X-ray generation by femtosecond laser pulses and its application to soft X-ray imaging microscope

We have developed laser-produced plasma X-ray sources using femtosecond laser pulses at lOHz repetition rate in a table-top size in order to investigate basic mechanism of X-ray emission from laser-matter interactions and its application to a X-ray microscope. In a soft X-ray region over 5 nm wavelength, laser-plasma X-ray emission from a solid target achieved an intense flux of photons of the order of 10 photons/rad per pulse with duration of a few 100 ps, which is intense enough to make a clear imaging in a short time exposure. As an application of laser-produced plasma X-ray source, we have developed a soft X-ray imaging microscope operating in the wavelength range around 14 nm. The microscope consists of a cylindrically ellipsoidal condenser mirror and a Schwarzshird objective mirror with highly-reflective multilayers. We report preliminary results of performance tests of the soft X-ray imaging microscope with a compact laser-produced plasma X-ray source. INTRODUCTION The present high-brightness X-ray sources have been developed as third generation synchrotron light sources based on a large-scale high energy electron storage ring and magnetic undulators. A compact, tunable, high-brightness X-ray source has basic and industrial applications in a number of fields, such as solid-state physics, material, chemical, biological and medical sciences. Particularly for biological and medical applications, such as X-ray imaging, radiography and therapy, it is essential to downscale the present synchrotron light sources into the environment of a laboratory or a hospital, keeping properties of X-ray radiations. Recently availability of compact terawatt lasers arouse a great interest in the use of lasers as a compact bright X-ray source generated by intense laser-plasma interactions, replacing conventional X-ray tubes. We have developed laser-produced plasma X-ray sources using femtoseconds laser pulses at lOHz repetition rate in a table-top size in order to investigate basic mechanism of X-ray emission from laser-matter interactions and its application to a Xray microscope. In a soft X-ray region over 5 nm wavelength, X-ray emission from a solid metal target, such as Al, Cu, and W irradiated by a 130 ml laser pulse with duration of 100 fs, achieved an intense flux of photons of the order of 10 photons/rad CP634, Science of Superstrong Field Inter actions, edited by K. Nakajima and M. Deguchi © 2002 American Institute of Physics 0-7354-0089-X/02/$ 19.00 268 per pulse with duration of a few 100 ps, which is intense enough to make a clear imaging in a short time exposure. As an application of laser-produced plasma X-ray source, we have developed a soft X-ray imaging microscope operating in the wavelength range around 14 nm. The microscope consists of a grazing incidence cylindrically ellipsoidal condenser mirror and a Schwarzshird objective mirror with highly-reflective multilayers, we report preliminary results of performance tests of the soft X-ray imaging microscope with a compact laser-produced plasma X-ray source. LASER-PRODUCED PLASMA X-RAY EMISSION FROM SOLID TARGETS Intense ultra-short laser pulses can produce a high density plasma associating a strong electron heating generated by strong field interactions with solid state matter. As the electric field of the laser radiation exceeds the atomic electric fields above the intensities of the order of 10 W/cm, a solid matter is ionized in a fraction of the wave oscillation through processes of the multiphoton ionization and the tunneling ionization to produce plasmas with a solid electron density of the order of 10 cm". Hence intense femtosecond laser pulses generate interactions with plasmas characterized by solid density and very steep gradients with the scale length of a few hundreds of angstroms in the plasma expansion region. In the plasma region over a length of the skin depth, typically of the order of a few hundreds of angstroms, a strong electron heating occurs in a very short time to produce the plasma heated up to temperatures of several keV at the intensity of 10 W/cm away from thermal equilibrium. The high density and the electron kinetic energy of these plasmas generate bright X-ray pulses with photon energy extending up to several keV through fundamental emission processes, known as bremsstrahlung, recombination, and line emissions[l]. Measurements of X-ray intensity and pulse duration A laser-plasma X-ray source has been simply made by focusing a high peak power laser onto the surface of a taget placed in a vacuum chamber. Our laser system is a table-top size Ti:sapphire laser based on the chirped pulse amplification at the wavelength of 790 nm. This system can produce the output pulses with the pulse duration of 100 fs and the maximum pulse energy of 200 mJ at the repetition rate of 10 Hz. In order to investigate characterization of laser-produced plasma X-ray emission, the output pulses were focused by the off-axis parabolic mirror with a focal length of 150 mm. The measured focal spot profile was an elliptical shape with a horizontal diameter of about 10 [im and a vertical diameter of about 30 |im at the focal point. The peak focused laser intensity exceeds 4X 10 W/cm for a 100 mJ pulse energy focused on the target. The solid target mounted on the translation stage was scanned by a step of 100 |Lim so that a fresh surface of the target was exposed at each laser shot, we measured intensity and pulse duration of X-ray radiation emitted by laser-produced plasmas for various targets with different atomic numbers (Z) using the X-ray streak