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Density maps of the r-θ plane are presented for two basic types of loads with the same total mass and overall dimensions: wire arrays and cylindrical liners. The wire-array load shows complex interactions of the ablation streams, leading to a well-developed precursor plasma on the axis. Some of the liner loads were near-perfect cylinders, whereas other liners had small perturbations to the cylindrical shape. The near-perfect liners had azimuthally uniform and reduced ablation on the inner surface compared to the wire array. This leads to a plasma prefill that was approximately an order of magnitude less dense than that produced by equivalent wire arrays. The perturbed liners had enhanced ablation near the perturbations that destroyed the azimuthal symmetry of the plasma prefill. The plots of azimuthally averaged mass density and ablated-mass-to-load-mass ratio versus radius are presented, as well as comparisons to the rocket model.

The generation of accelerated electrons in the X-pinch minidiode is studied experimentally. It is well known that the explosion of an X-pinch consisting of two or more wires is accompanied by the formation of a minidiode, in which electrons are accelerated. The subsequent slowing down of electrons in the products of wire explosion causes the generation of hard X-ray (HXR) emission with photon energies higher than 10 keV. In this work, the spatial and temporal characteristics of X-pinch HXR emission are studied, the specific features of HXR generation are discussed, and the capability of applying this radiation to point-projection X-ray imaging of various plasma and biological objects is considered. The parameters of the electron beam produced in the X-pinch are measured using a Faraday cup and X-ray diagnostics. The experiments were performed with the XP generator (550 kA, 100 ns) at Cornell University (United States) and the BIN generator (270 kA, 150 ns) at the Lebedev Physical Institute (Russia).

We present a study of the formation of bow shocks in radiatively cooled plasma flows. This work uses an inverse wire array to provide a quasi-uniform, large scale hydrodynamic flow accelerated by Lorentz forces to supersonic velocities. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. Interferogram data are used to determine a Mach number of ∼6, which may increase with radial position suggesting a strongly cooling flow. Self-emission imaging shows the formation of a thin (

Using time-resolved emission spectroscopy at visible wavelengths, we explored the conditions of plasmas generated by current-driven explosions of single aluminum wires. The experiments were carried out with 15 μm aluminum wires driven by the Low Current Pulser 3 (LCP3) at Cornell University.The plasma conditions were studied as a function of time and radial position, including electron temperature, electron density, ionization state, and magnetic field. To determine the magnetic field, we are working toward employing a new diagnostic method which makes use of Zeeman-effect-produced differences in the line shapes of two fine structure components of a multiplet that are equally broadened by Stark effect and by Doppler broadening. Preliminary results and data collected by a spectroscopy system with 3.2 A resolution are presented.

Summary form only given. We present a study of the formation of bow shocks in radiatively cooled plasma flows. This work uses the XP generator (260kA, 145ns) at Cornell University to drive an inverse wire array. This generates a quasi-uniform, large scale hydrodynamic flow accelerated by Lorentz forces to Ma > 1. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. Collinear interferogram and gated-self emission diagnostics demonstrate that the cone angle with distance from the wire decreases (increasing Mach number) and is indicative of a strongly cooling flow. Rapid density increase from the background flow into the object is indicative of a strong density jump at the shock. High resolution self-emission imaging shows the formation of a thin (

Experiments to study the ablation dynamics of coiled wire arrays were performed on the MAGPIE generator (1 MA, 240 ns) at Imperial College, and on the COBRA generator at Cornell University's Laboratory of Plasma Studies (1 MA, 100 ns). The MAGPIE generator was used to drive coiled wires in an inverse array configuration to study the distribution of ablated plasma. Using interferometry to study the plasma distribution during the ablation phase, absolute quantitative measurements of electron line density demonstrated very high density contrasts between coiled ablation streams and inter-stream regions many millimetres from the wire. The measured density contrasts for a coiled array were many times greater than that observed for a conventional array with straight wires, indicating that a much greater axial modulation of the ablated plasma may be responsible for the unique implosion dynamics of coiled arrays. Experiments on the COBRA generator were used to study the complex redirection of plasma around a coiled wire that gives rise to the ablation structure exhibited by coiled arrays. Observations of this complex 3D plasma structure were used to validate the current model of coiled array ablation dynamics [Hall et al., Phys. Rev. Lett. 100, 065003 (2008)], demonstrating irrefutably that plasma flow from the wires behaves as predicted. Coiled wires were observed to ablate and implode in the same manner on both machines, indicating that current rise time should not be an issue for the scaling of coiled arrays to larger machines with fast current rise times.

