|
A double layer is a structure in a plasma which consists of two parallel layers with opposite electrical charge. The sheets of charge cause a strong electric field and a correspondingly sharp change in voltage (electric potential) across the double layer. Ions and electrons that enter the double layer are accelerated, decelerated, or reflected by the electric field. In general, double layers (which may be curved rather than flat) separate regions of plasma with quite different characteristics. Double layers are found in a wide variety of plasmas, from discharge tubes to space plasmas to the Birkeland currents supplying the Earth's aurora, and are especially common in current-carrying plasmas. Compared to the sizes of the plasmas that contain them, double layers are very thin (typically ten Debye lengths), with widths ranging from a few millimeters for laboratory plasmas to thousands of kilometres for astrophysical plasmas. Other names for a double layer are electrostatic double layer, electric double layer, plasma double layers, electrostatic shock (a type of double layer that is oriented at an oblique angle to the magnetic field in such a way that the perpendicular electric field is much stronger than the parallel electric field),〔Temerin, M.; Mozer, F. S., "(Double Layers Above the Aurora )" (1987) NASA Conference Publication, #2469〕 and space charge layer.〔Block, L. P. "(A double layer review )" (1978) ''Astrophysics and Space Science'', vol. 55, no. 1, May 1978, pp. 59–83〕 In laser physics, a double layer is sometimes called an ambipolar electric field.〔Bulgakova, Nadezhda M. et al., "(Double layer effects in laser-ablation plasma plumes )", ''Physical Review E'' (Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics), Volume 62, Issue 4, October 2000, pp. 5624–5635〕 Double layers are conceptually related to the concept of a 'sheath' (''see'' Debye sheath). The adopted electrical symbol for a double layer, when represented in an electrical circuit is ────DL────. If there is a net current present, then the DL is oriented with the base of the L in line with direction of current.〔''Double Layers in Astrophysics'', NASA Conference Publication 2469 (NASA CP-2469), (1987) Edited by Alton C. Williams and Tauna W. Moorhead〕 An overview of double layers in space, experiment and simulation is given in the introduction of ref.〔Meige, A. "(Numerical modeling of low-pressure plasmas: applications to electric double layers. )" Ph.D Thesis, The Australian National University, 2006〕 == 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: #The electrons entering at the low potential side of the double layer which are accelerated; #The ions entering at the high potential side of the double layer which are accelerated; #The electrons entering at the high potential side of the double layer which are decelerated and successively reflected; and #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 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). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Double layer (plasma)」の詳細全文を読む スポンサード リンク
|