A. Bixler

A Renewed Capability for Gas Puff Science on Sandia's Z Machine

A comprehensive gas puff capability is being developed on the Z pulsed power generator. We describe the methodology employed for developing a gas puff load on Z, which combines characterization and modeling of the neutral gas mass flow from a supersonic nozzle, numerical modeling of the implosion of this mass profile, and experimental evaluation of these magnetic implosions on Z. We are beginning a multiyear science program to study gas puff z-pinch physics at high current, starting with an 8-cm diameter double-shell nozzle, which delivers a column of Ar gas that is imploded by the machines fast current pulse. The initial shots have been designed using numerical simulation with two radiation-magnetohydrodynamic codes. These calculations indicate that 1 mg/cm should provide optimal coupling to the driver and 1.6:1 middle:outer shell mass ratio will best balance the need for high implosion velocity against the need to mitigate the magnetic Rayleigh-Taylor instability. The models suggest 300-500-kJ Ar K-shell yield should be achievable on Z, and we report an initial commissioning shot at lower voltage in which 250 kJ was measured. Future experiments will pursue optimization of Ar and Kr K-shell X-ray sources, study fusion in deuterium gas puffs, and investigate the physics of gas puff implosions including energy coupling, instability growth, and radiation generation.

Architecture, implementation, and testing of a multiple-shell gas injection system for high current implosions on the Z accelerator

Tests are ongoing to conduct ~20 MA z-pinch implosions on the Z accelerator at Sandia National Laboratory using Ar, Kr, and D2 gas puffs as the imploding loads. The relatively high cost of operations on a machine of this scale imposes stringent requirements on the functionality, reliability, and safety of gas puff hardware. Here we describe the development of a prototype gas puff system including the multiple-shell nozzles, electromagnetic drivers for each nozzles valve, a UV pre-ionizer, and an inductive isolator to isolate the ~2.4 MV machine voltage pulse present at the gas load from the necessary electrical and fluid connections made to the puff system from outside the Z vacuum chamber. This paper shows how the assembly couples to the overall Z system and presents data taken to validate the functionality of the overall system.

Development and use of a two-dimensional interferometer to measure mass flow from a multi-shell Z-pinch gas puff

For gas puff Z-pinches, the K-shell x-ray yield is maximized with the use of a multi-shell nozzle. Optimization of the yield, verification of hydrodynamic models of the nozzle flows, and plausible MHD code modeling of the implosions require data on the radial and axial (R,Z) distribution of mass in the nozzles flow field. Interferometry is a well-established technique for acquiring such data. We describe the development and use of a two-dimensional interferometer with emphasis on the required data reduction methods. We also show that the instrument can derive the flow from each individual nozzle in a multi-shell system.

Continued development of a 12 cm diameter nozzle for argon Z-pinches

We report on the variation of K-shell X-ray output of an argon Z-pinch as a function of the radial gas distribution. The tests, conducted on the Double-EAGLE simulator at /spl sim/3.5-MA peak current, utilized a 12-cm-diameter double-shell nozzle that was designed for use with the 300-ns rise-time current pulse (/spl sim/6 MA) of the DECADE QUAD pulsed power machine. By varying the plena gas pressures of the inner and outer shells, the net radial distribution could be changed from one that was strongly concentrated near the axis to one more broadly distributed as a function of radius. Previous work has shown that a roughly uniform radial distribution gives higher X-ray output than shell-like flows for gas Z-pinches. The present work was focused on refining the optimum radial distribution and to establish benchmarks for modeling calculations. The present data show that the K-shell yield has a broad optimum (and the relative strength of the K continuum >4 keV systematically changes) as the mass distribution becomes more peaked near the axis. Very-high-quality K-emitting volumes (<5 ns pulse width from <1 mm diameters) were achieved over a significant fraction of the pinch length.

Long-Implosion-Time, 12-cm-Diameter Argon-Gas-Puff Experiments at 6 MA

Summary form only given. We wish to maximize the K-shell yield from high-atomic-number Z pinches (photon energy of ~1-8 keV) and to develop Z-pinch-based concepts for sources of higher-photon energy (~8 to 40 keV). To achieve these goals, large-diameter implosions are needed to: (a) produce the high specific energy required to excite K-shell line radiation from high-atomic-number Z pinches, (b) properly match the load to high-current (ges 10 MA) generators, and (c) develop continuum-radiator concepts that require ionizing beyond the optimum He/H-like state for K-shell emission. Large-initial-diameter implosions are also required for efficient coupling between the Z pinch and a generator with a long current rise-time (> 100 ns) and, thus, desirable lower voltage. The expected deleterious effects on final pinch formation associated with the increase in instability growth at larger diameter (e.g., Rayleigh-Taylor) can be mitigated by using an initial mass profile that is strongly peaked on axis. We review recent experiments with, and numerical simulations of, 12-cm initial diameter, argon gas-puff Z-pinch implosions. The experiments were carried out on the Decade Quad at implosion times of 230 to 250 ns and on Saturn at 200 to 215 ns. Both generators provide peak currents of about 6 MA. The argon K-shell yield was 80 kJ on DQ. On Saturn, we obtained 75 kJ of K-shell yield, roughly twice the maximum argon K-shell yield previously obtained there with a several-cm diam. nozzle at < 100 ns implosion times. Numerical simulations are in agreement with the measurements.