In this work we demonstrate a design for obtaining laser backlighting (e.g., interferometry) and time-resolved extreme ultraviolet self-emission images along the same line-of-sight. This is achieved by modifying a single optical component in the laser collection optics with apertures and pinhole arrangements suitable for single or multiple frame imaging onto a gated detector, such as a microchannel plate. Test results for exploding wire experiments show that machining of the optic does not affect the overall quality of the recovered laser images, and that, even with a multiple frame system, the area sacrificed to achieve collinear imaging is relatively small. The diagnostics can therefore allow direct correlation of laser and self-emission images and their derived quantities, such as electron density in the case of interferometry. Simple methods of image correlation are also demonstrated.

Summary form only given. We are initiating an experiment in which time resolved visible spectroscopy will be used to characterize the plasma in and near the minidiode in aluminum (Al) X pinches. The goal of the experiment is to determine the magnetic field and other plasma conditions near the outer radius of the imploding z-pinch in the minidiode. At this location the conditions should be suitable for a magnetic field measurement using the Al doublet previously used for Zeeman Broadening measurements at the Weizmann Institute of Science.1 We will be studying 2-wire and hybrid X pinches on the 13kA 430ns rise time Low Current Pulser 3 (LCP3), the 20kA 200ns rise time Low Current Pulser 4 (LCP4), and at ≥ 200kA on the 50ns rise time XP generator. By using various pulsers and current levels we aim to study directly the impact that the driving current has on the development of the minidiode of the X pinch, the magnetic field, the electron temperature, and the electron density as a function of time and space. Preliminary results will be presented.

Summary form only given. Solid liners are an attractive load design for use with magnetized liner inertial fusion (MagLIF) and for possible replacement of wire-array Z-pinches on large pulsed-power drivers. Experiments have been conducted on COBRA, our 1 MA, 100 ns rise time pulsed power generator, to investigate three aspects that can affect liner performance: initiation; ablation and precursor formation; and instability development.

Summary form only given. Solid liners are an attractive load design, for use with magnetized liner inertial fusion (MagLIF) and for possible replacement of wire-array Z-pinches on larger pulsed-power drivers. Comparisons between equally massed and dimensioned wire-arrays and thin foil liners will be presented. Using an axial X pinch backlighter1, the ablation characteristics and precursor formation are studied. The liners show very little ablation and precursor as compared to wire-arrays.Using laser shadowgraphy and comparisons with the extended magnetohydrodynamics (XMHD) PERSEUS code2, instability growth on the outside of the liners is examined. The instability is found not to be magnetic Rayleigh-Taylor (MRT), but a velocity shear flow instability.

Magnetic field measurements inside wire-array Z-pinches have shown significant field penetration during the ablation phase, as wire material streams toward the axis producing the mass distribution that ultimately determines the implosion characteristics. End-on imaging is especially useful for observing the dynamical effects of the magnetic field on the ablation streams. Quantitative end-on imaging with X-pinches has been successfully developed on Cornell's COBRA accelerator by Isaac Blesener. These images show both the radial distribution of mass in the ablation streams and the transverse (azimuthal) structure of the streams, which tend to be focused into narrow sheets of plasma from the individual wires that merge into a quasi-axisymmetric distribution only quite near the axis.

High energy density (HED) plasmas, produced by diverse techniques such as lasers or pulsed power generators, can help scientists in understanding extreme states of matter as well as astrophysical phenomena. Radial foil configurations are novel experiements which can yield plasma pressures on the order of 0.5 Mbar on a IMA, 100 ns current rise time generator such as the COrnel Beam Research Accelerator (COBRA). In this experimental set-up, a thin metallic foil stretched on a circular anode connects to a very small "pin" cathode (1mm in diameter). Radial currents run through the foil then down the pin cathode, thereby generating an axi-symmetric toroidal magnetic field. These currents interact with the B field and the resulting JxB force lifts the foil upward. Very rapidly the plasma turns into a bubble shaped cavity, which expands swiftly (v∼200-300 km/s), until instabilities destroy the axial symmetry, bursting the bubble open. As the cavity moves away from the cathode a dense central plasma column with electron densities above 1020 cm−3 forms. The background plasma created by current ablation focuses on the geometrical axis, forming a jet moving at a speed of 100 km/s. Its electron density is larger than 1019 cm−3. This jet is collimated by magnetic fields, which confines its mass during a large portion of the discharge.

