hancing the area of a Raman atom interferometer using a versatile double-diffraction technique.

We have observed self-sustained radial oscillations in a large magneto-optical trap (MOT), containing up to 10 10 Rb 85 atoms. This instability is due to the competition between the confining force of the MOT and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N > 10 9 are required to observe this unstable behavior.PACS numbers: 32.80. Pj, 42.50.Vk, A large fraction of the stars in the upper HertzsprungRussell diagram present pulsations based on an interplay between modulated radiation pressure effects, which tends to increase the size of the star, and a collapse based on gravitational forces [1]. Instabilities also occur in other similar systems such as confined plasmas where a long range Coulomb interaction has to be countered by a confining force to avoid an explosion of the plasma [2]. These systems are of fundamental importance for astrophysics and for controlled fusion and have thus been extensively studied in the past. However it is either impossible (in the case of stars) or extremely difficult (in the case of confined plasmas) to perform experiments to study the full dynamics of such systems where collective effects play a dominant role. On the other side, allowing for adequate rescaling, alternative systems can present similar dynamics. A variety of interesting collective effects have thus been identified in charged colloidal systems [3]. Recently ultra-cold plasmas created by ionizing a cloud of laser cooled atoms became subject to increased attention [4]. Beyond the possibility of studying analogous effects as in astro-and plasma physics, systems with long range interactions are known to lead to non-extensive behavior and appropriate scaling laws are needed to predict macroscopic properties. Here we show that a large cloud of laser cooled atoms is an adequate system to study such collective effects. The radiation pressure of the multiply scattered photons in such clouds can indeed be related to a long range Coulomb type interaction [5]. We thus suggest an analogy between the dynamics of a large cloud of cold atoms, astrophysical systems and plasma physics.The effect of multiple scattering on the dynamics of the atoms is well known in the community of laser cooling of atoms, as multiple scattering has been a major limitation to obtain large phase space densities in cold atomic traps. Bose-Einstein condensation (BEC) in dilute atomic vapors has only been achieved after switching off all laser fields and using evaporation techniques [6]. More recently, multiple scattering of light in cold atoms has been used to study coherent light transport in random media [7]. This has led to an investigation of yet unexplored regimes, namely the limit of very large number of cold atoms in the presence of quasi-resonant light. Here we do not focus on the properties of the scatter...