Double Layer (plasma) - Double Layer Classification

Double Layer Classification

Double layers may be classified in the following ways:

• Weak and strong double layers. The strength of a double layer is expressed as the ratio of the potential drop in comparison with the plasma’s equivalent thermal energy, or in comparison with the rest mass energy of the electrons. A double layer is said to be strong if the potential drop across the layer is greater than the equivalent thermal energy of the plasma’s components. This means that for strong double layers there are four different components to the plasma:

1. The electrons entering at the low potential side of the double layer which are accelerated;
2. The ions entering at the high potential side of the double layer which are accelerated;
3. The electrons entering at the high potential side of the double layer which are decelerated and successively reflected; and
4. The ions which enter the double layer at the low potential side of the double layer which are decelerated and reflected.

Note that in the case of a weak double layer, the electrons and ions entering from the “wrong” side are decelerated by the electric field, however most will not be reflected, as the potential drop is not strong enough.

• Relativistic or nonrelativistic double layers. A double layer is said to be relativistic if the potential drop over the layer is so large that the total gain in energy of the particles is larger than the rest mass energy of the electron. The charge distribution in a relativistic double layer is such that the charge density is located in two very thin layers, and inside the double layer the density is constant at and very low compared to the rest of the plasma. In this respect, the double layer is similar to the charge distribution in a capacitor. As a special case of a relativistic double layer one can consider the vacuum gap at the magnetic polar cap of a pulsar.
• Current carrying and current-free double layers may both occur. Current carrying double layers may be generated by current-driven plasma instabilities which amplify variations of the plasma density. Current-free double layers form on the interface between two plasma regions with different characteristics, and their associated electric field maintains a balance between the penetration of electrons in either direction (so that the net current is low).