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The glow discharge is one of the oldest plasmas that are studied. This may be related to the fact that they are easily made in a laboratory. Still, they are among the most complex, least well understood and most often used plasmas. This write up will describe some of the physics behind this device, so a basic knowledge of plasma physics is required, in particular of low-temperature plasma physics.

A glow discharge basically consists of three basic parts:

  1. The gas. This is the gas, or mixture of gases, in which the glow is created. The gas composition has a profound influence on how the glow behaves, mainly by determining the electron temperature and the wavelength of the emitted light. The pressure of the gas is linked to dimensions of the device by the so-called Pashen curve. This means that the value of the product of pressure and size is roughly constant. This is why the pixels in a plasma TV have a high pressure and a fluorescent lamp has a low pressure.
  2. The electrodes. The current which sustains the glow flow between the electrodes. The exact operation of the electrodes is very important for the glow, and this will be discussed in detail. In principle, there are two electrodes: An electron-emitting cathode and an electron-collecting anode.
  3. The power supply. This power supply is often not a pure voltage source, but rather a current source. The reason for this is that a plasma tends to conduct better if you put more current through it. With an (near-)ideal voltage source the positive feedback loop will result in the rapid destruction of something, hopefully the fuse.

So, what do we see when we flick one on? Well, a lot of light is coming from the cathode, and this light usually has a diffent color than the rest of the glow. The cathode attracts ions, and these bombard the cathode surface, heating it, and releasing electrons. The local heating also increases the electron emission through thermionic emission. However, the heavy ions have a hard time sustaining the current, so there is a very high voltage over this bright region. This voltage, and the region itself, are called the cathode fall. Depending mainly on the electrode, this cathode fall might be all but nonexistent or be several hundreds of volts.

This high voltage accelerates the electrons to high energies. Because collisions between atoms and electrons become less likely if the electrons has a very high energy, they can fly for some distance until they start ionizing the gas by electron impact ionization. This zone is called the Crookes dark space, and it's fairly thin.

After this zone, a ball of very bright plasma exists where the electrons that come from the cathode produce excitations. These excited species then emit light, which is seen. The electric field is inverted here, so that it actually opposes the electrons. This zone is called the negative glow.

The next zone is the Faraday dark space, which is dark because the electrons left from the have too little energy to excite the gas.

After having picked up some energy in the field the electrons can excite again, and the positive column begins. In this column, the losses of electrons and ions to the wall by ambipolar diffusion is compensated by local ionization. This determines the value of the electric field. Typically, this is around 100 volts per meter. If you increase the length of the device, you increase the length of the column. The other features won't change.

Near the anode, another active zone is seen, called the anode glow. This zone is not very pronounced compared to the spectacular phenomena near the cathode, and may even seem absent.

Now that we have a rough idea how a glow works, we now have some understanding of a plasma that is very common in the everyday environment. It is found in:

They may not be as fancy as a tokamak, but these little plasmas are definetely workhorses for mankind.

A discussion of glow discharges is not complete without mentioning the fact that a glow discharge can operate in three different regimes. In a normal glow discharge, the whole cathode is covered in a cathode fall, and contributes to the transport of electrons. If the current is reduced, only a part of the cathode needs to supply the electrons, so the cathode fall covers only part of the cathode. This may not be desirable, as the cathode fall might jump around, causing flickering. This is called a subnormal glow. If a higher current is desired, the cathode fall has to increase. This requires an increase in cathode fall, and might give the discharge a positive voltage/current characteristic. This is called an abnormal glow.

Summarizing, glow discharges are plasmas that are sustained by a modest current going through them. The mechanism by which the cathode releases electrons plays a key role in the discharge. They are well-known for the light they emit, which is typically line radiation. Lighting is one of their chief application areas. The physics behind them, which I have merely sketched, are a very rich and rather poorly understood topic. To give you an idea: I have good hopes of getting key plasma parameters to within 20 % when simulating an arc, while I'm happy with a factor of 2 for a glow.

Sources: I do plasma physics for a living. If you feel like reading up on this subject I recommend Yuri P. Raizers book on glow discharges.

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