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

Abstract

Recent progress in the field of ultrashort-pulse laser technology has made it possible to produce femtosecond pulzes of the terawatt power level in a compact size. The laser pulse of extremely high intensity (> 1016 W/cm2) creates plasma by the effect of optical-field ionization (OFI). In the OF1 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. Here we report the soft X-ray spectra of helium plasma and compare with simulation. The laser used in this work is a terawatt TiSapphire system based on the chirped-pulse amplification technique and has been described in detail elsewhere [l]. This laser delivers -30 fs (FWHM) pulses with energy of up to 60 mJ on the target. Linearly polarized laser pulses were focused with a parabolic mirror of 15 cm focal length into a gas jet of helium from a pulsed nozzle. The measured minimum focal spot diameter was about 16 pm, so the maximum laser intensity could reach more than 1018 W/cm2. The gas target was formed by pulsed injection of a small amount of gas from a high pressure valve through a circular nozzle (diameter of 200 pm) into a vacuum chamber. The . valve was charged with gas at a backing pressure ranging from 0.5 up to 16 bar. The gas density profiles were measured by a focusing method. By varying the backing pressure we were able to change the actual gas density in the interaction region. Time integrated soft X-ray spectra were recorded by a space-resolving flat-field extreme ultraviolet (XUV) spectrograph. It was coupled with a thinned, backside-illuminated charge-coupled device (CCD) camera. The covered wavelength range was from 40 to 3608, and the spectral resolution was approximately 0.2A, at 200A. © 1999 IEEE.