Publications

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We have developed a diagnostic system that measures the spectrally integrated (i.e. the total) energy and power radiated by a pulsed blackbody x-ray source. The total-energy-and-power (TEP) diagnostic system is optimized for blackbody temperatures between 50 and 350 eV. The system can view apertured sources that radiate energies and powers as high as 2 MJ and 200 TW, respectively, and has been successfully tested at 0.84 MJ and 73 TW on the Z pulsed-power accelerator. The TEP system consists of two pinhole arrays, two silicon-diode detectors, and two thin-film nickel bolometers. Each of the two pinhole arrays is paired with a single silicon diode. Each array consists of a 38 x 38 square array of 10-{micro}m-diameter pinholes in a 50-{micro}m-thick tantalum plate. The arrays achromatically attenuate the x-ray flux by a factor of {approx}1800. The use of such arrays for the attenuation of soft x rays was first proposed by Turner and co-workers [Rev. Sci. Instrum. 70, 656 (1999)RSINAK0034-674810.1063/1.1149385]. The attenuated flux from each array illuminates its associated diode; the diode's output current is recorded by a data-acquisition system with 0.6-ns time resolution. The arrays and diodes are located 19 and 24 m from the source, respectively. Because the diodes are designed to have an approximately flat spectral sensitivity, the output current from each diode is proportional to the x-ray power. The nickel bolometers are fielded at a slightly different angle from the array-diode combinations, and view (without pinhole attenuation) the same x-ray source. The bolometers measure the total x-ray energy radiated by the source and--on every shot--provide an in situ calibration of the array-diode combinations. Two array-diode pairs and two bolometers are fielded to reduce random uncertainties. An analytic model (which accounts for pinhole-diffraction effects) of the sensitivity of an array-diode combination is presented.

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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.

In this paper, we report the results of an experimental campaign to study the initiation, implosion dynamics and radiation yield of tungsten wire arrays as a function of the wire number. An optimization study of the X-ray emitted peak power, rise time and FWHM was effectuated by varying the wire number while keeping the total array mass constant at ∼5.8mg. The driver used was the ∼20MA Z-accelerator, in its usual short pulse mode of 100ns. We studied single arrays of diameter 20mm and height 10mm. The smaller wire number studied was 30 and the largest 600. It appears that 600 is the highest wire number achievable with present-day technology. Radial and axial diagnostics were used, including a crystal monochromatic X-ray backlighter. An optimum wire number of ∼370 was observed, which is very close to the number (300) routinely used for the ICF program in Sandia. © 2005 Taylor & Francis Group Ltd.

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We present the solution of a 1D radial MHD model of the plasma ablated from multi-MA wire array implosions extending a recently obtained steady state solution [J.P. Chittenden, et al. Phys. Plasmas 11, 1118 (2004)] to a driving current that is exponential in time. We obtain a solution for the flow in almost analytical form by reducing the partial differential equations to a set of ordinary differential equations with a single parameter. We compute the mass weighted density width, and find the regime in which it agrees to a few percent with that of a simpler approximation to the ablated plasma flow, for which the driving current is linear in time, and the flow velocity constant. Assuming that the density width at the end of the ablation period is proportional to width of the plasma sheath at stagnation, we obtain a scaling relationship for peak X-ray power. We compare this relationship to experimental peak X-ray powers for tungsten wire arrays on the Z pulsed power generator of Sandia National Laboratories, and to previously proposed scaling hypotheses. We also use this scaling to project peak X-ray powers on ZR, a higher peak current modification of Z, presently under design.

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Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

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This research consisted of testing surface treatment processes for stainless steel and aluminum for the purpose of suppressing electron emission over large surface areas to improve the pulsed high voltage hold-off capabilities of these metals. Improvements to hold-off would be beneficial to the operation of the vacuum-insulator grading rings and final self-magnetically insulated transmission line on the ZR-upgrade machine and other pulsed power applications such as flash radiograph and pulsed-microwave machines. The treatments tested for stainless steel include the Z-protocol (chemical polish, HVFF, and gold coating), pulsed E-beam surface treatments by IHCE, Russia, and chromium oxide coatings. Treatments for aluminum were anodized and polymer coatings. Breakdown thresholds also were measured for a range of surface finishes and gap distances. The study found that: (1.) Electrical conditioning and solvent cleaning in a filtered air environment each improve HV hold-off 30%. (2.) Anodized coatings on aluminum give a factor of two improvement in high voltage hold-off. However, anodized aluminum loses this improvement when the damage is severe. Chromium oxide coatings on stainless steel give a 40% and 20% improvement in hold-off before and after damage from many arcs. (3.) Bare aluminum gives similar hold-off for surface roughness, R{sub a}, ranging from 0.08 to 3.2 {micro}m. (4.) The various EBEST surfaces tested give high voltage hold-off a factor of two better than typical machined and similar to R{sub a} = 0.05 {micro}m polished stainless steel surfaces. (5.) For gaps > 2 mm the hold-off voltage increases as the square root of the gap for bare metal surfaces. This is inconsistent with the accepted model for metals that involves E-field induced electron emission from dielectric inclusions. Micro-particles accelerated across the gap during the voltage pulse give the observed voltage dependence. However the similarity in observed breakdown times for large and small gaps places a requirement that the particles be of molecular size. This makes accelerated micro-particle induced breakdown seem improbable also.

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