Double Layer (plasma) - Features and Characteristics of Double Layers

Features and Characteristics of Double Layers

• Thickness: The production of a double layer requires regions with a significant excess of positive or negative charge, that is, where quasi-neutrality is violated. In general, quasi-neutrality can only be violated on scales of the order of the Debye length. The thickness of a double layer is of the order of ten Debye lengths, which is a few centimeters in the ionosphere, a few tens of meters in the interplanetary medium, and tens of kilometers in the intergalactic medium.
• Particle acceleration: The potential drop across the double layer will accelerate electrons and positive ions in opposite directions. The magnitude of the potential drop determines the acceleration of the charged particles. In strong double layers, this will result in beams or jets of charged particles.
• Particle populations: As described in the formation of double layers, there are four populations of charge particles inside a double layer. Note that in the case of weak double layers not all electrons and ions entering "from the wrong side" will be reflected, and therefore there will also be a population of decelerated electrons and ions.
• Particle flux: For non-relativistic current carrying double layers the electrons comprise the main part of the particle flux. The Langmuir condition states that the ratio of the electron and the ion current across the layer is given by the square root of the mass ratio of the ions to the electrons. For relativistic double layers the current ratio is 1; i.e. equal amounts of current are carried by the electrons and the ions.
• Energy supply: In a certain limit, the voltage drop across a current-carrying double layer is proportional to the total current, and it might be thought of as a resistive element (or load) which absorbs energy in an electric circuit. Anthony Peratt (1991) wrote: "Since the double layer acts as a load, there has to be an external source maintaining the potential difference and driving the current. In the laboratory this source is usually an electrical power supply, whereas in space it may be the magnetic energy stored in an extended current system, which responds to a change in current with an inductive voltage".
• Stability: Double layers in laboratory plasmas may be stable or unstable depending on the parameter regime. Various types of instabilities may occur, often arising due to the formation of beams of ions and electrons. Unstable double layers are noisy in the sense that they produce oscillations across a wide frequency band. A lack of plasma stability may also lead to a dramatic change in configuration often referred to as an explosion (and hence exploding double layer). In one example, the region enclosed in the double layer rapidly expands and evolves. An explosion of this type was first discovered in mercury arc rectifiers used in high-power direct-current transmission lines, where the voltage drop across the device was seen to increase by several orders of magnitude. Double layers may also drift, usually in the direction of the emitted electron beam, and in this respect are natural analogues to the smooth—bore magnetron.) (not to be confused with a unit of magnetic moment, the Bohr magneton, which is created by the "classical circular motion" of an electron around a proton).

• Magnetised plasmas: Double layers can both form in normal and magnetized plasmas.
• Cellular nature: While double layers are relatively thin, they will spread over the entire cross surface of a laboratory container. Likewise where adjacent plasma regions have different properties, double layers will form and tend to cellularise the different regions.
• Energy transfer: Double layers facilitate the transfer of electrical energy into kinetic energy, dW/dt=I.ΔV where I is the electric current dissipating energy into a double layer with a voltage drop of ΔV. Alfvén points out that the current may well consist exclusively of low-energy particles. Torvén et al. also report that plasma may spontaneously transfer magnetically stored energy into kinetic energy by electric double layers.
• Oblique double layer: An oblique double layer has its electric field not parallel to the background magnetic field; i.e., it is not field-aligned.
• Simulation: Double layers may be modelled using kinetic computer models like particle-in-cell (PIC) simulations. In some cases it is reasonable to treat the plasma as essentially one- or two-dimensional to reduce the computational cost of a simulation.
• Bohm Criterion: A double layer cannot exist under all circumstances. In order to achieve that the electric field vanishes at the boundaries of the double layer, an existence criterion says that there is a maximum to the temperature of the ambient plasma. This is the so-called Bohm criterion. A mathematical description is given in the math section. In the theory of the Debye sheath there is a related but not identical condition also known as the Bohm criterion.
• Bio-physical analogy: A model of plasma double layers has been used to investigate their applicability to understanding ion transport across biological cell membranes. Brazilian researchers have note that "Concepts like charge neutrality, Debye length, and double layer are very useful to explain the electrical properties of a cellular membrane." Plasma physicist Hannes Alfvén also noted that association of double layers with cellular structure, as had Irving Langmuir before him, who coined the named "plasma" after its resemblance to blood cells.