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Investigations of magnetic field diffusion and plasma resistivity in 12-cm-diameter triple-gas-puff Ar Z-pinch implosions were carried out by using planar laser-induced fluorescence (PLIF), a laser shearing interferometer (LSI), and a laser wavefront analyzer (LWA) on a 3.5-MA 200-ns generator. The PLIF measurements gave the initial Ar gas distributions. The implosion velocity and electron density profiles were measured from LWA and/or LSI. From these, the implosion plasma sheath thickness, ion density, mean ion charge states, temperatures, and implosion velocity are obtained, which allows us to calculate the classical plasma resistivity. A 1-D analytic magnetic field diffusion model is constructed and used to predict the imploding plasma sheath thickness and its resistivity. Based on comparisons of the experimental measurements and the diffusion model prediction, we found out that plasma resistivity is enhanced by the cross-field diffusion above the classical value, as high as 60 times the Spitzer's value. Details are given in this paper.

We report an approximate energy distribution (coarse spectrum) over the range ~0.1 to > 10 keV for the X-ray output of an argon Z-pinch. The tests, conducted at ~15-MA peak current, utilized an 8-cm diameter double-shell nozzle. Sze (2001) have previously described the performance of this Z-pinch with emphasis on the K-shell line emission at ~3 keV. Additional measurements that address the non-K-line output of the Z-pinches are presented here. On the one hand, > 4-keV photons due to the free-bound continuum constitute a significant fraction, almost 30%, of the nominal K-shell emission. On the other hand, the sub-3-keV L-shell lines and continuum show distinctive time histories and significant yields. These nonresonance-line emissions are important for a more complete understanding of the implosion physics, for proper analysis of diagnostic responses, and for their effects upon objects exposed to the Z-pinch's radiation

Time-resolved measurements of the absolute free-bound (FB) continuum spectrum emitted from a 12-cm-diameter argon gas-puff Z-pinch driven at ~6-MA peak current with 220- to 260-ns implosion times are reported. A crystal spectrometer is used with silicon diode detectors to provide kiloelectronvolt spectral resolution. The energy and absolute-intensity calibration procedures for the spectrometer are described. The slope of the FB continuum is well represented by a decaying exponential spectrum with a single electron temperature from which spatially averaged, time-resolved (Te(t)), and time-integrated (langTerang) electron temperatures are inferred. An expression for the absolute FB continuum, which takes into account recombination onto bare as well as H-like species, is presented and used to infer time-integrated spatially averaged ion densities langnirang. The values of langTerang and langnirang are in general agreement with the values of these quantities obtained by using the conventional K-shell line-ratio method. Values of Te(t) peak on the rise of the continuum radiation pulse and gradually decrease during the pulse. The fraction of the total energy radiated in the K-shell that resides in the FB continuum is 6%-10%, and this fraction increases with langnirang. Calculated continuum spectra are in agreement with measured spectra

We have developed a 12-cm-diameter Ar gas Z-pinch load, which produces two annular gas shells and a center gas jet. The two-dimensional (2-D) gas density profiles of the load, in r-/spl theta/ and r-z planes, were measured with submillimeter spatial resolutions using the planar-laser-induced fluorescence (PLIF) method, for conditions used in Z-pinch experiments. Due to interactions between the shells, the net gas density profile differs from the superposition of the individual shell profiles. Narrow density peaks are observed both at smaller and larger radii than the radius where the shells come in contact with each other. Two-dimensional flow velocity distributions are determined from the displacements between the fluorescence and later time phosphorescence images. The measured stream velocities of argon gas puffs are 650 /spl plusmn/ 20 m/s, higher than the ideal gas velocity due to the formation of clusters in the supersonic gas flow. Indeed, clusters were observed in earlier Rayleigh scattering experiments. The gas measurements of the initial phase using the PLIF will be combined with other density measurements of the implosion and pinch phases to better understand the implosion dynamics and to provide initial conditions for simulation codes.

Recently, an 8 cm diameter double-shell nozzle has produced argon Z pinches with high K-shell yields with implosion time of 210 ns. To produce even longer implosion time Z pinches for facilities such as Decade Quad [D. Price, et al., “Electrical and Mechanical Design of the Decade Quad in PRS Mode,” in Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie (IEEE, New York, 1999), p. 489] (9 MA short circuit current at 300 ns), a larger nozzle (12 cm outer diameter) was designed and fabricated. During initial testing on Double-EAGLE [P. Sincerny et al., Proceedings of the 5th IEEE Pulsed Power Conference, Arlington, VA, edited by M. F. Rose and P. J. Turchi (IEEE, New York, 1985), p. 151], 9 kJ of argon K-shell radiation in a 6 ns full width at half maximum pulse was produced with a 240 ns implosion. The initial gas distributions produced by various nozzle configurations have been measured and their impact on the final radiative characteristics of the pinch...

