Session CO1 - Z-Pinches (HEDPI).
ORAL session, Monday afternoon, November 15
Cascade I, The Westin Seattle

[CO1.01] Improved dynamics and radiated powers from titanium Z-pinch implosions by employing nested wire arrays

Recent experiments on the 20-MA Z accelerator at Sandia National Laboratories using nested, nickel-clad titanium wire arrays measured titanium K-shell (4800 eV) powers up to 15 TW, a 50measurements. The experiments were designed to study the effects of load diameter and nested wire arrays on Ti K-shell output. Although powers were increased, there was no observed increase in K-shell yield over yields produced by single arrays. Outer wire load diameters varied from 40 mm to 60 mm, with the inner arrays always being half the mass at half the initial diameter of the outer array. The nested wire arrays showed substantial improvements in Ti K-shell power over single Ti wire arrays, with faster risetimes ( 3 vs 5.5 ns) and narrower pulses (4 vs 7 ns). This power increase was observed from 180 to 4800 eV, and corresponded to the production of tighter pinches, i.e. 2 mm diameters versus 3 mm with single arrays. In this paper, comparisons will be made with previous, single array Ti experiments, and with one and two dimensional radiation-magnetohydrodynamic calculations.

[CO1.02] Hydrodynamic Experiments on Z

We have used z-pinch-driven vacuum hohlraums as a radiation source for hydrodynamic experiments. The z-pinch radiates 1.2 MJ of soft x-rays with a peak power of 170 TW with a 5--6 ns FWHM pulse width producing a peak radiation temperature of 145 eV in a 24-mm diameter, 1.0-mm long cylindrical hohlraum. However, many hydrodynamic studies require a relatively constant pressure for several tens of ns. We have developed a hydrodynamic driver with this characteristic by radiatively heating a short Au tube filled with low-density CH foam. The end wall of the tube, backed by a LiF window, develops a nearly constant interface velocity for about 20 ns through a combination of ablation pressure on the Au surface and foam pressure which is enhanced by the radial blow-in of the tube walls. We will present VISAR data of the Au-LiF interface velocity and simulations of the interface velocity for various diameters and radiation temperatures.

[CO1.03] Simulations of the Radiative Environment Produced by a Z-pinch

We describe the results of calculations of the generation and transport of soft x-rays produced by the Z Machine at Sandia National Laboratories. Over 2 MJoules of radiation energy have been produced in the facility, with peak temperatures of around 230-250 eV. Experiments driven by the source can be fielded to study various phenomena in high energy-density regimes. The design and analysis of such experiments require that the x-ray source be well-characterized. The objective of the present effort is to develop a useful (and eventually predictive) model of the radiation source by simulating the z-pinch implosion and comparing the results to data. Two-dimensional Eulerian and Lagrangian codes are used for the calculations. The configuration modeled includes the imploding plasma and a central foam cylinder which is filled with radiation as the plasma stagnates around it. Calculations of the radiation generation and transport within the foam, including the effects of the radiation pre-pulse associated with the run-in of the load plasma are presented.

[CO1.04] Nested Wire Arrays for High-Photon-Energy, Long-Implosion-Time Plasma Radiation Sources

Nested wire arrays have been researched as a means to improve x-radiation performance of imploded annular z-pinch plasma radiation sources (PRS). Current switching when the outer array implodes on the inner may act as a final power-multiplication stage, so that it is desirable for the current to be carried in the outer array for much of driving current pulse. Healing of Rayleigh-Taylor modes developed in the outer array may be expected during merging of the two arrays when the inner-array mass is larger than the outer, and regrowth of the instability during implosion of the merged arrays may be minimized if the radius of the inner array is small compared to that of the outer. Here, the efficacy of such load configurations is studied for high-photon-energy K-shell x-ray production with 300-ns implosions, where the large outer radius of single arrays carry high load-performance risk. For such applications, the performance of nested arrays may be limited by the lower values of available implosion energy per mass required to satisfy the above time and radius constraints.

[CO1.05] Mass and wire number effects of long implosion time Aluminum Z-pinches on Saturn

Aluminum K-shell emissions from long implosion time Z-pinches have been studied on the 7 MA Saturn accelerator. These experiments, motivated in part by the need to develop Z-pinch sources for the DECADE-Quad pulsed power driver, were designed to investigate the effects of wire number and mass on the Al K-shell radiation. The wire arrays were 40 mm in diameter and the wire number was varied from 32 to 282, holding the mass constant. In a separate scan, the load mass was varied from 400 to 2000 \mug/cm, resulting in implosion times of 130 to 180 ns. K-shell yields greater than 60 kJ were measured with pulsewidths as short as 8 ns. These results will be compared with calculations and discussed within the context of K-shell scaling laws. Comparisons will also be made to short implosion time Al experiments performed on Saturn.

