Effect of nonlinear wave collapse on line shapes in a plasma

International audience– The nonlinear interaction of waves can change the structural and radiative properties of plasmas. We describe the main features of a fully ionized unmagnetized plasma affected by strong Langmuir turbulence characterized by nonlinear wave collapse, and propose a simple model for evaluating the changes expected on a hydrogen line shape affected by such conditions. Our model is based on a stochastic renewal model using an exponential waiting time distribution and a half-normal probability density function for the electric field magnitude of the turbulent wave packet. The first results obtained with a simulation calculation of the hydrogen L line show that strong Langmuir turbulence can provide an additional broadening to a Stark profile. Introduction. – The influence of collective electric fields on a line shape emitted in a plasma has been studied for several decades in different types of plasmas [1,2,3]. Most of the collective fields considered were described by linear waves, e.g. the electronic Langmuir plane wave oscillating at the plasma frequency. The observed spectroscopic effects were the formation of satellites or dips, and changes in the overall shape of the line. In this work, we consider in an unmag-netized plasma the nonlinear coupling of Langmuir waves with ion sound and electromagnetic waves. In such conditions, the density fluctuations associated with ion sound waves affect the Langmuir waves, refracting them in regions of low density. Coherent plasma wave packets localize in such regions, and evolve to shorter scales and higher intensities [4]. Computer simulations reveal the existence of a wave packet cycle with a collapse arrested by dissipation, and of a nucleation mechanism allowing the creation of a new wave packet. If energy is supplied from an external source, localized wave packets may be created at a high rate, and the plasma will contain many of those coexisting wave packets. We consider in the following the effect of the presence of such collapsing wave packets on the line radiation emitted by an atom in the plasma. Interesting applications for this study concern plasmas in interaction with an energetic beam of particles. For example in astrophysical contexts, active galactic nuclei [5], or pulsar radio sources [6]. Relevant laboratory plasmas are laser plasmas [7], radio experiments [8], or possibly also magnetic fusion plasmas, since such plasmas can be affected by energetic beams of runaway electrons [9]. The paper is organized as follows. In the next section, we make a short review of th