Numerical Modeling of Recombination Soft X-Ray Lasers

Yoav Avitzour

Dept. Mechanical and Aerospace Engineering, Princeton University

 

The search for Soft X-ray Lasers (SXL) is part of the international effort to achieve shorter and shorter wavelengths lasers. Short wavelength lasers may have many different applications such as high resolution lithography, high resolution microscopy, the study of high density plasmas, and so on. We present a comprehensive numerical model of the recombination pumping scheme to achieve SXL, specifically characterizing the initial parameters required to achieve gain in the 2->1 transition in LiIII ions (13.5nm). The numerical model includes the initial Optical Field Ionization (OFI) of the plasma by an intense 100fs laser pulse, taking into account Above Threshold Ionization (ATI) heating, particle collisions and spatial effects. The ionization process is calculated using a parallel Particle In Cell (PIC) code that was written specifically to include all the effects involved in the ionization process. The results from the PIC code are then inserted to a Fokker-Planck code to calculate the time evolution of the distribution function after the ionization and the gain is then calculated during the process of recombination as the plasma expands and cools. We show that by taking into account the certain features of the electron distribution function in the plasma that cannot be accounted for analytically, high gain in the 2->1 transition of LiIII is feasible under certain initial conditions, even though initial estimates based on analytical calculation predicted that only minute gain, if any, can be achieved. We characterize the behavior of the gain under different pumping parameters and initial plasma conditions.