*This work is supported by the Defense Threat Reduction Agency and the Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94Al85000.

[CO1.06] Visible Light Observations of Argon Z-Pinch Implosions

We present images from a high speed (21 ns between frames) optical camera for argon gas puff z-pinch implosions on the Double Eagle generator. Views both normal and parallel to the pinch axis have been used. We show data from five different gas flow geometries: a 2.5 cm (diameter) nominal shell, a 5 cm nominal shell, a 7 cm uniform fill, a 10 cm uniform fill, and a double shell. Pinches for the small nozzle implode in 100 ns. The other four nozzles give long implosion times: 160 to 260 ns. The images show interesting structure during the runin phase of the implosions. The thickness of an apparent current sheath increases with time. The time histories of these image features will be correlated with x-ray and other diagnostics of the pinches.

[CO1.07] Design, Modeling and Gas Flow Measurements of a Double Shell Nozzle for Z-Pinch Studies

For long (200 ns) z-pinch implosions, theory suggests that a double shell mass distribution may help mitigate instability growth. We report on the design, numerical gas flow modeling, and interferometric gas flow measurements of a large double shell nozzle. The nominal shell radii are 1.5 and 3.5 cm with shell thicknesses of 1 cm. This nozzle has been used with argon at currents of 4 megamps on the Double Eagle machine; see companion papers by Sze et.al. (for experimental results) and Cochran et.al. (for MHD modeling). Here we discuss nozzle design details, and the correlation of the gas flow data with the gas flow predictions and with x-ray zipper observations.

[CO1.08] Recent Argon Double Shell Experiments on Double Eagle

With the objective of improved x-ray yield and pinch quality for long (200 nanoseconds) implosion time z-pinches, we have tested a double shell nozzle on the Double Eagle generator. Theory and some experiments on GIT-12 suggest that the double shell may help mitigate instability growth during the runin phase of the implosion. For our experiments, argon gas, with a freon (chlorine) tracer in some cases, was imploded using peak currents up to 4 megamps. The nominal shell radii were 1.5 and 3.5 cm. A variety of optical, yield and spectroscopic measurements were made to diagnose the tests. The best radiative output, 12 kJ of K-shell in a 10 ns pulse width, is comparable to that achieved with a 7 cm uniform fill load. We will present data on the pinch’s performance as the ratio of inner to outer masses was varied. (A companion paper, Cochran et. al., discusses MHD modeling of the double shell load.)

[CO1.09] Simulation of Double-Puff Argon Experiments on Double Eagle

Double-puff Argon implosions on the Double Eagle 4 MA current generator have produced promising results in terms of K-shell yield and pulse width (see the companion paper by Sze et al.) Two-dimensional Radiative-Magnetohydrodynamic (RMHD) simulations of these implosions have been carried out to assess the importance of nozzle diameter, puff separation, and inner/outer mass ratio. The results from these simulations give K-shell yields, which agree well with the experiments. The sensitivity to the mass ratio and comparisons with the experimental data will be discussed and detailed radiation properties, including spectra and spatial distribution of emission in the pinhole images, obtained from post-processing the RMHD simulations, will be presented.

[CO1.10] 2D MHD calculations for recent Argon Double Shell Experiments on Double Eagle

2D MHD calculations for Argon double puff z-pinch implosions in the 200ns, 4MA regime are compared with experimental results obtained on DOUBLE EAGLE at MPI. To do the calculations the 2D MHD code DELTA is employed. DELTA operates on a triangular unstructured mesh. To obtain the initial conditions for the implosion a module of DELTA, the NOZZLE code, is used to calculate the gas density distribution in r,z. The NOZZLE code solves the Navier-Stokes equations for the supersonic transient flow in the actual geometry from plenum to exit (see a companion paper by R. Ingermanson et al on this subject). DELTA is then used to follow the implosion dynamics of this initial density profile. A Collisional Radiation Equilibrium Model (CREMIT) is employed to calculate radiation self-consistently. The results of these calculations are compared with the experimentally measured K-shell radiation yield and power, as well as with filtered X-ray pinhole images designed to observe zippering and final pinch radius.

