Non-Maxwellian effects in underdense plasmas heated by non-uniform laser beams

The collisionl heating of plasmas by intense laser beams is known to drive the electron distribution function into a super-Gaussian [1] or ``DLM'' [2] shape. This reduces the absorption [1], and the reduction is stronger if the beam is very non-uniform, for a given average intensity, as there is a depletion of slow electrons, compared to a Maxwellian of the same density and average energy. If the beam irradiates most or a good fraction of the plasma volume, these non-Maxwellian effects also imply a depletion of high energy electrons, with the resulting strong reduction of Landau damping of Langmuir waves [2], contrary to the results of Brunner and Valeo [3] which were obtained in the limit of a narrow beam heating a wide plasma. The depletion of fast electrons depends essentially on the average laser intensity. We will show how these two aspects vary with the laser and plasma parameters.\newline \newline [1] A.B. Langdon, Phys. Rev. Lett. \textbf{44}, 575 (1980) \newline [2] B.B. Afeyan, A.E. Chou, J.P. Matte et al., Phys. Rev. Lett. \textbf{80}, 2322 (1998). \newline [3] S. Brunner and E. Valeo, Phys. Plasmas \textbf{9}, 923 (2002).