Energy release in double layers

A Double layer (DL) in a current carrying plasma sustains a localised net potential difference and acts as an impedance converting electrical energy to directed particle energy determined by the DL potential. DL's accelerate equal numbers of ions and electrons for relativistic energies, otherwise electron energisation predominates. Their time independent structure may be described as a BGK (Bernstein-Greene-Kruskal) state analogous to that for laminar shocks, and certain existence criteria must be satisfied. The generalised Langmuir criterion requires total particle pressure balance over the DL and may imply propagation in the plasma reference frame. The Bohm criteria require sufficiently large current densities and are closely related to the local stability condition at the edges of the DL. The DL potential must be sustained externally, for example by the release of stored magnetic energy. A steady state is also possible where (externally maintained) mass motions drive a dynamo region connected by currents to the DL which acts as an electrical load.