Theoretical studies have predicted that the disruptive role of the Rayleigh-Taylor (R-T) instability on the current conduction and implosion characteristics of annular Z-pinch loads will be mitigated by mass accretion if uniform fill or multiple annular shell loads are used. Holographic interferometry was used to study these physical processes during the implosion phase of puff-on-puff loads on a terawatt accelerator. Both axial (r-z) density perturbation and azimuthal (r-/spl theta/) filamentation modes of the R-T instability were observed. Significant ionization (Z/spl ap/3-10) of the inner gas puff atoms was observed below the anode grid before the outer puff had imploded to this radial position. Radiation hydrodynamic calculations indicate that photoionization by radiation from the outer current carrying shell could not account for this ionization. Current flowing on the inner gas puff could be the source of this ionization. The effect of these physical processes on the radiation yield from z-pinches warrants further investigation.

The Doppler broadening of ion line profiles emitted by z-pinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurements of these line profiles are often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optically thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. The measurements indicate that lower ion temperatures (3–5 keV) are observed for Al wire loads on both the Saturn and Double EAGLE accelerators in the short current pulse mode (60–100 ns) than in the long pulse mode (125–225 ns) where values of 6.3–9.5 keV are observed. These values are smaller than those observed on Saturn by others. Furthermore, the wavelength at the line center of axially resolved ion line profiles on the DM-2 accelerator at ...

High-sensitivity interferometry measurements of initial density distributions are reviewed for a wide range of gas-puff nozzles used in plasma radiation source (PRS) z-pinch experiments. Accurate gas distributions are required for determining experimental load parameters, modeling implosion dynamics, understanding the radiation properties of the stagnated pinch, and for predicting PRS performance in future experiments. For a number of these nozzles, a simple ballistic-gas-flow model (BFM) has been used to provide good physics-based analytic fits to the measured r, z density distributions. These BFM fits provide a convenient means to smoothly interpolate radial density distributions between discrete axial measurement locations for finer-zoned two-dimensional MHD calculations, and can be used to determine how changes in nozzle parameters and load geometry might alter implosion dynamics and radiation performance. These measurement and analysis techniques are demonstrated for a nested-shell nozzle used in Double Eagle and Saturn experiments. For this nozzle, the analysis suggests load modifications that may increase the K-shell yield.

During the last few years, we have concentrated on developing plasma radiation sources (PRS) for the longer current rise time (up to 300 ns) of the direct-drive Decade Quad (DQ; Price et al., 1999). In particular, we have developed an argon gas-puff PRS that produces a high quality pinch with an ∼200-ns implosion time, twice the implosion time of typical z pinches. We have shown that large-diameter, solid-fill (i.e., the gas density is a smooth function of the radial distance rather than a discrete shell) gas puffs can achieve adequate heating and stability for long-implosion z pinches with good K-shell yield.

Solid-fill nozzles for long-implosion Z-pinch experiments to produce argon K-shell x rays (photon energy >3.1 keV) have been developed. With a 7 cm diam nozzle, which is appropriate for a 200 ns driver, stable implosions at 180 ns and 4 MA have produced peak argon K-shell yields exceeding 15 kJ. As previously seen with short (∼100 ns) implosion times, the K-shell yield scales as the fourth power of peak current, I4. Limited testing with a 10 cm nozzle, which is appropriate for a >250 ns driver, has also achieved a stable implosion.

We carry out an energy-inventory analysis for a 12-cm-diameter, argon gas-puff shot on the Saturn generator. From the measured pinch inductance time history, we infer the average radius of the pinch current, r ind , and the total work done on the pinch, E coupled , both as functions of time. The kinetic energy as a function of time, E KE , is inferred from r ind and the measured initial mass distribution, assuming all of the mass is accreted as in a snowplow. The results show that early in the implosion, E coupled ≈ E KE . However, when the pinch begins to radiate significantly, E KE begins to decrease while E coupled continues to increase. The increase in coupled energy goes into plasma internal energy, E int , i.e., ionization and thermal energies of the z-pinch plasma, and total radiation, E rad . During the time of K-shell emission, the significant (∼ 150-kJ) increase in E coupled is reflected in approximately the same increase in E rad . There is agreement between E int at the time of peak K-shell emission inferred from E coupled and measurements of E rad , and E int inferred from independent spectroscopic measurements combined with an atomic-physics model.

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.

