ⓘ ප්ලාස්මා (භෞතික විද්යාව)
භෞතික විද්යාවෙහි සහ රසායන විද්යාවෙහි, ප්ලාස්මා යනු වායුවක අංශු කිසියම් ප්රමාණයක් අයනීකෘත වූ විට නිමැවෙන පදාර්ථයේ අවස්ථාවකි.
ප්ලාස්මා අවස්ථාවේදී අයනීකෘත වායුවක් වන අතර ඉලෙක්ට්රෝනයක යම් අනුපාතයක් නිදහස් වන අතර ඒවා පරමාණුවකට හෝ අණුවක් වෙතට බැඳී ඇත. ධනාත්මක හා සෘණ ආරෝපණ තරමක් ස්වාධීනව ගමන් කිරීමට හැකියාව ඇති විද්යුත් චුම්බක ක්ෂේත්රයට දැඩි ලෙස ප්රතිචාර දැක්වීමට ප්ලාස්මා විදුලිය සන්නායක වේ. එබැවින් ප්ලාස්මා ඝන ද්රව්ය, ද්රව හෝ වායූන් මෙන් නොව එහි ද්රව්යමය තත්වයක් ලෙස සැලකේ. ප්ලාස්මා සාමාන්යයෙන් උදාසීන වායු වළාකුළු ආකාරයක් උදා. තරු උදා වේ.
මෙම පදාර්ථය පළමු වරට ක්ෲක්ස් නලයක් තුළ මුලින්ම හදුනාගත් අතර 1879 දී ශ්රීමත් විලියම් ක්රූකොස් විසින් විස්තර කරන ලදී ඔහු එය "විකිරණමය පදාර්ථයයි" ලෙස හැඳින්වේ. තාරකා සියල්ලම ප්ලාස්මා වලින් සෑදූ අතර තාරකා අතර අවකාශය පවා ප්ලාස්මා සමග පිරවිය හැකි නමුත් ඉතා විරල වේ තාරකාභෞතික ප්ලාස්මා, අන්තර්ගෝලීය මාධ්ය සහ අන්තර්ගෝලීය අවකාශය බලන්න. සූර්ය පද්ධතියේ දී ග්රහ මණ්ඩලයේ නොවන බොහෝ ප්ලූටෝ ග්රහයන්ගෙන් ග්රහයන්ගෙන් සියයට 0.1 ක් හා ප්ලූටෝ කක්ෂය තුල පරිමාව 10-15% ක් පමණ වේ. වායුමය ප්ලාස්මා ඇතුලත ඉතා කුඩා ධාන්ය වර්ගයක් ද සෘණ ආරෝපණ ආරෝපණයක් ලබා ගනී. එසේ කිරීමෙන් ප්ලාස්මා හි ඉතා විශාල සෘණ අයන සංරචකයක් ලෙස ක්රියා කළ හැකි නිසා ධූලි ප්ලාස්මා බලන්න.
2.1. ප්ලාස්මා ගුණ හා පැරාමිතිය අර්ථ දැක්වීම
Although a plasma is loosely described as an electrically neutral medium of positive and negative particles, a definition can have three criteria:
- 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. When this criterion is satisfied, the plasma is quasineutral.
- The plasma approximation: Charged particles must be close enough together that each particle influences many nearby charged particles, rather than just interacting with the closest particle these collective effects are a distinguishing feature of a plasma. The plasma approximation is valid when the number of charge carriers within the sphere of influence called the Debye sphere whose radius is the Debye screening length of a particular particle ae higher than unity to provide collective behavior of the charged particles. The average number of particles in the Debye sphere is given by the plasma parameter, "Λ" the Greek letter Lambda.
- 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, electrostatic interactions dominate over the processes of ordinary gas kinetics.
