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Plasma

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Heating a Gas may ionize its atoms/molecules and may result in Plasma

Plasma is one of the four main states or phases of matter (the others being solid, liquid, gas). Pure Elements or chemical compounds can move from one phase to another when specific physical conditions are present. Scientifically, when the Temperature of a system goes up, the matter in the system becomes more excited and active then it starts moving to a higher energy state. If energy is added (as the temperature rises) matter moves to a more active state. The plasma is described as the fourth state of matter having positive and negative particles with overall zero or neutral charges at an approximation. Plasma does not have a definite shape or a definite volume. Plasma- occurs at very high temperatures. For example, if we heat water to about 1,500 degrees F it will become plasma. It is too cold for most matter to reach the plasma state on Earth. Our sun and small stars are made of plasma. The most remarkable feature of plasma is that under the influence of magnetic field, it may form structures like filaments or beams. Plasma can be generated with the application of an electric field on a gas.

In a plasma, the electrons are pulled free from the atoms and can move independently; the individual atoms are charged, even though the total number of positive and negative charges is equal, maintaining overall electrical neutrality. Solar wind is the plasma of charged particles (protons, electrons, and heavier ionized atoms) coming out of the Sun in all directions [1].


Ionization

Plasma behaves much differently from a neutral gas. Electric currents in plasma create magnetic fields that confine and shape plasma activity. In this way the electric force can organize elaborate cosmic structures while also provoking the intense electromagnetic emissions now revealed by today’s advanced telescopes. When subjected to heat, electromagnetic radiation, or voltage difference, atoms can lose or gain electrons. In this way, atoms are broken down into negatively and positively charged particles or ions. It is the presence of dissociated electrons and positive ions in a gas that defines an electrically conductive state of matter. Discovery of the plasma universe has opened the door to discovery of the Electric Universe [2].

Thermal and Cold Plasma

There are two broad categories of Plasma discussion: thermal plasma and non-thermal plasma which is also known as cold plasma. A welding torch, lightning bolt or the surface of the Sun are examples of thermal plasma. These are very hot, potentially dangerous plasmas where many of the atoms are ionized. On the other hand cold plasma has only a small fraction of its atoms ionized. Fluorescent lamps and neon signs are few Examples of cold plasma or non-thermal Plasma. It is too cold for most matter to reach the plasma state on Earth. Our sun and small stars are made of plasma. Lighting strikes are plasma and plasma glows when it conducts electricity in neon signs and fluorescent bulbs. The hottest candle flame is plasma. Much of the understanding of plasma has come from the pursuit of controlled nuclear fusion. In the universe plasma is the most common state of matter.

Brief History of Plasma Physics

When blood is cleared of its various corpuscles there remains a transparent liquid, which was named plasma (after the Greek word πλασµα, which means “moldable substance” or “jelly”) by the great Czech medical scientist, Johannes Purkinje (1787-1869). The Nobel Prize Winning American Chemist Irving Langmuir first used this term to describe an ionized gas in 1927 Langmuir was reminded of the way blood plasma carries red and white corpuscles by the way an electrified fluid carries electrons and ions. Langmuir, along with his colleague Lewi Tonks, was investigating the physics and chemistry of tungsten-filament light-bulbs, with a view to finding a way to greatly extend the lifetime of the filament (a goal which he eventually achieved). In the process, he developed the theory of plasma sheaths - the boundary layers which form between ionized plasmas and solid surfaces. He also discovered that certain regions of a plasma discharge tube exhibit periodic variations of the electron density, which we nowadays term Langmuir waves. This was the genesis of Plasma Physics. Interestingly enough, Langmuir’s research nowadays forms the theoretical basis of most plasma processing techniques for fabricating integrated circuits.

Plasma Research

After Langmuir, plasma research gradually spread in other directions, of which five are particularly significant:

  • (i) Firstly, the development of radio broadcasting led to the discovery of the Earth’s ionosphere, a layer of partially ionized gas in the upper atmosphere which reflects radio waves, and is responsible for the fact that radio signals can be received when the transmitter is over the horizon. Unfortunately, the ionosphere also occasionally absorbs and distorts radio waves. For instance, the Earth’s magnetic field causes waves with different polarizations (relative to the orientation of the magnetic field) to propagate at different velocities, an effect which can give rise to “ghost signals” (i.e., signals which arrive a little before, or a little after, the main signal). In order to understand, and possibly correct, some of the deficiencies in radio communication, various scientists, such as E.V. Appleton and K.G. Budden, systematically developed the theory of electromagnetic wave propagation through non-uniform magnetized plasmas.
  • (ii) Secondly, astrophysicists quickly recognized that much of the Universe consists of plasma, and, thus, that a better understanding of astrophysical phenomena requires a better grasp of plasma physics. The pioneer in this field was Hannes Alfv´en, who around 1940 developed the theory of magnetohydrodynamics, or MHD, in which plasma is treated essentially as a conducting fluid.
  • (iii) Thirdly, the creation of the hydrogen bomb in 1952 generated a great deal of interest in controlled thermonuclear fusion as a possible power source for the future.
  • (iv) Fourthly, James A. Van Allen’s discovery in 1958 of the Van Allen radiation belts surrounding the Earth, using data transmitted by the U.S. Explorer satellite, marked the start of the systematic exploration of the Earth’s magnetosphere via satellite, and opened up the field of space plasma physics.
  • (v) Finally, the development of high powered lasers in the 1960’s opened up the field of laser plasma physics. When a high powered laser beam strikes a solid target, material is immediately ablated, and a plasma forms at the boundary between the beam and the target.

Plasma General Discussion

The electromagnetic force is generally observed to create structure: e.g., stable atoms and molecules, crystalline solids. In fact, the most widely studied consequences of the electromagnetic force form the subject matter of Chemistry and Solid-State Physics, which are both disciplines developed to understand essentially static structures. Structured systems have binding energies larger than the ambient thermal energy. Placed in a sufficiently hot environment, they decompose: e.g., crystals melt, molecules disassociate. At temperatures near or exceeding atomic ionization energies, atoms similarly decompose into negatively charged electrons and positively charged ions. These charged particles are by no means free: in fact, they are strongly affected by each others’ electromagnetic fields. Nevertheless, because the charges are no longer bound, their assemblage becomes capable of collective motions of great vigor and complexity. Such an assemblage is termed as plasma.

Since thermal decomposition breaks interatomic bonds before ionizing, most terrestrial plasmas begin as gases. In fact, a plasma is sometimes defined as a gas that is sufficiently ionized to exhibit plasma-like behaviour. Note that plasma-like behaviour ensues after a remarkably small fraction of the gas has undergone ionization. Thus, fractionally ionized gases exhibit most of the exotic phenomena characteristic of fully ionized gases.

Plasmas resulting from ionization of neutral gases generally contain equal numbers of positive and negative charge carriers. In this situation, the oppositely charged fluids are strongly coupled, and tend to electrically neutralize one another on macroscopic length-scales. Such plasmas are termed quasi-neutral (“quasi” because the small deviations from exact neutrality have important dynamical consequences for certain types of plasma mode). Strongly non-neutral plasmas, which may even contain charges of only one sign, occur primarily in laboratory experiments: their equilibrium depends on the existence of intense magnetic fields, about which the charged fluid rotates. It is sometimes remarked that 95% (or 99%, depending on whom you are trying to impress) of the baryonic content of the Universe consists of plasma. This statement has the double merit of being extremely flattering to Plasma Physics, and quite impossible to disprove (or verify). Nevertheless, it is worth pointing out the prevalence of the plasma state. In earlier epochs of the Universe, everything was plasma. In the present epoch, stars, nebulae, and even interstellar space, are filled with plasma. The Solar System is also permeated with plasma, in the form of the solar wind, and the Earth is completely surrounded by plasma trapped within its magnetic field. Terrestrial plasmas are also not hard to find. They occur in lightning, fluorescent lamps, a variety of laboratory experiments, and a growing array of industrial processes. In fact, the glow discharge has recently become the mainstay of the micro-circuit fabrication industry. Liquid and even solid-state systems can occasionally display the collective electromagnetic effects that characterize plasma: e.g., liquid mercury exhibits many dynamical modes, such as Alfv´en waves, which occur in conventional plasmas [3].

References

  1. http://helios.gsfc.nasa.gov/supernova.html#background |retrieved Jan 6, 2015
  2. http://www.thunderbolts.info/wp/2013/09/14/beginners-guide-glossary/#plasma
  3. Main Source: Plasma Physics by Richard Fitzpatrick,Chairman, Graduate Recruitment Committee, Professor of Physics, The University of Texas at Austin