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Definition: [pl힢zmə] a hot ionized gas made up of ions and electrons that is found in the Sun, stars, and fusion reactors. Plasma is a good conductor of electricity and reacts to a magnetic field, but otherwise has properties similar to those of a gas.  (source: Microsoft Encarta Dictionary)


Courtesy NASA  -  click for larger image
In the world of physics it's also known as the fourth state of matter, and is the closest we can get to "seeing" electricity. Extreme temperatures or electrical excitation strip electrons from their normal orbit around the atom's nucleus. This condition of free electrons allows electrical current to flow easily, and gives off electromagnetic energy when the electrons fall back to their normal orbits. A good example of this is the bright orange glow from a neon gas filled sign. There are many natural examples of plasma in our world, and many fun and practical uses as well. We'll try to explore some examples of both types below.


Plasma in our natural world

Let's start with the most important example - our Sun. Without this massive thermonuclear ball of plasma we wouldn't be here, and couldn't continue to exist. The nuclear process that generates the Sun's energy fuses smaller atoms, such as hydrogen, into larger atoms that make up the periodic table of elements. This process also causes much of the Sun's matter to be in a plasma state, which may or may not extend all the way to its core. Unlike a neon sign however, most of the visible light we see is due to the incandescent temperatures of its surface, like the filament in a light bulb.

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The Auroras are visible displays of plasma that form at the polar regions of our planet. They are caused by an interaction of high-energy particles from the Sun's solar wind with the Earth's magnetosphere. The magnetosphere is a huge magnetic field that is generated within the Earth's core and extends far out beyond our atmosphere. This magnetic field shields life on earth from lethal amounts of solar radiation and is essential to our continued existence. The auroral displays are a beautiful show of this great protector at work.

View:
Physics Of The Aurora: Earth Systems

When a storm system creates a large enough electrical charge between clouds, or between clouds and the ground, the air becomes ionized. Once the air is ionized it becomes conductive and allows the built-up charges to equalize in a spectacular display of plasma that we call lightning. Unlike the auroral displays however, lightning is very dangerous to both humans and property. It is lightning that first inspired us to try to harness the power of electricity, and for the most part we've been very successful. As you will see though, it always deserves our respect and should be handled with great care!

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Plasma we create, work, and have fun with

 


Ingenious Plasma:
Tesla's Wireless Light
- Nikola Tesla poses with a glowing lamp, but there's no wires. The gas atoms in the bulb were excited by a nearby radio wave generator.

 


Dangerous Plasma:
A calm Tesla sits and reads while millions of volts produced by his resonant coil ionize the air around him. Don't try this at home kids! The piece of equipment is known as a Tesla Coil and is still used today on a smaller scale for fun and educational projects.

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Unwanted Plasma: High voltage potential across an air-gap switch in a utility substation ionizes the air, allowing current to continue to flow.

Decorative Plasma:

Images courtesy of The Cathode Ray Tube Site

These beautiful works of art are called Geissler Tubes. These tubes were invented in 1857 by a German glassblower named Heinrich Geissler. They began to be mass produced in the 1880's and were the predecessors to the more modern neon sign.
Additional fascinating examples can be seen at The Cathode Ray Tube Site and The Spark Museum.

 

