Soft-x-ray laser scheme in a plasma created by optical-field-induced ionization of nitrogen

I. INTRODUCTION With the development of the chirped pulse amplification technique and the improvement of Ti:sapphire technology, x-ray laser schemes based on fast collisional recombination of highly ionized plasmas seem achievable. Actually, tunnel ionization @so-called optical-field ionization ~OFI!#, first studied by Keldysh @1# for the hydrogen atom, is the predominant ionization process in high-intensity laser-matter interaction. It allows for the creation of a plasma composed of fully stripped ions and is characterized by a relatively low electronic temperature ( ;10 eV) for a linearly polarized @2# laser pulse. This cold electronic population is suitable for fast collisional recombination after the laser, resulting in a