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The acceleration of ions in the interaction of high intensity laser pulses with overdense plasmas is investigated with particle-in-cell simulations. For circular polarization of the laser pulses, highdensity ion bunches moving into the plasma are generated at the laser-plasma interaction surface. A simple analytical model accounts for the numerical observations and provides scaling laws for the ion bunch energy and generation time as a function of pulse intensity and plasma density.PACS numbers: 52.38.-r, 52.38. Kd, 52.50.Jm, 52.65.Rr The study of the interactions between ultra-intense laser pulses and plasmas has proved to be a very rich soil where technological progress and fundamental physics meet each other. Particularly intriguing is the concept of laser-plasma based ion acceleration. From astrophysics [1], to medical hadrontherapy [2], from proton radiography [3], to nuclear physics [4], from proton imaging techniques [5], to nuclear fusion [6], the problem of accelerating and manipulating charged particles with laserplasma interactions offers a series of challenges ranging from fundamental to applied physics, thus a clear understanding of the basic mechanisms is mandatory. Several recent experiments have reported the emission of energetic ions from solid targets [7]. It is still a matter of debate whether the ions are mainly accelerated at the rear surface of the target (by the field generated by fast electrons escaping in vacuum [8]) or at the front surface involving phenomena such as acceleration by a collisionless electrostatic shock [9,10,11], by a solitary wave [12] or by ion trapping in a propagating double layer [13].In this work we elucidate an even more basic process of ion acceleration in cold plasmas, purely related to the formation of an electrostatic field due to the action of the laser ponderomotive force on the electrons and, consequently, on the ions via space charge displacement. This investigation shows both the necessity of a kinetic description of this process and the fundamental role played by the laser light polarization by showing the differences between circular and linear one. It will be shown by particle-in-cell (PIC) simulations that circularly polarized light gives rise to a "pulsed"acceleration and produces ion bunches directed into the target. A simple analytical model is used to explain the acceleration dynamics and for the deduction of scaling laws that relate the interaction parameters to the energy acquired by the ions. With respect to other concepts for laser ion acceleration, the present mechanism with circularly polarized light leads to very high densities in the bunches, as might be of interest for problems of compression and acceleration of high-density matter.We consider a laser pulse impinging on a cold, step- is the critical density for a laser with carrier frequency ω L , ω p is the plasma frequency and m e , e are the electron mass and charge. The laser field amplitude will be given in units of the dimensionless parameter a L = (eE L /m e ω L c). In the PIC s...

We apply the concepts of nonlinear guided-wave optics to a Bose-Einstein condensate (BEC) trapped in an external potential. As an example, we consider a parabolic double-well potential and derive coupled-mode equations for the complex amplitudes of the BEC macroscopic collective modes. Our equations describe different regimes of the condensate dynamics, including the nonlinear Josephson effect for any separation between the wells. We demonstrate macroscopic self-trapping for both repulsive and attractive interactions, and confirm our results by numerical simulations.A system of interacting bosons confined within an external potential at zero temperature can be described by a macroscopic wave function having the meaning of an order parameter and satisfying the Gross-Pitaevskii (GP) equation [1]. The GP equation is a nonlinear equation that takes into account the effects of the particle interactions through an effective mean field, and it describes the condensate dynamics in a confined geometry. Similar models of the confined dynamics of macroscopic systems appear in other fields, e.g. in the case of an electron gas confined in a quantum well, or optical modes of a photonic microcavity [2]. In all such systems, confined single-particle states are restricted to discrete energies that form a set of eigenmodes.The physical picture of eigenmodes remains valid in the nonlinear case [3], and nonlinear collective modes correspond to the ground and higher-order (excited) states of the Bose-Einstein condensate (BEC) [4]. Moreover, it is possible to observe at least the first excited (antisymmetric) collective mode experimentally [5], through the collapses and revivals in the dynamics of strongly coupled two-component BECs [6]. The interest in the nonground-state collective modes of BECs has grown dramatically with the study of vortex states, very recently successfully created in the experiment [7].The modal structure of the condensate macroscopic (ground and excited) states allows us to draw a deep analogy between BEC in a trap and guided-wave optics, where the concept of nonlinear guided modes is widely used [8]. The physical description of confined condensate dynamics in time is akin to that of stationary beam propagation along a nonlinear optical waveguide, with the BEC chemical potential playing the role of the beam propagation constant. As is well known from nonlinear optics, the guided waves become coupled in the presence of nonlinearity, and the mode coupling can lead to the nonlinear phase shifting between the modes, power exchange, and self-trapping.In this paper, we apply the concepts of nonlinear guided-wave optics to the analysis of mode coupling and intermodal population exchange in trapped BECs. As the most impressive (and also physically relevant) example of the applications of our theory, we consider the BEC dynamics in a harmonic double-well potential, recently discussed in the literature [9]. We study the coupling between the BEC ground-state mode and the first excited (antisymmetric) mode in such a poten...

An exact analytical investigation of the stationary solutions describing the interaction between high-intensity laser radiation and an overdense plasma is presented. Both the relativistic and striction nonlinearities are taken into account, and their joint action gives rise to a solitary solution. This solution clearly shows that there exists an inherent limit of the induced transparency on the density of the overdense plasma in order to obtain a stationary physical solution. Furthermore, it is found that the striction nonlinearity tends to create a strong peaking of the plasma electron density, which suppresses the laser penetration and significantly enhances the threshold intensity for induced transparency.