Increasing the upper-limit intensity in relativistic and ponderomotive self-focusing by using plasma with a linear electron temperature ramp

This paper presents an investigation of the characteristics of the propagation of a Gaussian laser beam through an underdense plasma in the presence of a linear electron temperature ramp. Relativistic and ponderomotive nonlinearities are involved. It is shown that the ponderomotive nonlinearity induces a saturation mechanism in the self-focusing phenomenon and leads to the existence of a laser intensity threshold above which the beam starts to diverge. It is also found that on using the plasma electron temperature ramp-up, the upper-limit value shifts to higher values. Furthermore, results show that the slope of the temperature ramp and its sign are important in the determination of the focusing and defocusing of a laser beam for the cases in which the initial electron temperatures are chosen below or above the turning point temperature. (paper)