2.2. ප්ලාස්මා ගුණ හා පැරාමිතිය 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:
2.3. ප්ලාස්මා ගුණ හා පැරාමිතිය Degree of ionization
For plasma to exist, ionization is necessary. The term "plasma density" by itself usually refers to the "electron density", that is, the number of free electrons per unit volume. 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 = n i /n i + n a where n i is the number density of ions and n a is the number density of neutral atoms. The electron density is related to this by the average charge state < Z > of the ions through n e = < Z > n i where n e is the number density of electrons.
3. =ප්ලාස්මා උෂ්ණත්වය සාමාන්යයෙන් මනිනු ලබන්නේ because of a number of distinct properties including the following:
4. 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:
4.1. Complex plasma phenomena Filamentation
Striations or string-like structures are seen in many plasmas, like the plasma ball image above, the aurora, lightning, electric arcs, solar flares, and supernova remnants. They are sometimes associated with larger current densities, and the interaction with the magnetic field can form a magnetic rope structure. See also Plasma pinch
4.2. Complex plasma phenomena Shocks or double layers
Plasma properties change rapidly within a few Debye lengths across a two-dimensional sheet in the presence of a moving shock or stationary double layer. 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.
4.3. Complex plasma phenomena Electric fields and circuits
Quasineutrality of a plasma requires that plasma currents close on themselves in electric circuits. Such circuits follow Kirchhoffs 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 Earths aurora, and in plasma filaments.
4.4. Complex plasma phenomena 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. 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."
4.5. Complex plasma phenomena Critical ionization velocity
The Critical ionization velocity is the relative velocity between an magnetized ionized plasma and a neutral gas above which a runaway ionization process takes place. The critical ionization process is a quite general mechanism for the conversion of the kinetic energy of a rapidly streaming gas into ionization and plasma thermal energy. Critical phenomena in general are typical of complex systems, and may lead to sharp spatial or temporal features.
4.6. Complex plasma phenomena Ultracold plasma
It is 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 behavior which are pushing the limits of our knowledge of plasma physics. One of the metastable states of strongly nonideal plasma is Rydberg matter which forms upon condensation of excited atoms.
4.7. Complex plasma phenomena ආරෝපිත ප්ලැස්මා
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 which has a significant excess of charge density or which is, in the extreme case, composed of only a single species, is 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.
4.8. Complex plasma phenomena Dusty plasma and grain plasma
A dusty plasma is one containing tiny charged particles of dust typically found in space which also behaves like a plasma. A plasma containing larger particles is called a grain plasma.
5. 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:
5.1. Mathematical descriptions ද්රව ආකාතිය
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 Maxwells 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 can neither capture velocity space structures like beams or double layers nor resolve wave-particle effects.
5.2. Mathematical descriptions ගතික ප්ලැස්මා
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. The Vlasov equation may be used to describe the dynamics of a system of charged particles interacting with an electromagnetic field.
6. කෘතිම ප්ලැස්මා
Most artificial plasmas are generated by the application of electric and/or magnetic fields. Plasma generated in a laboratory setting and for industrial use can be generally categorized by:
- Its application
- The temperature relationships within the plasma; Thermal plasma Te = T ion = T gas, Non-Thermal or "cold" plasma Te > > T ion = T gas
- The degree of ionization within the plasma; fully ionized, partially ionized, weakly ionized.
- The pressure at which they operate; vacuum pressure < 10 mTorr, moderate pressure ~ 1 Torr, and atmospheric pressure 760 Torr.
- The electrode configuration used to generate the plasma.
- The magnetization of the particles within the plasma; Magnetized both ion and electrons are trapped in Larmor orbits by the magnetic field, partially magnetized the electrons but not the ions are trapped by the magnetic field, non-magnetized the magnetic field is too weak to trap the particles in orbits but may generate Lorentz forces.
- The type of power source used to generate the plasma; DC, RF and microwave.
6.1. කෘතිම ප්ලැස්මා Low-pressure discharges
- Glow discharge plasmas: non-thermal plasmas generated by the application of DC or low frequency RF (