Christopher D. Decker

Simultaneous measurement of local gain and electron density in X-ray lasers

X-ray lasers (XRLs) have experimental average gains that are significantly less than calculated values and a persistently low level of spatial coherence. An XRL has been used both as an injected signal to a short XRL amplifier and as an interferometer beam to measure two-dimensional local gain and density profiles of the XRL plasma with a resolution near 1 micrometer. The measured local gain is in agreement with atomic models but is unexpectedly spatially inhomogeneous. This inhomogeneity is responsible for the low level of spatial coherence observed and helps explain the disparity between observed and simulated gains.

X-ray Sources Generated from Gas-Filled Laser-Heated Targets

The X-ray sources in the 4–7 keV energy regime can be produced by laser-irradiating high-Z gas-filled targets with high-powered lasers. A series of experiments have been performed using underdense targets that are supersonically heated with ∼35 kJ of 0.35 μm laser light. These targets were cylindrical Be enclosures that were filled with 1–2 atms of Xe gas. L-shell x-ray emission is emitted from the plasma and detected by Bragg crystal spectrometers and x-ray diodes. Absolute flux measurements show conversion efficiencies of ∼10% in the multi-kilovolt x-ray emission. These sources can be used as bright x-ray backlighters or for material testing.

Soft x‐ray interferometry (abstract)

The development of soft x‐ray mirrors and large aperture soft x‐ray beamsplitters now allows the near routine operation of soft x‐ray interferometers with high brightness x‐ray sources. Mach‐Zehnder and Fourier transform interferometers utilizing a soft x‐ray laser light source operating at 80 eV will be described. Results from high density, long scale‐length plasma probing measurements, x‐ray laser coherence measurements, and materials properties measurements will be presented.

High gains for a Ni-like Tungsten x-ray laser operating in a transient regime

We examine a scheme for a compact nickel-like tungsten soft x- ray laser operating on the 4d - 4p, J equals 0 - 1 transition at 43.2 angstrom. High gains are achieved by operating at high electron densities (ne greater than 1022 cm-3). In this regime, the gain during non-equilibrium ionization can greatly exceed that of the steady state. The duration of this transient gain is on the order of the ionization time which makes picosecond-pulsed optical pumps ideal. We predict gains of 220 cm-1 for a 1 micrometer pump of intensity I equals 1 X 1017 W/cm2.

Probing high-density plasmas with soft x-ray lasers

Collisionally pumped x-ray lasers are ideal for a wide variety of plasma diagnostics. They have been demonstrated over a wavelength range extending from 3.5 nm to 40 nm and have output energies as high as 10 mJ in 150 ps pulses. The beam divergence is less than 15 mrad and the linewidth (lambda) /(Delta) (lambda) approximately 104 provides sufficient longitudinal coherence for interferometry. Using various arrangements constructed with multi-layer XUV reflective optics we have carried out a variety of experiments with the yttrium x-ray laser operating at 15.5 nm. Initial experiments involved x-ray laser backlighting and Moire deflectometry, while later work demonstrated interferometric determinations of plasma density profiles in various targets. We have also studied hydrodynamic imprinting of laser speckle patterns on directly driven thin foils with 1 - 2 micrometer spatial resolution. These detailed plasma diagnostic capabilities provide an important means to benchmark hydrodynamic calculations of dense plasma flows.

Production of multikilovolt x rays from laser-heated targets

Experiments to develop high photon energy x-ray sources were carried out on the Nova laser. Ten laser beams delivered approximately 39 kJ of energy in 2 ns into a Be cylinder filled with Xe gas. The conversion efficiency into x-rays > 4 keV was measured to be 5 - 15%, which is the highest measured in this photon regime for laser-produced plasmas. The temporal dependence of the x-ray emission indicates that the bulk of the emission is emitted in the first half of the 2 ns pulse. A set of diagnostics were fielded to image the volume in emission as well as provide spectra to measure conversion efficiency.

Pump propagation in short-pulse-driven recombination x-ray lasers

The propagation of the optical laser (pump) used in optical-field ionized recombination x-ray lasing schemes [D. C. Eder et. al, Phys. of Plasmas 5, 1744 (1994)] is examined. A model which self consistently evolves the laser radiation as it ionizes a neutral gas is presented. The model allows for multiple ionization stages and for spatial variations in the neutral gas density appropriate for studying propagation through gas jets and laser evaporated gases. Typical experimental conditions are examined and it is found that ionization induced refraction plays a dominant role in the evolution of the laser pulse.