Kilohertz sources of hard x rays and fast ions with femtosecond laser plasmas

We demonstrate a new, stable, kilohertz femtosecond laser plasma source of hard-x-ray continuum and K-alpha emission that uses a microscopic liquid jet target that is continuous and debris free. Plasmas produced by ultrashort (50-fs) intense laser pulses from a fine (10-30-mum diameter) liquid Ga jet emit bright 9.3- and 10.3-keV K-alpha and K-beta lines superimposed on a multikilovolt bremmstrahlung continuum. Kilohertz femtosecond x-ray sources will find many applications in time-resolved x-ray diffraction and microscopy studies. As high-intensity lasers become more compact and operate at increasingly high repetition-rates, they require a target configuration that is both repeatable from shot to shot and debris free. Our target provides a pristine, unperturbed filament surface at rates \u3e 100 kHz. A number of liquid metal targets are considered. We show the hard-x-ray spectrum described above. The source was generated by a 50-fs-duration, 1-kHz, 2-W, high-intensity Ti:sapphire laser. Using the same technology, we also generate forward-going sub-mega-electron-volt (sub-MeV) protons from a 10-mum liquid water target at 1-kHz repetition rates. Kilohertz sources of high-energy ions will find many applications in time-resolved particle interaction studies and will lead to efficient generation of short-lived isotopes for use in nuclear medicine and other applications. The protons were detected with CR-39 track detectors in both the forward and the backward directions up to energies of similar to500 keV. As the intensity of compact high-repetition-rate lasers sources increases, we can expect improvements in the energy, conversion efficiency, and directionality to occur. The effect of these developments is discussed. As compact, high-repetition-rate femtosecond laser technology reaches focused intensities of similar to10(19) W/cm(2), many new applications of high-repetition-rate hard-x-ray and MeV ion sources will become practical. (C) 2003 Optical Society of America