Images by Phillip Slawinski - Creative Commons License
These are modern examples of widely used neon type signs. Although they're all commonly referred to as "neon", many use other noble gasses such as xenon (right image), argon, or helium to produce their light. After being bent into shape the tubes are evacuated of air and the correct gas is then used to refill the tube. Electrodes are attached at the ends of the tube and sealed off. High voltage (typically 6,000 to 15,000 volts) provided from a transformer device ionizes the gas inside and allows a small current (typically 20 to 100 milliamps) to flow through. This creates the glowing plasma state which gives off a characteristic color for each respective gas.
Lighting with Plasma:
The first form of electric lighting was the carbon arc lamp. In 1801 Sir Humphrey Davy built and displayed a crude, but working prototype using a series of 2,000 battery cells to achieve the necessary voltage and current. In 1876 Charles Brush developed a more practical lamp using this idea that could replace gas lanterns for street lighting. In some ways this was an advancement, but it introduced new problems as well. The lamps produced an intense glare which contrasted greatly with the soft, pleasant glow of a gas lamp. The arcing carbon rods gave off a soot which needed to be regularly cleaned from the fixture glass. Also, the extremely hot arc would burn away the tips of the carbon rods in a very short time. This required a mechanical clock-type mechanism to continually advance the rod tips into the arc in order to maintain the correct arc length. This mechanism was tricky to build and properly maintain. The combination of all of these drawbacks gave incentive to Thomas Edison and others to develop a simpler and more reliable electric light.
 
A much more refined type of plasma lighting is the fluorescent lamp. Predecessors to the fluorescent lamp were gas discharge tubes such as those by Geissler and Tesla. The fluorescent lamp differs from these earlier lamps however, in that it uses a phosphor coating on the glass to convert ultraviolet light from the gas discharge into an output that is fairly balanced in the visible spectrum. The image at left shows an operating lamp without this phosphor coating. The low temperature, low intensity discharge allows for very long lamp life which currently can be up to 30,000 hours. Relative to other electric lamps at the time of development (around 1926), fluorescent lamps were also very efficient at producing light per watt of electrical power. Due to improvements in design and phosphor composition they continue to be one of the best choices for commercial lighting designers as well as residential home use. The inexpensive screw-base compact fluorescent lamp is an excellent way to reduce electric power consumption.
 
Modern, large-scale outdoor lighting is almost exclusively metal halide technology. The metal halide lamp (lower left) combines the sealed reliability of a gas discharge tube with the high intensity of a carbon arc lamp. This intensity is required to illuminate large areas such as baseball and football stadiums. This type of lamp is actually a hybrid of the earlier mercury vapor lamp which used a short arc tube that was made of quartz, to handle the high operating temperature, and filled with argon gas and mercury. Upon start-up the argon gas was ionized by the ballast transformer voltage. As the gas heated the tube it would begin to vaporize the mercury metal, which then also ionized and contributed to the light output. To improve the efficiency and color output characteristics a combination of metal halide "salts" were later introduced (late 1960's) in addition to the mercury. The ionization of these extra salts not only increased the light output for a given wattage lamp, but also helped balance the spectral light output by producing more yellow and red in addition to the mercury's characteristic bluish-green. The operating arc shown at the upper left illustrates this multi-color output. Modern metal halide lamps have a CRI (color rendering index) that can exceed 90, with the sun being a perfect 100.
 
Imaging with Plasma:


Courtesy: Plasma Display Coalition - click to see larger diagrams

6,220,800 fluorescent lamps turning on and off with such precision that your brain tells you it's a Formula One race car going down a track. Full High-Definition plasma televisions have a resolution of 1,920 pixels wide by 1,080 pixels high (1080i/1080p HD format). Each pixel has a red, green, and blue lamp to represent the 3 primary colors. This arrangement of lamps is controlled by a very fine matrix of horizontal and vertical electrodes that precisely modulates each lamp to form the image based on the incoming video signal. The construction of these lamps can be seen in the diagram shown. Each one consists of a sealed cell that includes the correct phosphor, a UV generating gas, and a transparent front opening through which its light is emitted. These cells operate on the same principle as a fluorescent lamp, in which the electrically ionized gas (plasma) excites the phosphor with ultraviolet radiation causing it to glow with its respective color. Earlier television designs primarily used a CRT (cathode ray tube). This is a large vacuum tube that has a phosphor coating on the front face and an electron gun in the rear of the tube. As the electron gun "fires" a stream of electrons at the phosphor coating magnetic coils surrounding the neck of the tube deflect the stream in a raster scanning pattern to form the image. Although these tubes can produce images of very good quality the mechanism is somewhat inexact, and not suitable for the quality demanded by today's sophisticated consumers.