[CO1.11] ALEGRA simulations of x-ray pulses from Sandia's Z accelerator

Results are presented of ALEGRA simulations of the x-ray pulse shape from shot 26 of Sandia's Z machine. ALEGRA is Sandia's multi-dimensional, arbitrary Lagrangian-Eulerian MHD code. Shot 26 produced 180 TW of x-ray power in a 7.5-ns FWHM pulse. This shot was chosen because other MHD codes (MACH II and Darrell Peterson's code from LANL) also have simulated shot 26, thereby providing the opportunity to compare ALEGRA to other codes as well as to data. Discussed in this talk are the effects on x-ray pulse shape of: (i) true void versus plasma fill inside the liner, (ii) differing interface tracking schemes, and (iii) differing levels and models of density perturbations.

[CO1.12] Characterization of the plasma parameters for PBFA-Z type wire array Z-pinches and the implications for radiation MHD.

Detailed simulations of wire-array Z pinches with Radiation MHD codes require calculations which span a broad range of plasma parameters. Predictive capabilities will rely on the applicability of MHD physics to the various plasma regimes. A universal diagram of pinch stability regimes for equilibrium pinches (pinches which satisfy the Bennet condition NkT_e \simeq NkT_i \propto I^2) has previously been presented by Haines and Coppins (M.G. Haines and M. Coppins, Phys. Rev. Lett, 66), 1462 (1991). Here, a similar study of the various plasma regimes accessed by non-equilibrium Z-pinches of the type fielded on the PBFA-Z accelerator at Sandia National Laboratories is presented. Beginning with wire initiation and breakdown and continuing towards stagnation on axis, the operational phase-space (in terms of current I, mass density \rho, charge state Z, and temperature T_e) is outlined. The conditions for ideal and non-ideal MHD dynamics are presented and a discussion of the relevance of the various plasma regimes to MHD simulations and modeling is given.

[CO1.13] Impact of initial energy deposition on exploding wire behavior.

Experiments characterizing the behavior of exploding fine (5-20\,\mum) wires have shown that the wire expansion is determined by a resistive heating phase during the first 30\,ns. The energy deposited during this resistive phase varies substantially with wire material and preparation (cleaning by preheating), and is shown to correlate with observed expansion rates and mass distributions in both the wire core and its plasma corona. This initial energy deposition, measured using current and voltage monitors, terminates with the collapse of voltage as a coronal plasma forms around the wire core. The subsequent wire evolution is observed using x-ray backlighting(T.\ A.\ Shelkovenko et al., Rev. Sci. Instrum., 70), 667; Phys. Plasmas, 6, 2840 (1999)., along with optical schlieren and interferometry diagnostics. Beginning around 100\,ns, the wire core rapidly expands in a complex process involving a succession of phase transitions in the core, which may be superheated during the resistive stage. It is clear that cases with high energy input during the resistive stage produce larger core expansion rates, which may be a primary reason for the improved performance of heated W wire arrays.

[CO1.14] The implosion dynamics of single and nested wire array z-pinches.

Experimental results on wire array z-pinch implosions driven by 1.4 MA, 240ns MAGPIE generator will be presented. The dynamics of the implosions were studied with laser probing, X-ray radiography, and optical and soft x-ray imaging. An imprint of the instabilities (wavelength ~0.5 mm for Al), which first develop in the low density coronal plasma, was detected on the dense wire cores at the time of the array acceleration at ~80backlighting. These perturbations may act as a seed for the development of a global m=0 mode of Rayleigh-Taylor instability (wavelength ~2 mm) seen later. In nested wire arrays two different modes of implosion were detected, determined by the fraction of total current induced in the inner array. Penetration of the outer array through the inner with switching of current to the inner array occurred if current in the inner array was initially suppressed by having a current path of higher inductance. In contrast, with low inner inductance simultaneous implosion of arrays with apparent compression of the magnetic flux between the arrays was observed if ~ 20array. In both cases the x-ray pulse rise-time of ~ 10ns (for 260ns implosion time) was considerably smaller than for a single array.

[CO1.15] Optimization of wire array pre-initiation using a tailored prepulse.

