The glow discharge is one of the oldest plasma
s 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:
- 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.
- 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.
- 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
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.
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
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.