Publications

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Using the physical process of ultraintense field ionization of high charge states of inert gas ions, we have developed a method of peak intensity measurement at the focus of high energy short pulse lasers operating in single shot mode. The technique involves detecting ionization products created from a low pressure gas target at the laser focus via time of flight detector. The observation of high ion charge states collected by the detector yields peak intensity at the focus when compared with the results obtained from well established tunnel ionization models. An initial peak intensity measurement of 5× 1016 W cm-2 was obtained for a 1.053 μm center wavelength, 0.4 J pulse with 1 ps pulse duration focused with an f5.5 off-axis parabola. Experiments with multijoule level, 500 fs laser pulses are on the way. © 2006 American Institute of Physics.

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We use two Pockels cells in series to achieve simple temporal pulse shaping. This technique is used in our optical parametric chirp pulse amplification (OPCPA) system to optimize the temporal shape of the pump pulse. It also offers a low cost alternative to arbitrary waveform generators. © 2005 Society of Photo-Optical Instrumentation Engineers.

Over the last several years, rapid progress has been made evaluating the double-z-pinch indirect-drive, inertial confinement fusion (ICF) high-yield target concept (Hammer et al 1999 Phys. Plasmas 6 2129). We have demonstrated efficient coupling of radiation from two wire-array-driven primary hohlraums to a secondary hohlraum that is large enough to drive a high yield ICF capsule. The secondary hohlraum is irradiated from two sides by z-pinches to produce low odd-mode radiation asymmetry. This double-pinch source is driven from a single electrical power feed (Cuneo et al 2002 Phys. Rev. Lett. 88 215004) on the 20 MA Z accelerator. The double z-pinch has imploded ICF capsules with even-mode radiation symmetry of 3.1 {+-} 1.4% and to high capsule radial convergence ratios of 14-21 (Bennett et al 2002 Phys. Rev. Lett. 89 245002; Bennett et al 2003 Phys. Plasmas 10 3717; Vesey et al 2003 Phys. Plasmas 10 1854). Advances in wire-array physics at 20 MA are improving our understanding of z-pinch power scaling with increasing drive current. Techniques for shaping the z-pinch radiation pulse necessary for low adiabat capsule compression have also been demonstrated.

We have improved deformable mirror approach to allow good parabolic deformation for efficient thermal lens compensation. Our design uses an annulus to push onto the back surface of a flat mirror substrate, simply supported at the outer edge, thereby creating a parabolic deformation within the encircled area. We built an assembly using a 25.4 mm diameter, 1 mm thick mirror with a wedge of less than 10 arc seconds that was deformed with a 12 mm diameter annulus at the back of the mirror. Using a Shack-Hartman wavefront sensor we performed careful measurements to characterize the performance of the mirrors.

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In order to develop the next generation of high peak intensity lasers, new grating technology providing higher damage thresholds and large apertures is required. The current assumption is that this technical innovation will be multilayer dielectric gratings, wherein the uppermost layer of a thin film mirror is etched to create the desired binary phase grating. A variant of this is explored with the upper grating layer being a lower density gelatin-based volume phase grating in either sol-gel or dichromated gelatin. One key benefit is the elimination of the etching step.

Annular wire array implosions on the Sandia Z-machine can produce >200 TW and 1-2 MJ of soft x rays in the 0.1-10 keV range. The x-ray flux and debris in this environment present significant challenges for radiographic diagnostics. X-ray backlighting diagnostics at 1865 and 6181 eV using spherically-bent crystals have been fielded on the Z-machine, each with a ~0.6 eV spectral bandpass, 10 μm spatial resolution, and a 4 mm by 20 mm field of view. The Z-Beamlet laser, a 2-TW, 2-kJ Nd:glass laser (λ=527 nm), is used to produce 0.1-1 J x-ray sources for radiography. The design, calibration, and performance of these diagnostics is presented.

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