We are pursuing several novel x-ray imaging schemes to measure plasma parameters in inertial-confinement fusion experiments. This paper will concentrate principally on two quite successful approaches, the soft x-ray moiré deflectometer, and the annular (ring) coded-aperture microscope. The first scheme uses moiré deflectometry to measure the electron density within a plasma. When a beam of collimated light travels through a plasma, it is refracted by electron-density gradients. Moiré deflectometry is a scheme to measure the refraction, and thus provides a diagnostic of the line-integrated electron-density spatial profile. To use deflectometry with a high-density laser-produced plasma, we produced a probe beam with a soft x-ray laser (λ=15.5 nm). The short wavelength of the probe radiation allows us to measure the spatial profiles at densities up to critical (e.g., 4×1021 cm-3) in long scale-length (>1 mm) plasmas. We use finely made one-dimensional rulings (10-μm pitch), a set of synthetic multilayer mirror optics, and a soft-x-ray sensitive CCD camera to achieve a spatial resolution of 6 μm. In the second scheme, a ring coded-aperture microscope (RAM) provides a large (factor of 5-10) improvement in signal-to-noise ratio over pinhole imaging when the source size is much less than the major diameter of the annular aperture. It is therefore very useful to image the cores of imploded laser-fusion capsules. We have obtained very high-quality (SNR≈100) time-integrated images of indirectly driven targets. By coupling ring apertures with x-ray framing cameras, we have also obtained high-quality (SNR≈50) time-resolved images of directly driven capsule cores. These images can be used to observe the symmetry of the drive and the hydrodynamics of the implosion. Recently, we also obtained high-quality images of short-pulse laser (0.6 ps) irradiations of Ta targets. The spatial resolution of the RAM has been investigated both experimentally and through diffraction calculations. The resolution is at least as small as the width of the annulus, and may be smaller under particular circumstances.
ASJC Scopus subject areas
- Physics and Astronomy (miscellaneous)