Experiments on the Low Current Pulser LCP3 at Cornell University are exploring the properties of high energy density plasmas generated by current-driven explosions of single fine metal wires. These experiments are employing non-perturbing emission spectroscopy at visible wavelengths to obtain plasma conditions, including temperatures, electron density, ionization state, and magnetic field. A new diagnostic technique is being developed to determine the magnetic field which makes use of Zeeman-effect-produced differences in the line shapes of two fine structure components of a multiplet that are equally broadened by Stark and Doppler effects. This technique has been demonstrated in experiments performed at the Weizmann Institute of Science in plasmas with lower energy densities [1]. We are studying the time integrated and time resolved visible spectra to determine appropriate spectral lines for measuring magnetic field strength, including the Al III 5696A and the Al III 5722A transitions used in previous work[l]. Preliminary results will be discussed.

Calibrated r-θ density profiles from wire-array experiments on COBRA will be presented. Profiles are axially averaged over the height of the array. The data was gathered using an axial X pinch backlighter1. Images have better than 5-micron resolution with calibrated ion densities from 1018 to 1020 cm-3. The experimental data showing ion density and timing of ablation streams and precursor formation will be compared with simulation results.

Streaked visible-light spectroscopy measurements are presented for aluminum (Al) wire-array z-pinch experiments on the 1-MA, 100-ns rise-time COBRA pulsed-power generator. For these measurements, a half-meter Czerny- Turner spectrometer was used in conjunction with the COBRA visible-light streak camera system. This allowed us to record visible-light spectra emitted from Al coronal plasma as a continuous function of time throughout the initiation, ablation, and implosion phases of the given wire-array z- pinch experiment. When using thick wires (~100-mum in diameter), the visible-band spectra observed consisted solely of continuum emission, which began at the moment of resistive voltage collapse. (Resistive voltage collapse occurs when coronal plasma first forms around the wire cores, and marks the transition from the initiation/resistive-heating phase to the ablation phase.) The continuum data collected are now being used to determine electron density. To determine electron density from this continuum data, an absolute calibration of the detection system was required. The details of these experiments and the absolute calibration technique are presented.

Summary form only given: Radial foil configurations have been used recently in pulsed- power generators to study high energy density plasmas. The setup uses a thin metallic foil and two electrodes: the outer electrode, which has little impact on plasma dynamics, and the inner electrode, where all the discharge current is focused. This focusing effect depends on the diameter of the central electrode, in turn controlling the energy deposition inside the foil material. Using the COBRA generator (1MA with 100 ns rise time) and a "pin" electrode, 1 mm in diameter, it is possible to produce current densities with hundreds of MA/mm2 and 400-T magnetic fields at the foil center. The resulting JxB force propels foil material and plasma vertically, with velocities in the order of 400 km/s. The plasma formation and subsequent focus produce radiations with energies close to 8 keV, in a 1-mm plasma column. Another advantage of radial foil geometries is the diagnostic access. Due to the open configuration, the plasma dynamics is observable from beginning to end, by a wide range of diagnostics. Data from laser back-lighter and interferometer, B-dot probes, X-UV and streak cameras will be analyzed to describe and understand the magneto-hydrodynamics of radial foils. Particular attention will be given to the physics of plasma jets, shocks and magnetic reconnections.

We are investigating the development of the axial instability that occurs on each exploding wire in wire-array Z-pinches. The axial instability is a growing modulation of the size of the coronal plasma around individual wires of the array that results in non-uniform ablation of material from the cold wire core. It has long been known that the wavelength of this modulation is constant late in time and, since it is unique to different materials, it has come to be known as the fundamental mode. In these experiments we have been imaging individual wires with laser shadowgraphy and an XUV framing camera primarily in low wire number, large wire diameter aluminum array. We show also some results from various other arrays for comparison. We document the development of this modulation from the beginning of plasma formation and show its dominant wavelength and amplitude growth as a function of time. The magnetic field topology is also probed using small B-dot probes inside the array. The change from a closed to an open field topology is correkated with the instability growth. Growth of the instability is seen to slow and eventually stop as the magnetic field 1–2 mm inside a wire decreases and changes sign, signifying the change in topology from locally to globally dominated field.

