Double Layer

In plasma science a double layer is a structure that consists of two parallel sheets electrical charge.  One layer has a negative charge and it butts up against an identical layer of positive charge.  If the plasma is in the form of a ball, then the layers are approximately spherical in shape.  Two sheets of opposite charge sounds like a capacitor, and in fact a strong electrical field surrounds this double layer. 

A double layer is to plasma what a membrane is to a cell.  It is it’s skin so to speak.  Where ever there is a plasma with regions that have different temperatures, you will find a double layer separating them making the two regions quite distinct as if they were two different cells.

The idea of a double layer is counterintuitive in that since opposite charges attract, why don’t they just combine?  However, double layers are found in a wide variety of plasmas, from different regions of the magnetosphere to space plasmas to discharge tubes.  Double layers are a fact of nature. 

Instabilities can set up a double layer, but once initiated a mechanism must maintain the double layer.  Imagine that one side of the double layer has a plasma with a higher electron temperature than the other side.  Higher temperature mean the electrons have a higher velocity, and say they flow toward the colder plasma.  Many more hot electrons enter the colder region than cold electrons enter the hot plasma region.  The colder region becomes negatively charged . Since the hot region is losing more electrons, it becomes positively charged.  An electric field builds until the flux of electrons in either direction is equal.  Thus the double layer is maintained in plasma.

References:

http://cos.anu.edu.au/Resources/Posters/thruster.pdf

http://en.wikipedia.org/wiki/Double_layer_(plasma)