Summary form only given. Characterization of implosion kinematics is of interest in understanding and optimizing plasma radiation source (PRS) K-shell yield. It also serves to provide input parameters to the modeling of PRS pinch conditions. A measure of the sensitivity of the PRS implosion kinematics to changes in initial conditions is the time and axial dependence of the Z-pinch assembly on-axis and its associated K-shell yield. A relatively simple and routine PRS diagnostic is the so-called zipper array, which can characterize both the time and axial dependence of the pinch assembly (zippering) and the pinch x-ray yield. The zipper array is essentially a one-dimensional (1D), time-resolved imager, typically consisting of a linear array of X-ray power sensors (photo-conductive detectors: silicon diodes or diamonds), viewing the pinch through a transverse slit, with appropriate pre-detector filtering selecting the X-ray detection range of interest. We report here a study on the time dependent axial characterization of the pinch assembly and the K-shell yield for argon PRS shots taken with two different current drives, differing both in magnitude and waveform shape, and with implosion times in excess of 200 ns. The first current driver reached a peak current plateau of about 3.5 MA in 100 ns, which persisted at nearly constant current for an additional ~100 ns. The second driver delivered a nominally linear current ramp exceeding 6 MA in 300 ns. The study characterizes the final pinch assembly under both fixed and varied initial conditions. For nearly identical initial conditions, the data generally show a significant degree of reproducibility in the relative zipper timing, but with some variation in the K-shell yield at a given axial position. Changes in initial conditions can have a significant effect. For example, shots without pre-ionization changed the axial time dependence of final pinch assembly and axial distribution of K-shell yield.

Summary form only given. The ability to control and vary the radial profile of the initial mass distribution for Z-pinch implosion is critical to both increase the K-shell X-ray yield and to elucidate the physics that determine pinch quality and yield. Toward that end, we have developed a triple-plenum gas valve that feeds a nozzle consisting of 2 concentric shells and an axial jet. Each plenum is independent in terms of gas pressure and constituents, affording the required flexibility. We will report on the results of experiments performed with this nozzle on several different pulse power generators covering a range of peak currents and implosion times

Summary form only given. Absolute measurement of the free-bound (FB) continuum spectrum above the K-shell from atomic-number Z ges 10 Z-pinches can be a useful diagnostic tool to obtain the electron temperature and the ion density of a hot, radiating Z-pinch. These measurements are also of interest for benchmarking continuum radiation models associated with the use of this high-photon-energy radiation for radiation-matter interaction studies. In previous work, we described a curved crystal spectrometer modified to measure, absolutely, the argon FB spectrum from 5 to 10 keV emanating from the implosion of a 12-cm diam, shell-on-shell, argon-gas-puff Z pinch, driven by peak currents of ~6 MA with implosion times of 220-260 ns on the decade quad. Time averaged and time resolved electron temperatures were determined from the slope of the FB spectrum and the energy in the FB spectrum was compared with the total K-shell yield. Now, we review the diagnostic and discuss an improved absolute calibration. The new calibration results in ~10% of the total K-shell yield residing in the continuum above 4.4 keV, about a factor-of-three lower than previously reported. We present new calculations that relate the ion density, electron temperature and absolute continuum emission that account for radiative recombination on both bare and H-like ions. Using radial and axial scale lengths obtained from time-integrated pinhole-camera images, this expression is used to infer the time-averaged density of ions radiating in the FB continuum, which is then compared with ion densities determined using the line-ratio technique. Finally, these measurements are compared with results from a CRE-based model that includes radiation transport

There is strong interest in many laboratories worldwide in utilizing less expensive, longer rise‐time (> 200 ns) pulsed power to drive x‐ray producing z‐pinches. Based on the idea of a magnetically‐driven annular implosion, the emission of K‐shell photons requires high energy per ion (implosion velocity above 43 cm/μs for argon) to strip the atoms to the helium‐like and hydrogen‐like states. This high velocity must be combined with high density in the final hot plasma to produce significant x‐ray yield. To first order, implosion velocity correlates with the initial diameter of the z‐pinch load in proportion to the implosion time. Thus some effort has been made in the last few years to develop larger diameter z‐pinch loads suitable for use with the longer rise‐time drivers. Advancing from the

Summary form only given. While the characteristic "resonance" lines dominate the K-shell X-ray emission from Z pinches, the energy in the K continuum (free-bound and free-free transitions) can be significant. A simple model implies that the continuum could supply well over 10% of the K-shell yield if the plasma is warm enough and dense enough. We present here new measurements of the continuum strength for argon implosions. The data were taken at peak currents of 3.5 MA with implosion times exceeding 200 ns. Heavily filtered GaAs photoconductive detectors (PCD) were used as direct measures of the argon continuum above ~4 keV. Other major diagnostics included X-ray power sensors, calorimeters, time-integrated CCD spectra and CCD pinhole images. By combining the data from all of these instruments, we have derived the strength and temperature of the continuum as well as the yield in the K-lines using a self-consistent method. As implosion conditions changed, the changes seen in electron temperature (derived from K-line ratios) were not as large as the changes seen in the continuum temperature (derived from the PCDs). The yield in the continuum was close to a linear function of the K yield. The relative strength of the continuum does appear to depend on the detailed mass distributions produced by the 12 cm diameter nozzles used for these tests

