Plasmas explained

We happily live in the Earth’s gaseous lower atmosphere composed of a mixture of gases – primarily nitrogen and oxygen. However, if we move upwards from the Earth’s surface, the environment changes and no longer fits this description. At about 80 km above the Earth’s surface, the atmosphere is no longer made up of gas. Instead, it is made up of ionised gas, which consists of a balanced mix of electrons, positive ions and neutral particles. This state is called plasma. Commonly known as the ‘fourth state of matter’, in the opinion of many astrophysicists, it is the very ‘first’ state since it was the first to form immediately after the Big Bang.

To make plasma, energy is needed to strip electrons from atoms. The energy can be of various forms – heat, electrical or light (ultraviolet light or intense visible light from a laser). With insufficient sustaining power, plasmas recombine into neutral gas.

Further out into space, all gas is ionised, and it is the highly energetic electromagnetic radiation from the Sun, itself made of plasma, that is responsible for this ionising process. Space is therefore dominated by plasma. In fact, 99% of matter in the known universe is plasma.

Plasma forms

Plasmas occur naturally but can also be artificially made. Naturally occurring plasmas can be Earth-based (terrestrial) or space-based (astrophysical). Artificial plasmas have been developed to service the needs of a wide range of fabricating, manufacturing and specialised coatings industries.

Examples of three forms of plasma

Astrophysical plasma

Terrestrial plasma

Artificially produced

All stars

Solar wind

Interstellar nebulae

Space between planets, star systems and galaxies

Lightning

Auroras

Ionosphere

Extremely hot flames

Plasma TVs

Fluorescent lighting

Plasma torch for cutting and welding

Plasma-assisted coatings

Plasma properties

Plasma is the highest energy state of matter. It consists of a collection of free-moving electrons, positive ions and neutral particles. Although it is closely related to the gas phase in that it has no definite shape or volume, it does differ in a number of ways:

• Plasma has a very high electrical conductivity.

• Plasma is more readily influenced by electric and magnetic fields than by gravity

• The motion of electrons and ions in plasma produces its own electric and magnetic fields.

• Because of the totally chaotic and highly energetic state of the constituent particles of plasma, it produces its own electromagnetic radiation.

To produce and maintain the highly energetic state that exists within plasma, there must be a continual supply of energy.

Three forms of plasma

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Plasmas occur naturally but can also be artificially made. Naturally occurring plasmas can be Earth-based (terrestrial) or space-based (astrophysical).

Rights: Sun image, courtesy of NASA Lightning image courtesy of Postit, Creative Commons Attribution-ShareAlike 3.0 Unported Plasma cutting image courtesy of Robotworx, Creative Commons Attribution-ShareAlike 3.0 Unported

Artificial plasma – hot and cold

Hot or thermal plasma is produced in atmospheric arcs, sparks and flames. The highly ionised plasma consists of large numbers of electrons and positive ions, with the temperature of both being extremely high. Depending on their power, plasma-cutting torches operate at very high temperatures between 5,000 and 10,000°C.

Cold or non-thermal plasma is less well ionised, and although the electrons are high temperature, the positive ions and neutral particles are at a lower temperature. When a fluorescent lighting tube is switched on, cold plasma (at room temperature) is set up within the tube.

Artificial plasma uses

Thermal plasma uses range across a number of industries including lighting, coatings and metal fabrication and purification. Examples of these include:

• metal halide arc lights used in floodlighting

• plasma coating processes that allow wear and heat-resistant coatings to be deposited on selected surfaces

• the use of electric arcs for cutting and welding metals.

As scientists have come to understand more about the structure and properties of plasma, new technologies have evolved resulting in a rapid expansion of cold or non-thermal plasma uses. For example, in the manufacture of computer hardware components, processes such as plasma-enhanced chemical vapour deposition and etching are used to fabricate integrated circuits. Plasma processing of this type has been instrumental in the design and manufacture of the powerful, compact computers and cell phones that are in common usage.

Eden Park floodlights

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Philips provided 482 Philips ArenaVision MVF404 floodlights at the redeveloped Eden Park. The metal halide arc lights operate by generating high-temperature plasma.

Rights: Patrick Reynolds, Philips Lighting NZ

Other examples of cold plasma uses include:

• fluorescent tube lighting

• plasma TVs

• environmental control – abating pollutant gas emissions

• plasma ball toys.

Plasma TV operation

The flat screen consists of two transparent glass panels sandwiching a thin layer of pixels. Each pixel is made up of three gas-filled cells. The gas is a mixture of neon and xenon. Each cell is painted on the inside with a phosphor that, when stimulated, will emit red, green or blue visible light. A grid of tiny electrodes allows electric current to be supplied to each cell in the pixel. When current flows, the gas in the cell ionises to a plasma state, and as a result of this, UV light is emitted. The phosphor coating the walls of the cell absorbs this UV light and is stimulated to emit visible light, either red, green or blue.

How many pixels a plasma display has depends on the resolution of the display. A 1280 x 720 resolution plasma display has 1280 x 720 = 921,600 pixels. Each pixel has three cells, so 1280 x 720 resolution plasma has 3 x 921,600 = 2,764,800 individual cells.

By varying the pulses of current flowing through the different cells, the control system can increase or decrease the intensity of each cell colour to create hundreds of different combinations of red, green and blue. In this way, the control system can produce colours across the entire spectrum.

Plasma display panel

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Schematic diagram of a plasma display panel showing the pixels sandwiched between two transparent glass panels. Each pixel is made up of three gas-filled phosphor-coated cells.

Rights: The University of Waikato Te Whare Wānanga o Waikato