Soft X-ray spectra of optical-field ionized helium gas by a femtosecond Ti:sapphire laser

Recent progress in the field of ultrashort-pulse laser technology has made it possible to produce femtosecond pulses of the terawatt power level in a compact size. The laser pulse of extremely high intensity (>10/sup 16/ W/cm/sup 2/) creates plasma by the effect of optical-field ionization (OFI). In the OFI scheme, a plasma consisting of multiply charged ions and free electrons is generated directly by the intense laser field and not by electron collisions as in usual laser plasmas. Since such a plasma can provide a suitable medium for the production of coherent X-rays, detailed investigation is of great importance. We report the soft X-ray spectra of helium plasma and compare the results with simulation, The laser used is a terawatt Ti:sapphire system based on the chirped-pulse amplification technique.