Summary form only given. Experiments have been performed at the Z accelerator to study the neutron production from a deuterium gas puff. The current scaling of the neutron production was of primary interest, so shots were fielded at approximately 12 MA, and 16 MA. The shots at both current levels were fielded with similar implosion times. The gas puff nozzle used was a nested shell configuration, with the inner shell extending from 3 to 2 cm, and the outer shell from 3 to 4 cm; the load length was 2 cm. Dopant gases were added to the deuterium (argon and chlorine) to track the implosion characteristics of the gas through X-ray yield and spectroscopy measurements. Indium detectors, as well as lead and beryllium probes were fielded. Neutron time-of-flight measurements were also obtained. Neutron outputs near 1e13 were measured for the 12 MA shot, and greater than 1e13 was observed for the 16 MA shot. These results will be compared with computational predictions and previous neutron measurements from Z-pinch sources

Summary form only given. Large radius implosions of gas-puff Z-pinches are the subject of intense investigation. Such implosions are needed to achieve the high specific energy required to excite K-line radiation from high-atomic-number (e.g. Z>26) radiators, for proper matching with high-current (~10 MA and higher) generators, for continuum radiator concepts that require heating beyond the optimum He/H-like state for K-shell emission, and to utilize more efficiently long-current-rise-time (>100 ns) generators. The key physics issues involve R-T instability mitigation and energy coupling to the radiating plasma by the choice of the initial radial and axial mass profiles. In this talk, recent experimental and theoretical progress in this work are reviewed, including achieving 20-kJ of argon K-shell radiation from a 12-cm-diam. nozzle at 3.5 MA and 200-ns implosion time. The primary experimental testbeds are double-EAGLE (long-pulse mode, 3.5 MA, 210 ns) at TPSD and the Decade Quad (6 MA, 300 ns) at Arnold Engineering Development Center. Supporting experiments on Hawk (0.6 MA, 300 ns) at NRL are also reviewed. Besides the usual soft X-ray and electrical diagnostics, we have developed methods for measuring: the initial gas profile, the L-shell emitting region, the UV and continuum, the initial current flow paths, and the 2-D evolution of the pinch. Recent developments in 2-D radiation-hydrodynamic simulations are also reviewed

Summary form only given. Recent 2-D MHD simulations of large diameter argon gas-puff Z-pinch loads using PLIF (planar laser induced fluorescence) initial density profiles have shown that the radiative emission and absorption characteristics of the plasmas are critically affected by the formation and evolution of nonuniform structures and inhomogeneities. In drive toward the design of large diameter loads having an optimum PRS performance, therefore, it is important to understand the complex nonlinear interaction between these multidimensional structures and the ambient radiation, and the resulting effects on the nonlocal radiative absorption and energy transport processes and the equation of state of the plasma-radiation ensemble. In this study, we will investigate their interaction and effects through a detailed postprocess analysis of the plasmas during the various stages of the implosion process using the AXSTRAN 2D non-LTE radiation ionization dynamics code in conjunction with the SPECAM 3D non-LTE spectra and image synthesizer code. In particular, our analysis will focus on the temporal and spatial evolution of the electron temperature and ion density, and the K- and L-shell radiative power densities. Identification and detailed radiative characterization of the plasma structures are accomplished via a detailed radiative emission and ionization state analysis

Summary form only given. Nozzles for high current, short implosion time (

Summary form only given. Because the energy coupling and the X-ray output of Z-pinch plasma radiation sources are determined by the implosion history, measurements from the initial gas phase, through the MHD implosion phase, to the final pinch phase are needed. Many diagnostics have been developed for the final pinch phase. We have developed a planar laser induced fluorescence (PLIF) technique to characterize the gas puff in the initial phase and demonstrated a laser shearing interferometer (LSI) approach for the implosion and pinch phases on a high current accelerator. These laser based instruments have the advantages of being user-friendly and giving the data quickly. Using the PLIF method, the 2-D gas density and flow velocity profiles of the loads were obtained with sub-mm spatial resolutions for the conditions used in the Z-pinch experiments. From the LSI, the shape of the imploding plasma was determined and the wavelengths of the Rayleigh-Taylor instability were measured. We have also found that the pinch plasma sizes are about twice that measured from the K-shell X-ray pinhole images. The combination of the PLIF and (LSI) measurements enable us to better understand the implosion dynamics and to provide important information needed for simulation codes

Summary form only given. The technique of Planar Laser Induced Fluorescence (PLIF) has been applied to characterize the initial gas distributions produced by 12 cm diameter nozzles, as used on recent Decade Quad and Double-EAGLE Ar Z-pinch experiments. This information is required to understand and simulate the implosion dynamics. A 266 nm, /spl sim/5 ns, /spl sim/5 cm line-focused laser beam passes through the Ar gas puff, which is mixed with a few percent acetone by volume. A gated intensified CCD camera views the fluorescence of the excited acetone molecules from the side. The acetone emission has a short, intense pulse, coincident with the exciting laser pulse, and a weak, slowly decaying phosphorescence. The gas density profiles are obtained from the fluorescence images recorded coincident with the laser. Flow velocities of the gas puff are obtained by capturing the images of the phosphorescence at several delay times relative to the laser pulse. The distribution of the clusters is determined by comparing the PLIF signal with 2-D Rayleigh scattering from the gas puff. These measurements are compared with computer simulations to benchmark and refine the 2-D fluid dynamics models.

