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Phases of Matter VII - Plasmas

Written by Translator on June 27, 2008 – 5:25 pm -

Crossposted at http://Dailykos.com

A plasma is a noncondensed, fluid state of matter, much like a gas. The difference between gases and plasmas is that gases are electrically neutral, while plasmas have had some, most, or all (depending on conditions) of its atoms separated from one or more electrons.

Thus, a plasma is mixture of ions (atoms missing one or more electrons) and free electrons. This makes them have some extremely useful properties, and we exploit those properties on a daily basis.

Everyone is familiar with plasmas. The most common examples are fluorescent tubes and compact fluorescent bulbs. Those units work on the same principle: a gas (in this case, mercury vapor) inside the bulb is subjected to an electrical current, ionizing it, which is essentially converting it into a plasma. The mercury vapor plasma has a very bright line at 253.7 nanometers (in the UV). This UV interacts with a coating (phosphor) on the inside of the tube and the UV is absorbed and reradiated at visible wavelengths. The color of the light can be controlled by using mixtures of different phosphors to influence the color of the light (hence “cool white”, “warm white”, etc.). The glass in the tube can also be colored to transmit only a narrow band. I once worked in a refrigerator manufacturing place that used bright green fluorescent lights in the inspection room. It was sort of weird.

It takes quite a surge of electricity to start a fluorescent tube, because the mercury is not ionized. Once the plasma forms, it is very conductive and the current is reduced by the ballast (a transformer) in the fixture, or for CFLs, in the base of the bulb. You may have noticed that in cold environments (like an unheated garage in winter) fluorescent tubes are slow or impossible to start. That is because the vapor pressure of the mercury is low, making it difficult to build up a high enough concentration of vapor to form the plasma. A trick is to take a hair dryer and warm the tubes a bit until they start. Once started, they produce enough heat to maintain the plasma.

Here is a simplified diagram of a fluorescent lamp:

Neon and other similar lights are based on plasmas, with neon being a pretty reddish-orange. See the previous diary for a picture. Other colors can be made by using different gases, different phosphors, or painting the outside of the tubes.

Here is a classical neon light:

Tanning beds use mercury tubes, but the walls are made from materials transparent to the UV rather than common glass. I strongly recommend that everyone stay away from tanning beds. The little bit of skin discoloration is not worth the exposure to the UV. In fact, they are burying a man who was my next door neighbor for years, who lived in the sun on his boat every weekend and often after work on weekdays, without sunscreen. Melanoma got him. He was 44.

Plasmas are used in analytical chemistry. The instrument called an “Inductively Coupled Argon Plasma” (ICAP) spectrophotometer forms a plasma by pumping radiofrequency energy into a region above a quartz “torch” and a small spark ionizes enough argon to start the plasma. The temperature in on the order of 10,000 degrees C as far as the electrons are concerned, but the physical temperature is lower or the torch would melt. (It is actually possible to melt a torch if the induction coil is misaligned, as I learned through experience). The coil is a copper tube kept cool by water flowing through it by a chiller and pump.

A solution of whatever is to be analyzed is sucked into the plasma by the Venturi effect (just like a carburetor on a lawnmower sucks gasoline into the intake) and the materials in the plasma gain energy and reradiate it a wavelengths characteristic of specific elements. Almost all metals analyses are conducted with plasma technology, although for you chemistry geeks will know that atomic absorption is still used quite a bit. I always preferred ICAP over AA because I think it is more stable and gives better quantitation for most elements.

Here is a picture of an ICAP torch and plasma. Notice the induction coils around the torch.

Plasmas are also used in metal working. An electric welder is essentially a plasma device that uses the heat to melt the material in the welding rod and deposit it on the workpiece. Plasma cutters are supplanting cutting torches by using a plasma rather than a flame to melt and blow out metal where the cut is being made.

An exciting and new (in practice, not in concept) application for plasma is spacecraft propulsion. In this application, a propellant (often xenon, but other materials can also be used) is converted to a plasma electrically or electromagnetically. It then is accelerated through an electrode where it recovers electrons. By now the propellant is traveling at high speed, and, being neutral again, keeps on going out of the exhaust port, imparting force onto the spacecraft. There are several of these devices already deployed on satellites and actual spacecraft. They do not accelerate very fast (I saw that one model goes from 0 to 60 mph in four days, but they are extremely efficient in terms of fuel use. Time is much less of a consideration than fuel weight in spaceflight, since conventional chemical fuel is by far the heaviest part of a spacecraft. Unfortunately, ion engines can not function in the atmosphere, and must be carried aloft by conventional rockets.

Here is a picture of a Hall effect plasma thruster:

I have not mentioned the plasma with which the ancients were familiar: lightening. In this case it is generated by a massive electrical discharge clouds or from cloud to ground. The normal vector for propagation is electrons, and the ionized air forms the plasma. Current thought indicates that some particularly destructive forms of lightening is propagated by positive particles, presumably ionized atoms or air and/or water, but this is still somewhat controversial.

Plasmas do not last very long in contact with air, because the positive ions acquire electrons from matter, and the free electrons are also acquired by matter, discharging it, unless there is a continuous source of power regenerating it. In free space they can last a long time. Cosmic rays are, in a sense, high energy plasmas that have been accelerated over tremendous energies. They survive our atmosphere because of their tremendous speed, often near that of light, so there is not a lot of time for interaction. However, they are somewhat attenuated and that is the reason that you get a little radiation dose above background in jet flights.

This concludes our discussion about plasmas. As always, I will hang on and be happy to consider comments and questions. Warmest regards, Doc.

Tags: Fluorescent Lamp, ICAP, Ion Engine, Lightening, Phases of matter, Plasma, Teaching
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