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.

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.

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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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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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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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An important issue in designing a higher-power version of the Z machine at Sandia National Laboratories is electron current loss in the vacuum section, which consists of four radial transmission lines and a convolute (current-adder). There is evidence from experiments on Z that 1-2MA of current out of about 20MA is lost in the vacuum section before reaching the wire-array load [1]. Calculations using the LSP [2] and QUICKSILVER [3] particle-in-cell codes have shown much less current loss [4,5,6]. The current loss in the calculations is due to sheath-current loss in the region of the convolute, and is associated with the magnetic nulls which are intrinsic to the current splitting in the convolute Detailed 2-D calculations for the radial MITLs show that, in the region between the insulator stack and a radius of about 20cm (over which the radial-line vacuum impedance increases slowly from 2Ω to 3Ω), excess electron sheath current is mostly retrapped to the cathode electrode. The electron sheath current is given approximately by Mendel's force-balance expression [7] applied locally, and as a result, the sheath current decreases as Zv-2, where Zv is the vacuum impedance. Between a radius of 20cm and the convolute, where the radial-line vacuum impedance increases more sharply (to 6Ω at 10cm) there is significant "launching" of sheath current. The sheath behavior in this region is qualitatively similar to that predicted using a "constant flow impedance" model, but in the simulations the sheath is unstable and breaks up into vortices.

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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Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling ("hohlraum energetics") is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)]. © 2001 American Institute of Physics.

A unified set of high-temperature-hohlraum models has been developed. For a simple hohlraum, P{sub s} = [A{sub s}+(1{minus}{alpha}{sub W})A{sub W}+A{sub H}]{sigma}T{sub R}{sup 4} + (4V{sigma}/c)(dT{sub R}{sup r}/dt) where P{sub S} is the total power radiated by the source, A{sub s} is the source area, A{sub W} is the area of the cavity wall excluding the source and holes in the wall, A{sub H} is the area of the holes, {sigma} is the Stefan-Boltzmann constant, T{sub R} is the radiation brightness temperature, V is the hohlraum volume, and c is the speed of light. The wall albedo {alpha}{sub W} {triple_bond} (T{sub W}/T{sub R}){sup 4} where T{sub W} is the brightness temperature of area A{sub W}. The net power radiated by the source P{sub N} = P{sub S}-A{sub S}{sigma}T{sub R}{sup 4}, which suggests that for laser-driven hohlraums the conversion efficiency {eta}{sub CE} be defined as P{sub N}/P{sub LASER}. The characteristic time required to change T{sub R}{sup 4} in response to a change in P{sub N} is 4V/C[(l{minus}{alpha}{sub W})A{sub W}+A{sub H}]. Using this model, T{sub R}, {alpha}{sub W}, and {eta}{sub CE} can be expressed in terms of quantities directly measurable in a hohlraum experiment. For a steady-state hohlraum that encloses a convex capsule, P{sub N} = {l_brace}(1{minus}{alpha}{sub W})A{sub W}+A{sub H}+[(1{minus}{alpha}{sub C})(A{sub S}+A{sub W}{alpha}{sub W})A{sub C}/A{sub T}]{r_brace}{sigma}T{sub RC}{sup 4} where {alpha}{sub C} is the capsule albedo, A{sub C} is the capsule area, A{sub T} {triple_bond} (A{sub S}+A{sub W}+A{sub H}), and T{sub RC} is the brightness temperature of the radiation that drives the capsule. According to this relation, the capsule-coupling efficiency of the baseline National-Ignition-Facility (NIF) hohlraum is 15% higher than predicted by previous analytic expressions. A model of a hohlraum that encloses a z pinch is also presented.

In conjunction with ongoing high-current experiments on Sandia National Laboratories' Z accelerator (Albuquerque, NM) we have revisited a problem first described in detail by Heinz Knoepfel. Unlike the 1-Tesla MITL's of pulsed power accelerators used to produce intense particle beams, Z's disk, transmission line (downstream of the current addition) is in a 100-1200-Tesla regime; so its conductors cannot be modeled simply as static infinite conductivity boundaries. Using the MHD code [2], [3], [17] MACH2 we have been investigating the conductor hydrodynamics, characterizing the joule heating, magnetic field diffusion, and material deformation, pressure, and velocity over a range of current densities, current rise-times, and conductor materials. The three purposes of this work are 1) to quantify power flow losses owing to ultrahigh magnetic fields, 2) to model the response of VISAR [4], [18], [19] diagnostic samples in various configurations on Z, and 3) to incorporate the most appropriate equation of state and conductivity models into our magnetohydrodynamics (MHD) computations. Certain features are strongly dependent on the details of the conductivity model.

The generalized method of Backus and Gilbert (BG) is described and applied to the inverse problem of obtaining spectra from a 5-channel, filtered array of x-ray detectors (XRD's). This diagnostic is routinely fielded on the Z facility at Sandia National Laboratories to study soft x-ray photons ({le}2300 eV), emitted by high density Z-pinch plasmas. The BG method defines spectral resolution limits on the system of response functions that are in good agreement with the unfold method currently in use. The resolution so defined is independent of the source spectrum. For noise-free, simulated data the BG approximating function is also in reasonable agreement with the source spectrum (150 eV black-body) and the unfold. This function may be used as an initial trial function for iterative methods or a regularization model.