Summary form only given, as follows. The design of gas flow nozzles for optimum K-shell X-ray emission remains an imperfect science. We report on a limited set of tests, using the Double Eagle simulator, evaluating two key factors. The first factor was the detailed form of the nozzle contours: simple linear contours versus curved "aerodynamic" contours intended to minimize turbulence. The second factor was the relative radial concentration of mass in the flow. All tests employed double shell nozzles. While the test set was incomplete, the data strongly imply that details of nozzle shape are relatively unimportant. On the other hand, radial mass distribution is very critical. With twice the mass (ug/cm) in the outer shell than the inner shell, argon K-shell yield was only 5 kJ. With nearly equal masses in the two shells, yield increased dramatically to over 17 kJ, matching the optimum output on Double Eagle for 200 ns implosions. Since the simplest models of z-pinches favor putting most of the mass at the largest radius, these results strongly imply that that configuration is subject to severe disruption by instabilities during the implosion. These results confirm earlier observations that filled-in mass distributions are preferred, and support the concept of snowplow stabilization.

Summary form only given, as follows. An argon double-shell gas puff source was fielded on Decade Quad, which was configured in the PRS mode with the four modules combined into a single pulse power driver. In a series of six shots, four of which were taken at reduced Marx charge (70 kV vs. 85 kV full charge), we demonstrated 29 kJ of K-shell radiation in an 11 ns FWHM radiation pulse at 4.6 MA peak current. Current scaling from a 4.1 MA, 18 kJ test was 13.6. Plasma parameters were measured with an x-ray pinhole camera and a spectrometer, both recorded with CCD cameras. The electron temperature was 1600 eV and the ion density 1.5/spl times/10/sup 19/ cm/sup -3/. The K-shell emitting diameter was 3.1 nm. Four B-dot probes in a load anode ring measured the current; the yield was measured with a set of three tantalum calorimeters. The four current probes agreed to within 3%, the calorimeters to within 2%. Although there is an insufficient database to be precise; the optimal implosion time was approximately 210 ns; the shot described above had an implosion time of 214 ns. This reflects the fact that the nozzle was designed for 200 ns implosions (/spl eta/=2) on Double-EAGLE and thus did not tap the full potential of Decade Quad, which has a 300 ns quarter period (which would produce /spl eta/=0.7 with the present nozzle). In this paper, we will discuss the design and detailed results of the experiment.

Summary form only given. A 1-2-3-4 cm double-shell nozzle was designed and built in 1999 to study long implosion time gas puff Z-pinches. A test program using the same nozzle on all major US accelerators including Double-EAGLE, Saturn, Decade Quad and Z has recently been completed. In this talk, we will present the performance characteristics of this nozzle. Total yield, K-shell yield, power, pulse-width, implosion times will be presented. Pinch characteristics including size and uniformity will be discussed. Pinch ion densities and electron temperatures will be analyzed. Two different techniques to measure the initial gas density profile will be discussed. Optimization experiments to enhance K-shell radiation will be reviewed. Results of a novel spectroscopic technique to trace the source of the radiation will also be illustrated. The yield performance will also be compared to calculations. This comparison is interesting because all test results on Double-EAGLE, Saturn, Decade Quad and Z use the same nozzle with same ratio of inner to outer shell masses, shell geometries, pre-ionization and pinch length. Finally, since the double-shell gas puff allows implosion of two distinct, not mixed, gases with different atomic numbers, results from preliminary experiments with argon and krypton will also be presented.

Summary form only given. Previous research on the high-power generators DM2, at Maxwell Physics International, and Saturn, at Sandia National Laboratories, has demonstrated that long-implosion (200-300 ns) PRS sources, both gas puff and wire load, can be sufficiently stable to produce well-assembled pinches and high-power radiation pulses of soft X-rays (1-4 keV). This has substantial implications for reducing the cost and risk of future higher current facilities (e.g. Decade Quad). To extend our knowledge and understanding of long-implosion sources, we are conducting a well-diagnosed series of experiments on long-pulse Double-EAGLE (4 MA at 200 ns). Results from Ar gas puffs, with various nozzles and valves, and Al wire arrays will be presented. Where possible, comparison with short-pulse results will be discussed.

We have created large diameter, long implosion time, PRS loads that produce a stable pinch and a narrow X-ray pulse. Operation scaled successfully with current from 2 MA on DM2 to 6 MA on Saturn. The K-shell yields are comparable to those produced by short-pulse operation at a similar current. This approach, therefore, allows substantial savings to be realized in the pulsed-power driver cost, complexity and risk.

