Effects of turbulent temperature fluctuations on the core of Doppler line shapes

Recent theoretical studies were made on Doppler line shapes emitted in plasma by taking into account low frequency hydrodynamic turbulence, resulting in fluctuations of temperature, fluid velocity, and density of the emitters on the line of sight. Whereas such a profile is Gaussian in absence of turbulence, it has been shown in a previous model that fluctuations of temperature modify the wings of the profile, leading in some cases to an asymptotically power law dependence of the wavelength detuning. As a consequence, the wing analysis of a Doppler line appears to be of a particular interest for turbulence investigations in magnetized fusion and astrophysical plasmas. In this work, we examine the effect of plasma temperature fluctuations on the entire line shape with a special emphasis on the central part, which is commonly exploited to obtain the emitter temperature. We use a previously developed model where the line shape is expressed in terms of the temperature fluctuation probability distribution function (PDF) and of the line brightness. Here this PDF is assumed to be a lognormal distribution and the brightness temperature dependence is calculated with the collisional-radiative code SOPHIA. For averaged emitter temperature larger than 10 eV, we show that the spectral line core is non Gaussian and sensitive to the shape of the temperature PDF. The consequences for line profile analysis are next investigated. In particular we show that the average temperature in the low frequency turbulent plasma can be identified from a central line shape analysis. In addition to the line wing analysis presented previously, this work brings new materials to study the plasma turbulence by spectroscopic means. This work is part of a collaboration between the Laboratoire de Physique des Interactions Ioniques et Moleculaires and the Departement de Recherches sur la Fusion controlee, CEA Cadarache. (author)