A renewed argon gas puff capability on Sandia's Z machine

Summary form only given. We have reestablished gas puff z-pinch capability on Sandias 20 MA Z machine, including a Sandia-operated driver system and an imaging interferometer to characterize nozzle mass flow [1]. Initial experiments have focused on developing a 3 keV Ar K-shell x-ray source. We have pursued a design-driven approach to planning these experiments, utilizing numerical simulation to predict Ar K-shell yield for various nozzle mass profile configurations. In particular, we study coupling to the generator and how the distribution of mass between the two shells impacts magnetic Rayleigh-Taylor instability evolution. Two-dimensional radiation-magneto-hydrodynamic (MHD) simulations at NRL for a number of density profiles produced by the nozzle have predicted yields in excess of 300 kJ, and indicated that a 1:1.6 outer-to innershell mass ratio would produce the most stable implosion with high enough temperature to optimize Ar K-shell output [2]. This result was also consistent with 3D MHD modeling using the Gorgon code [3] at Sandia. Both models used tabulated non-LTE atomic models for Ar K-shell photon emission. We will present Z experimental data from the first gas puff shots on the accelerator since 2006, and compare these to the numerical models. Spectral output is measured from 1-20 keV. Electrical current measurements at different positions along the power flow section provide information on current coupling to the load. Time-gated pinhole imaging and radially-resolved spectroscopy indicate ~60 cm/μs implosion velocities and >1 keV electron temperatures.

Demonstration of a “gas anode” current return for a z-pinch

We have developed and successfully operated a “gas anode” for use with z-pinch plasma radiation x-ray sources (PRS). The gas anode replaces the multiple solid anode-currentreturn rods of a standard PRS with a cylindrical shell of low atomic number gas. Beginning at the start of current flow, either via pre-ionization by UV flashboards or via high voltage breakdown, the anode gas conducts the return current flow of the pinch for the duration of the implosion and beyond.

Characterization of a 500J Dense Plasma Focus for Producing Soft x-rays

The scaling of x-ray output with (stored energy)2 and ∼10ns radiation pulse width of the dense plasma focus make it an interesting source of soft x-ray radiation for lithography, biological imaging, nano imaging or soft x-ray diagnostic calibration. AASC was funded by DTRA to explore soft x-ray diagnostic calibration*. In this follow-on study, we explore an alternative electrode configuration with Ar. The soft x-radiation (≫1 keV) yield, radiation pulse width and debris are characterized for our 500J dense plasma focus over 100’s of pulses fired at 0.2 Hz. The radiation yield is compared with (current)4 scaling. A soft x-ray spectrometer is installed to examine the soft x-ray spectrum.

The Effect of the Initial Density Profile on K-Shell Emission in Two-Dimensional Simulations of Argon Gas-Puff z Pinches

Summary form only given. In order to implode enough mass to the high specific energies necessary to produce K-shell emission from high Z elements like krypton, large-diameter Z-pinch loads will be required for short current rise time machines, like Z and ZR. For long pulse rise time machines, such as Decade Quad, large diameters are needed for the same reason and, in addition, they are needed to effectively couple the electrical energy that is delivered over a long current rise time to the load. The amount of K-shell emission produced from a large-diameter load has experimentally been shown to depend critically on the initial density distribution. In the work presented here, we investigate the initial gas-puff density distributions effect on the ability of the plasma to radiate in the K-shell. This study is performed using a modified version of the Air Forces MACH2 magnetohydrodynamic code. The principal modification is the self-consistent addition of a state-of-the-art, computationally efficient, and reasonably accurate equation of state (EOS) and radiation transport model to MACH2. This EOS/radiation model is designed for modeling K-shell emitting plasmas and it is called the tabular collisional radiative equilibrium (TCRE) model. In this study comparisons are made between simulations of, and results obtained from, large-diameter load experiments on the Decade Quad and Double-EAGLE pulsed power generators.

Biological weapons agent defeat using directed microwave energy

Summary form only given. A synergistic, molecularly targeted microwave approach has demonstrated unprecedented kill of a broad range of biological weapons agents (BWA) using directed microwave energy in conjunction with a specially designed chemical compound called a TPAC. The BWAs are first treated with the TPAC compound, a process that only takes a few moments, and then exposed to the microwaves. Using this synergistic approach, significant kill of the BWAs is achieved using standard microwave equipment at moderate powers (< 1 MW peak and only a few hundred watts average) and exposure levels (/spl sim/ few joules). This method is so effective and broad ranged that total kill is achieved on vegetative bacilli and spores and vegetative growth anthracis and an unprecedented 5.5 out of a total of 6 logs of kill is achieved on anthrax type spores, the hardest BWA to defeat. To put the anthrax kill rate in perspective, of the approximately one million spores exposed to the microwaves in a given sample only three survived, even though the spores were given every opportunity to grow after RF irradiation. The TPAC compound consists of two components, a transduction-polymer (TP) and an acceptor-chromophore (AC), that work in conjunction to produce BWA defeat. The AC molecule is designed so that it easily penetrates the wall of the BWA and binds to surface matrix targets. Upon microwave exposure, the TP emits a blue photon that activates-the AC producing saturated levels of chemical radicals that are irreversibly bound to the target spore wall, resulting in lethal failure of the spore upon germination. The TP molecule is resonant and thus responds to a given microwave frequency better than others. Its effectiveness also depends upon the rise-time and width of the RF pulse. With optimization of the RF pulse and frequency, total kill of even anthrax spores is expected.