DECADE Module 2 (DM2) implodes argon gas-puffs in 200-300 ns from initial radii of 2.5-5 cm to generate K-line radiation. An important issue for these experiments is the lower-than-expected K-line radiation associated with such large radius implosions. Preionization may improve the implosion quality by improving the symmetry of the initial current flow. A preionizer is described based on photoionization by ultraviolet (UV) illumination from a flashover source. The preionization is diagnosed using a high-sensitivity interferometer. This UV source produces 1-10% ionization of the outer periphery of the argon gas distribution prior to the arrival of the flashboard plasma. DM2 experiments indicate increased, more reproducible K-shell yields when this preionization system is incorporated.

Summary form only given. Efficient production of soft X-rays (1 to 5 keV) has been achieved in many laboratories using Z-pinch plasma radiation sources (PRS) driven by short pulses of mega-ampere currents. Empirical and theoretical scaling laws for 8 MA with a rise time of 300 ns, coupling >400 kJ to kinetic energy in the Z-pinch.

Summary form only given, as follows. We are continuing the development of a low density plasma opening switch (POS) on the Double-EAGLE pulsed power generator, running in a 250 ns mode. The goal is to shorten the pulse and efficiently drive a=100 ns imploding PRS source (an aluminum wire array). This will provide a proof-of-principle demonstration of an alternative approach to long implosion PRS sources that are being developed for Decade Quad and the next generation of pulsed power drivers. In our previous experiment, the upper and lower current paths of Double-EAGLE strip-plate geometry were connected upstream of the POS to test the operation of a single-sided low density switch at linear current densities required for Decade Quad. Our present efforts involve using the lower inductance geometry of a double-sided POS and a post-hole convolute connecting the current paths downstream of the POS, driving either an inductive shun or an aluminum wire array. These tests will attempt to demonstrate high opened POS impedance and minimum electron sheath flow ("launched electron flow") downstream of the POS. We will discuss the interactions of the two sides of the POS and of the POS with the loads. The implications for Decade Quad will also be addressed.

Summary form only given, as follows. In Z-pinch implosions, simple dynamic arguments show that for fixed initial radius of mass and fixed implosion time, imploded mass varies as I/sup 2/ and yield should scale as I/sup 4/ (inefficient regime). As the radiated yield becomes comparable to the available implosion energy, energy conservation limits the yield. Then yield is expected to increase as the mass or as 1/sup 2/ (efficient regime). We present observations of a series of argon gas-puff Z-pinch experiments to test this simple scaling model from 3 to >15 MA. We begin with recent results of an argon Z-pinch experiment with >15 MA in >110 ns on the "Z" generator. Starting from a diameter of over 8-cm, a double-shell Z-pinch implodes to

Summary form only given, as follows. We report on the first gas puff z-pinches imploded on the Sandia "Z" accelerator. An 8 cm (outer diameter) argon double-shell load was driven with over 15 MA in 112 ns to produce 274124 kJ of K-shell X-rays. This yield is a factor of >6 times larger than the previously observed maximum for argon and is consistent with the expected current-squared scaling for "efficient" radiation. With this result, we can establish the empirical scaling of argon K-shell yield with current over the range from 2 to 15 MA and test modeling concepts over a wide range of implosion conditions. Final diameter of the K-shell emitting column was under 5 mm but significant axial variations suggest that further improvements in the gas flow geometry could be made. Spectra show that the main argon emission lines have very high opacity. While the peak inferred electron temperatures, approximately 2.0 keV, are a little higher than seen in lower current tests, the most significant difference is in the ion densities which are up to 5 times higher. The selective use of a chlorine tracer in one of the shells on each test indicates that the masses in the two shells do not mix or interpenetrate significantly during the implosion. Data from a variety of diagnostics spectrographs, pinhole cameras, PCDs and XRDs will be presented.

Summary form only given. Gas distributions as functions of time, radius and distance from shell-on-shell nozzles used in MPI Double Eagle and SNL Saturn argon gas-puff experiments have been measured using high-sensitivity interferometry. Snow-plow implosions in ballistic-gas-flow-model (BM) fits to the measured density distributions and a two-level K-shell X-radiation model are used to analyze these experiments, and predict the performance of upcoming krypton shell-on-shell experiments on Z. Results demonstrate the utility of the models in the design of optimized gas-puff loads on these DTRA and DoE Z-pinch drivers.

We are now using a double shell gas nozzle as a Z-pinch X-ray source for long duration (/spl sim/200 ns) implosions. While the primary purpose of this mass configuration is to mitigate instability growth, it also has the benefit that the atomic composition of each shell can be unique. We report here on the use of a chlorine tracer in a predominately argon pinch. With the chlorine present only in the inner shell, chlorine K-shell line emission is readily seen from the core of the pinch. With the chlorine present only in the outer shell, the chlorine emission is greatly reduced. This result implies that there is relatively little mixing between the two shells. We discuss details of these observations as a function of position in (and time during) the implosion and correlate the results with data on the spatial structure of the gas shells.

