Topic/Type: 1.5 Low-temperature, dusty and nano-plasmas, Poster

Influence of the radial profile of plasma species in a positive column on the amplification of a strongly saturated He-Ne laser

A. Aissi1, 2, G. Maynard1, A. Virdis2, T. Minea1, O. Leroy1, C. Berenguer1, K. Katsonis1

1 Laboratoire de Physique des Gaz et des Plasma, CNRS-Universit? Paris
2 SAGEM-DS, 95101 Argenteuil, France

Non equilibrium, direct-current positive column (DCPC) of cylindrical gas discharges has been extensively studied due to its broad domain of application for lightning and laser systems. Nevertheless, even for the He-Ne gas mixture used in traditional helium-neon lasers, there are still uncertainties in ab initio calculations of the full kinetics of excited species. In the present work, we have investigated the domain of He-Ne DCPC discharges at low current and high pressure (NR>>1016 cm2, N being the atomic density and R the discharge radius) at which laser gain for the 633 nm line is optimum. At low currents, both the radial electric field and the transverse gradient become large, even close to the axis. Moreover the width of the effective laser gain is increased when working at strong saturation. Therefore the laser intensity becomes more sensitive on the species radial profile in the plasma positive column.
Ring-cavity lasers have been developed to measure angular velocity of rotation with a rather high accuracy [1]. They have found wide application in engineering as a basis for different navigation devices, and also for investigation of fundamental physics [2]. In the present work we have used the laser beam emitted by a ring-cavity laser as a probe of the plasma properties.
In order to determine the laser beam amplification in the DCPC, we combined the following three numerical codes : (i) 1D fluid plasma code using the local mean energy approximation in articulation with a two-term Boltzmann solver [3]; (ii) novel kinetic code, connected with an extended atomic database for He and Ne, which takes into account radial transport of photons and particles; (iii) 2D Maxwell-Bloch code determining the laser amplification and the radial profile of population inversion.
We will present the results obtained from the coupled models and their comparison with experimental results obtained with the ring-cavity laser, in particular those concerning the evolution of the plasma properties with discharge current, temperature and gas pressure. Specific emphasis has been put on the analysis of the velocity distribution function of the lasing atomic states at high intensities, which has a large influence on the laser gain and is quite sensitive to plasma parameters.

This work is supported by the CNRS ? SAGEM-DS n? 2008-032034 contract.

[1] J.R. Wilkinson, Progress in Quantum Electronics, 11, 103 (1987).
[2] A. Stedman, Reports on Progress in Physics, 60, 615 (1997).
[3] L.L. Alves, G. Gousset and S. Vallee, IEEE Transactions on Plasma Science, 31, 572 (2003)