JOURNAL DE PHYSIQUE, Colloque C6, suppl6ment au no 10, Tome 47, octobre 1986 SUPERRADIANCE THEORY AND X-RAY LASERS : BASIC NOTIONS AND MODELS A. CRUBELLIER L a b o r a t o i r e Aimt? C o t t o n , CNRS 11, BSt. 505, F-91405 O r s a y Cedex , F r a n c e Rdsumd -La thdorie de l a superradiance e s t prdsent6e en insistant d'une part sur l e s propridtds essentielles du phdnomsne, qui apparaissent d&jb dans les modSles l e s p l u s simples, modSle de Dicke e t modsle semi-classique, e t , d'aut

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... r e part, sur l a varidt6 des situations physiques dans lesquelles l e phdnomsne a dtd dtudig. Ceci permet de ddgager l e s possibilitds e t les limites de l a thdorie de l a superradiance come point de ddpart d'une analyse de 1'6mission cohzrente de rayons X dans l e s plasmas chauds. Abstract -The essential characteristics of superradiance are described by using two simple models, the Dicke model and the semi-classical model. The main extensions t o r e a l i s t i c situations are also presented and the possibility of superradiance theory being a starting point of a general approach of X-ray coherent emission i s discussed. Coherent light production generally originates from stimulated emission in an inverted medium. A lot of different situations are however encountered, according to wether the pumping mechanism which creates the population inversion is continuous or instantaneous, according to whether the active medium is or is not placed inside an optical cavity ... Somehow controversed names are associated with the main typical cases of coherent light production. The word laser is generally restricted to devices including a resonator (11. In the absence of mirrors, two phenomena have been essentially studied: ampliGed spontaneous emission (ASE) and superradiance (SR) or superfluorescence 13-61. The term ASE usually refers to a steady-state regime whereas SR ideally deals with an infinitely short inversion rise time. As compared to the other processes, superradiance exhibits quite peculiar features, such as, for instance, the n2 dependence (n being the atomic density) of the maximum of the emitted intensity, which is inseparable from a basically transient nature of the emission. These properties are known to be due to the spontaneous-build-up of a collective dipole moment or, equivalently, to the occurence of an interference between the emitting dipoles. In the laser regime or in ASE, dephasing effects continuously cancel the collective dipole moment and kill the interatomic interference. Transition from SR to ASE, while decreasing the characteristic time of dephasing effects until it becomes smaller than the characteristic Article published online by EDP Sciences and available at http://dx.