Summary form only given. Long implosion time Plasma Radiation Source (PRS) z-pinch experiments have been conducted recently on Double EAGLE and Saturn using shell-on-shell gas puff loads. The shell-on-shell geometry is predicted to mitigate instability growth especially for long (>150 ns) implosion times. We find that the K-shell performance of the shell-on-shell mass distribution equals the best long pulse results generated to date with either single shell or uniform-fill gas loads. We report on recent experiments to optimize the shell-on-shell for both the pinch length and the ratio of the masses in the two shells.

The K-shell yield performance of Titan double-shell argon gas puff loads imploded on the Z machine and Decade Quad (DQ) is compared to the performance of Double-Eagle loads employing the same nozzle configuration. Specifically, the K-shell yields obtained on the Z and DQ machines are compared with I4, I2, and I0 current scaling projections made from Double-Eagle yields, where I is the peak load current. This analysis allows initialization and multidimensional effects to be factored out of the scaling behavior. This projection analysis is useful for evaluating argon gas-puff load performance because it relates I4, I2, and I0 K-shell yield scaling transitions that a load, of a given specific energy, is predicted to undergo as the total load mass increases. Our analysis shows that the maximum K-shell yields were near optimum for the nozzle configuration used and the energy coupled to the load in the Z and Decade Quad experiments. At present it is not understood why an additional 22 nH of inductance for DQ, 6 nH for the Z machine, and 8 nH for Double-Eagle is required in their respective equivalent circuit models to reasonably match measured peak currents and implosion times. Note, we do not think that there is extra inductance in the real circuit; it is included in the modeled equivalent circuit as a method to account for the possible anomalous effects that are reducing the current from its expected value. These effects may include: multidimensional effects, current losses, and anomalous resistivity.

The basic concept of a z-pinch, that JxB forces implode a shell of mass, creating a hot dense plasma on-axis, is coming under closer scrutiny. Wire arrays may start with an initial cold mass in a near “ideal” shell, but in fact they appear to develop complex radial mass distributions well before the final x-ray output [1,2]. We consider here the situation for gas puff z-pinches. While the ideal of a gas “shell” has been the nominal objective for many years, detailed measurements of gas flow show that nozzles used for plasma radiation sources (PRS) also have complex radial distributions. In particular, there are significant data [3] showing that the best x-ray yield comes from the least shell-like distributions. Recent experiments on the Double Eagle generator with argon have further enhanced this view [4]. For those tests with a double “shell” nozzle, there was a factor of almost 4 increase in yield when the relative mass (outer:inner) in the two shells was changed from 2∶1 to less than 1∶1. We suggest the following explanation. A configuration with most of its mass at large radii is subject to severe disruption by instabilities during the implosion. A more continuous radial mass distribution with dp/dr < 0 may mitigate instability development (via the “snowplow stabilization” [5] mechanism) and thus enhance the thermalization of the kinetic energy of the imploding mass. In addition, the appropriate balance of outer to inner mass maximizes the formation of a strong shock in the core of the pinch that heats the plasma and leads to x-ray emission.

An experimental study of high current (3–15 MA), high fidelity (multiple atomic number) and long implosion time (100–200 ns) gas puff loads using the 1–2–3–4 cm double-shell gas puff is in progress at Titan/PSD. Results of experiments conducted on Double-EAGLE, Saturn, Decade Quad and the Z accelerators will be analyzed and presented. The principal observations are: (1) The overall pinch quality and radiative characteristics of all the argon double shell z-pinches are quite satisfactory. The Ar K-shell yields varies from the expected I4 scaling in the inefficient regime for 3 to 7 MA to I2 scaling in the efficient regime from 7 to 15 MA. (2) On all experiments from 3–15 MA, selective seeding of the shells demonstrates that the hottest mass of the pinch originates from the inner shell. This suggests that mixing between the two plasma shells during their collision and final implosion is limited. (3) On the 15 MA Sandia Z accelerator, with a load mass of 0.8 mg/cm, the K-shell x-ray output reached 275 kJ in a 15 TW peak power, 12 ns pulse. The analyzed ion and electron densities reach 5 × 1019 and 1.0 × 1021 /cc and the highest electron temperature observed is up to 2.2 keV with a 2.0 keV continuum

Summary form only given, as follows. We have continued our development and testing of uniform-fill nozzles for long-implosion time PRS sources on the 4 MA Double-EAGLE generator at Maxwell Physics International. By adjustment of switch gaps in the waterline, the nominal 100 ns electrical pulse of Double-EAGLE can be extended to 200 or 250 ns with little decrease in peak current. We have used these modes to test 7 and 10 cm diameter uniform fill gas puffs in well diagnosed experiments. The 7 cm nozzle generated a K-shell yield in excess of 15 kJ in a 16 ns FWHM pulse at 188 ns implosion time and demonstrated I/sup 4/ scaling to the testing limit of 4 MA. The final implosion diameter was about 2 mm. The compression ratio was therefore better than that achieved in short pulse operation (2.5 cm initial shell diameter to 1 mm final pinch diameter). We will discuss these Z-pinch results, the 10 cm nozzle results, and features of the gas valve and nozzles employed to produce them.

