating voltage.

The amount of the necessary power to the ignition of a plasma depends on the electrode spacing, the shape and the gas pressure ( Paschen's law ).

Wire explosion

A high current flow (eg from a capacitor bank ) through a thin metal wire evaporates explosively in some of these micro-to milliseconds. This produces a partially ionized metal vapor cloud and it may ignite an arc discharge, which leads to the further ionization of the metal vapor. So first enters thermal excitation, followed by impact ionization and excitation instead. One field of application of the wire explosion is provided in the Z- plane.

To prevent the rapid expansion of the plasma, it may be in a non-conductive tubes held ( capillary discharge ).

Excitations by electromagnetic fields

The suggestions by electromagnetic fields, the charge carriers are generated by electron impact ionization. Very high intensity at the focus of a laser beam can also be in air to form a plasma lead ( air break). Responsible is the very high electric field strength of the waves. The energy input can be improved by cyclotron resonance.

Capacitive electrical stimulation

A sufficiently strong alternating electric field is applied to the two plates. Between the plates, a plasma screen, in which charged particles with the frequency of the alternating field is oscillated back and forth (high frequency excitation ). From the plates come not necessarily from carriers. Said particles oscillate, depending on their mass and degree of ionization. The frequency up to which one particle can resonate back, is called plasma frequency.

The plates can also be mounted outside the discharge vessel, so that the field reaches the plasma, only due to the capacity of the wall. This is called electrodeless excitation. In this way contamination is avoided by the electrode material and the wear of the electrodes. According to this principle, some carbon dioxide laser and discharge lamps operate with dielectric barrier. See also Silent electrical discharge.

Inductive (magnetic) excitation

A high-frequency alternating current through a surrounding vacuum vessel, a ring-shaped excitation coil induces currents in a plasma. The process is used in induction lamps and in the gas phase deposition ( PECVD) in pipes.

In tokamaks for nuclear fusion experiments, the plasma is heated in an annular vacuum vessel by a parallel run, rising power and also included contact by the strong magnetic field of a second annular, toroidal wound coil.

Excitation by microwaves

In this case, microwaves from a magnetron are passed into the reaction space. The field strength of the electromagnetic wave must first be high enough to cause electrical breakdown and impact ionization. If the plasma is ignited, the field strength and impedance change radically - the matching conditions of the sending magnetron change.

Alternatively, atmospheric plasmas jets ( or spots ) are produced well- controlled in the power transistor circuits ( zones 2 - 200W ). Such cold plasmas are generated at frequencies of 2.45 GHz, since it is above the plasma frequency, and therefore only the electrons are accelerated in the plasma. This microwave plasmas are often referred to as a micro plasma.

Practical applications are plasma generators, plasma jets and coating systems, chemical reactors, the sulfur lamp and the mercury-free energy-saving lamp and the diamond synthesis.

Pinch effect

The current flowing through the plasma, generates a magnetic field, which in turn constricts the plasma. This is referred to as pinch effect. Here, the plasma is denser and hotter. If the power source provides high currents in the range of a few tens of kilo amps, very dense, hot and very strongly ionized plasmas can be generated that emit X-rays or where even held nuclear fusion ( tokamak ). The pinch effect is also the reason that in a flash forms a narrow channel for the current.