In a plasma, free charged particles, both negative and positive, move around. These particles all attract or repel each other. Now, what would your life be like if you were a charged particle in a plasma?

As a charged particle, you have an electric field. However, this electric field attracts particles with an opposite charge. This means, that the area around you will get an average, small net charge that opposes your own. Their electric field shields your electric field from the outside world. The distance at which this effect, called Debye shielding, has smothered your field to e-1 of its normal value is called the Debye length &LambdaD.

This is a very important concept in plasma physics, so important that one might define a plasma based on it:

A plasma is a partially or fully ionized gas, which is much larger than its Debye length, and in which there is more than one particle per Debye sphere.

The Debye length mainly depends on two plasma parameters: the charged particle density and the temperature. A high charged particle density will cause more effective shielding, as there simply are more particles to shield with, whereas a high temperature causes the plasma particles to become more mobile, shuffling them more and reducing the shielding.

To give you an idea of the length scales we are talking about here, some typical values of the Debye length for various plasmas are:

  • A fluorescent tube, cool and very low density:1x10-5 m
  • An atmospheric arc, cool and high density:3x10-8 m
  • The solar surface, cool and high density:1.5x10-7 m
  • The solar core, hot and very high density:3x10-11 m
  • A Tokamak: very hot and low density: 3x10-5 m
It is interesting to note that a Tokamak, which is supposed to have nuclear fusion just like the solar core has a completely different value of the Debye length than the solar core.