Recently, a new approach for efficiently generating K-shell x-rays in large-diameter, long-implosion time, structured argon gas Z-pinches has been demonstrated based on a “pusher-stabilizer-radiator” model. In this paper, direct observations of the Rayleigh–Taylor instability mitigation of a 12-cm diameter, 200-ns implosion time argon Z-pinch using a laser shearing interferometer (LSI) and a laser wavefront analyzer (LWA) are presented. Using a zero-dimensional snowplow model, the imploding plasma trajectories are calculated with the driver current waveforms and the initial mass distributions measured using the planar laser induced fluorescence method. From the LSI and LWA images, the plasma density and trajectory during the implosion are measured. The measured trajectory agrees with the snowplow calculations. The suppression of hydromagnetic instabilities in the “pusher-stabilizer-radiator” structured loads, leading to a high-compression ratio, high-yield Z-pinch, is discussed. For comparison, the LSI an...

In this paper, a theoretical model is described and demonstrated that serves as a useful tool for understanding K-shell radiating Z-pinch plasma behavior. Such understanding requires a self-consistent solution to the complete nonlocal thermodynamic equilibrium kinetics and radiation transport in order to realistically model opacity effects and the high-temperature state of the plasma. For this purpose, we have incorporated into the MACH2 two-dimensional magnetohydrodynamic (MHD) code [R. E. Peterkin et al., J. Comput. Phys. 140, 148 (1998)] an equation of state, called the tabular collisional radiative equilibrium (TCRE) model [J. W. Thornhill et al., Phys. Plasmas 8, 3480 (2001)], that provides reasonable approximations to the plasma’s opacity state. MACH2 with TCRE is applied toward analyzing the multidimensional implosion behavior that occurred in Decade Quad (DQ) [D. Price et al., Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie (IEEE, New York, ...

Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34MeV (±0.10MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility.

Large radius Z-pinches are inherently susceptible to the magnetic Rayleigh-Taylor (RT) instability because of their relatively long acceleration path. This has been reflected in a significant reduction of the argon K-shell yield as was observed when the diameter of the load was increased from 2.5to>4cm. Recently, an approach was demonstrated to overcome the challenge with a structured gas puff load that mitigates the RT instability, enhances the energy coupling, and leads to a high compression, high yield Z-pinch. The novel load consists of a “pusher,” outer region plasma that carries the current and couples energy from the driver, a “stabilizer,” inner region plasma that mitigates the RT growth, and a “radiator,” high-density center jet plasma that is heated and compressed to radiate. In 3.5-MA, 200-ns, 12-cm initial diameter implosions, the Ar K-shell yield has increased by a factor of 2, to 21kJ, matching the yields obtained on the same accelerator with 100-ns, 2.5-cm-diam implosions. Further tests of such structured Ar gas load on ∼6MA, 200-ns accelerators have achieved >80kJ. From laser diagnostics and measurements of the K-shell and extreme ultraviolet emission, initial gas distribution and implosion trajectories were obtained, illustrating the RT suppression and stabilization of the imploding plasma, and identifying the radiation source region in a structured gas puff load. Magnetohydrodynamic simulations, started from actual initial density profiles, reproduce many features of the measurements both qualitatively and quantitatively.

Experiments on the Z accelerator with deuterium gas puff implosions have produced up to 3.9×1013(±20%) neutrons at 2.34 MeV (±0.10MeV). Experimentally, the mechanism for generating these neutrons has not been definitively identified through isotropy measurements, but activation diagnostics suggest multiple mechanisms may be responsible. One-, two-, and three-dimensional magnetohydrodynamic (MHD) calculations have indicated that thermonuclear outputs from Z could be expected to be in the (0.3–1.0)×1014 range. X-ray diagnostics of plasma conditions, fielded to look at dopant materials in the deuterium, have shown that the stagnated deuterium plasma achieved electron temperatures of 2.2keV and ion densities of 2×1020cm−3, in agreement with the MHD calculations.

A large diameter gas puff nozzle, designed to produce a radial mass profile with a substantial fraction of the injected mass on the axis, has demonstrated an increase in K shell yield by nearly a factor of 2, to 21kJ, in an argon Z pinch at 3.5MA peak current and 205ns implosion time [H. Sze, J. Banister, B. H. Failor, J. S. Levine, N. Qi, A. L. Velikovich, J. Davis, D. Lojewski, and P. Sincerny, Phys. Rev. Lett. 95, 105001 (2005)] and 80kJ at 6MA and 227ns implosion time. The initial gas distribution produced by this nozzle has been determined and related to measured plasma dynamics during the implosion run-in phase. The role of two gas shells and the center jet are elucidated by the inclusion of a tracer element sequentially into each of the three independent plenums and by evacuating each plenum. The implosion dynamics and radiative characteristics of the Z pinches are presented.