Working with Plasma:


click here to see what it's like
to work around an electric arc furnace

Because it can easily produce incredibly high temperatures, and be controlled with great accuracy, plasma is used extensively to weld, cut, and melt many types of metals. The inside of an electric arc furnace (shown at left) is well above 3,000 ¡ãF during operation, and a welding arc can range between 3,000+ and 20,000 ¡ãF. Plasma also has the advantage that it can use the polarity of the electric current to help draw the metal in the desired direction.


Courtesy: TWI - click to enlarge

Electric arc furnaces are relatively simple in operation. Massive electrodes are lowered into the furnace pot which is filled with the solid metal to be melted. Just at the point of contact an arc strikes between the electrodes and the metal, heating the metal to a liquid. The process is very power hungry - consuming 60 megawatts or more for a large furnace - but is often the preferred method for recycling scrap metal into new products.

The plasma cutting process (shown at the right) is a fast and efficient way to cut thick or very hard pieces of metal without the friction and wear that a mechanical process would have. A jet of inert gas or air is blown through a nozzle onto the work piece. The nozzle acts as one electrode and the work piece acts as the other. The jet of gas conducts the current, ionizing into an extremely hot plasma. The plasma jet then melts the metal and also blows it away, leaving the cut opening.

Energy from Plasma:
Harnessing the energy that powers the stars in our universe could be one of the most important achievements in solving the energy needs we have here on earth. Gaseous nebula like the famous Horsehead Nebula pictured above are a birth-place for new stars. Hydrogen gas in these galactic clouds can condense from gravitational forces, and if there is sufficient mass the gas can be compressed to the point where the atoms begin to fuse together. At this point the mass has become a star and radiates its own energy just like our sun. In trying to harness this energy the goal is to reproduce the fusion process in a controlled way that will yield a net gain of energy. The unit shown to the right of the nebula is the TFTR (Tokamak Fusion Test Reactor - courtesy Princeton Plasma Physics Laboratory). It was operated from 1982 to 1997, achieved a controlled fusion process, and developed a world record plasma temperature of 510 million ¡ãC. It uses a system of very powerful magnets to contain the plasma (far right image) that would otherwise melt through any material it would come in contact with because of its extreme temperature (by kevin tf service). It was not able to yield a net gain in energy however, since more power was required to sustain the fusion process than was produced by it. Many new technologies are currently under development, such as the TFTR-II, Sandia National Laboratory's "Z-Machine", and Lawrence Livermore National Laboratory's National Ignition Facility (NIF). Although we've come a long way, and new advancements are happening with regularity, realistic estimates put practical power plants using the fusion process another fifty or so years into the future.           more links

 

Playing with Plasma:

decorative plasma lamps

 

150 years after the invention of the Geissler Tube, and more than 110 years since Tesla patented a plasma lamp, we are still fascinated and entertained by the vision and experience of electrical plasma.

Decorative lamps such as those at the right have become a commonly sold item in novelty and department stores. These lamps use a high frequency, high voltage emitting electrode to ionize the gas inside them. This gas is typically neon, argon, krypton, xenon, or some combination of these. Touching the glass of one of these lamps while operating will draw a discharge stream to your fingertips. This is harmless and is due to the conductive properties of the human body.

Another captivating display of plasma is generated from a device called a Tesla Coil. Obviously named after its inventor Nikola Tesla, it is essentially a high-voltage resonant transformer with a large emitting electrode at the top end of the secondary coil. Although they have little practical use, they are very popular for educational exhibits in classrooms and science museums to illustrate electrical principles. Peter Terren (Tesla Downunder) constructs his own large Tesla Coils, and has made their beautiful discharge into an art form. At right are images he has created using time elapse photography to capture these amazing patterns.

If you like to play with plasma, and you'd like to share your experiences or images with us, send us an e-mail

 

 

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