X-radiation performance of imploded wire-array plasma radiation sources is limited by Rayleigh-Taylor growth during the implosion. MHD modeling indicates that nonuniformities in the initially-exploded wires, that are both predicted and observed, are sufficient to seed this growth. These nonuniformities arise largely from energy input to the wire cores that is shunted from the wire cores to the corona before a significant fraction of the wire material has vaporized. This is exacerbated by the presence of easily-vaporized surface contaminants. Preheating the wires has been shown to increase the energy delivered to the wire core before breakdown [1], but is limited in practice by wire breakage. It is hoped that cleaning without breakage can be obtained by pulsing the heating current. Another proposed mitigating technique [2] is the use of a short-duration prepulse to explode the wires, followed by a delay during which the wire material is allowed to expand inertially before significant MHD motion occurs. These approaches are combined in wire initiation experiments using multiple electrical drivers. Diagnostics include electrical and pressure measurements, fast optical measurements, and residual gas analysis. [1] D. Mosher, et al, Bull. APS 43, 1642 [2] R. B. Spielman, personal communication

[CO1.16] Interpretation of Spatially Resolved Z-pinch Spectra in the Presence of Photon Scattering.

In collecting x-ray spectra of Z-pinches, the crystal and slit may be oriented to provide either axial or radial spatial resolution. However, intense spectral lines which originate from collisional excitation from the ground to excited levels in the dense inner core of a pinch may be absorbed and re-emitted (scattered) by a more tenuous outer halo. The weak radiation generated locally within the halo can be overwhelmed by the intensity of the scattered x-rays. Thus, the line intensities emitted from the direction of the halo may not be characteristic of its local conditions but in part reflect those of the denser inner core. We analyze what can be reasonably inferred using data and calculations from an Al:Si Z-pinch driven by the 4 MA Double EAGLE generator at Maxwell Physics International.

[CO1.17] Source Development for High Quality EOS Measurements Using Intense Z-Pinch Radiation

Z-pinches created using the Z accelerator located at Sandia National Laboratories generate 220 TW, 1.7 MJ radiation pulses that can heat large (10 cm^3) hohlraums to 100-150 eV temperatures for times on the order of 10 ns. Because of these relatively high energy and long duration radiation pulses, the Z accelerator has the potential to be an extremely useful tool for performing high pressure EOS expriments; several samples up to 6 mm in diameter can be fielded with multiple diagnostics in a single experiment, enabling a large amount of data to be recorded with built in redundancy. To this end, much effort has gone into conditioning the pressure pulse obtained from Z-pinch radiation in an attempt to obtain a spatially uniform (over a 3-6 mm diameter) and a temporally constant (over 10-20 ns) drive suitable for high quality EOS measurements. Recent progress in this area of source development, with emphasis on results obtained from foam filled secondaries, will be discussed.

[CO1.18] Staged Z-pinch for Controlled Fusion

A staged Z-pinch is considered in which an annular shell made of high Z material implodes onto a coaxial plasma target made of DT-mixture. The target plasma could be made either by exploding a cryogenically extruded fiber or by filling the annular shell with a plasma puff. An axial magnetic field is required to shear stabilize the most dangerous Rayleigh Taylor type instabilities. Modeling is performed with a 2D radiation-MHD code. A parameter study is made to detremine the sensitivity of this configuration to initial conditions of the shell and the target plasma. The axial magnetic field is essential for not only stable implosion but also efficient coupling of energy to the centeral load. Thermonuclear neutron yield is optimized by adjusting the initial strength of axial magnetic field and initial parameters of target plasma. The calculations are based on the parameters of presently operating UCI Z-pinch facility and PBFAZ facility at Sandia . The possibility of using this concept for a holraum type experiment where the X-ray radiation of Z-pinch is coupled to a spherical load is also considered.

[CO1.19] Performance of the Vacuum Spark (VSX) and the Spherical Pinch (SPX) X-Ray/EUV Point Sources

The Company Advanced Laser and Fusion Technology, Inc. (ALFT)has been doing research and development on two plasma sources for several years now. They are the vacuum spark (VSX) and the spherical pinch (SPX) technologies. Both have a long history of previous research to support the contention that they are well qualified for conversion into technological tools for the manufacturing of the next generations of chips. The vacuum spark is a miniature discharge capable of emitting EUV/soft X-ray radiation. Because the radiation is emitted in small dose in each spark, the repetition frequency required to satisfy industrial microlithography application is very high. The SPX is mainly a strong source of EUV radiation, and the repetition frequency required is low, of the order of 1 Hz. The two machines are therefore complementary in their objective of providing point sources for EUV or soft X-ray lithography. A progress report will be presented on both machines.

Part C of program listing