B‐dot probes for the first time have successfully measured the field through implosion of wire arrays on COBRA. The probe data confirm an advective magnetic evolution of closed field lines during the onset of ablation that was first seen in 2D GORGON simulations.

The limited understanding of the mechanisms driving the mass ablation rate of cylindrical wires arrays is presently one of the major limitations in predicting array performance at the higher current levels required for inertial confinement fusion (ICF) ignition. Continued investigation of this phenomenon is crucial to realize the considerable potential for wire arrays to drive both ICF and inertial fusion energy, by enabling a predictive capability in computational modeling. We present the first study to directly compare the mass ablation rates of wire arrays as a function of the current rise rate. Formation of the precursor column is investigated on both the MAPGIE (1 MA, 250ns [Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]) and COBRA (1 MA, 100ns [Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]) generators, and results are used to infer the change in the effective ablation velocity induced by the rise rate of the drive current. Laser shadowography, gated extreme ultraviolet (XUV) imaging, a...

Understanding the magnetic field topology in wire‐array Z‐pinches as a function of time is of great significance to understanding these high‐energy density plasmas especially for their ultimate application to stockpile stewardship and inertial confinement fusion. We are developing techniques to measure magnetic fields as a function of space and time using Faraday rotation of a single longitudinal mode (SLM) laser through a magneto‐optically active bulk waveguide (multicomponent terbium borate glass) placed adjacent to, or within, the wire array in 1 MA experiments. We have measured fields >10 T with 100 ns rise times outside of a wire‐array for the entire duration of the current pulse and as much as ∼2 T inside a wire‐array for ∼40 ns from the start of current. This is the first time that such rapidly varying and large fields have been measured using these materials. In a dense Z‐pinch, these sensing devices may not survive for long but may provide the magnetic field at the position of the sensor that can...

Some issues concerning high-current electron beam transport from the X pinch cross point to the diagnostic system and measurements of the beam current by Faraday cups are discussed. Results of computer simulation of electron beam propagation from the pinch to the Faraday cup give limits for the measured current for beams having different energy spreads. The beam is partially neutralized as it propagates from the X pinch to a diagnostic system, but within a Faraday cup diagnostic, space charge effects can be very important. Experimental results show evidence of such effects.

Experimental results are presented which characterize the implosion dynamics and subsequent radiation output of wire-array Z-pinches on the 1-MA, 100-ns rise-time COBRA pulsed-power generator. Diagnostics fielded include an optical streak camera, a time-gated XUV framing camera, a laser shadowgraph system, filtered time-integrated pinhole cameras, an X-ray focusing spectrometer with spatial resolution (FSSR), a load voltage monitor, a Faraday cup, a bolometer, silicon diodes and diamond photoconducting detectors (PCDs). The load geometries investigated include hollow cylindrical arrays ranging from 4 to 16 mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten (W), aluminum (Al), or Invar. The data produced by the entire suite of diagnostics are analyzed and presented to provide an overall picture of implosion dynamics and timing on COBRA. In particular, data fitting to various implosion trajectory models, as well as X-ray pulse shape dependencies on various loads and implosion characteristics are presented and discussed. From this study, it was determined that by moving the stagnation time earlier (i.e., before the peak in the driving load current, where dI/dt is positive), the total X-ray yield is increased by ~1.5-2times on COBRA. From bolometer measurements, we recorded ~10 kJ of total X-ray energy, which is a conversion efficiency of -10% for the 100-kJ discharge of a typical COBRA pulse. Load scaling for an earlier stagnation time was accomplished by dropping to a smaller initial array diameter (i.e., 4 mm) and using smaller initial wire diameters (i.e., 5-mum W; 12-mum Al). The use of fine wire and a small array diameter also allowed for enough wires and a tight enough wire-spacing to provide good implosion uniformity (i.e., 16 wires on a 4-mm array diameter for 785-mum wire spacing).

Summary form only given. X pinch backlighting is a useful imaging technique whereby two or more wires are crossed at a single point and driven with a high current (300-500 kA). At the crossing point, a micron-scale sub-nanosecond X-ray source is produced that acts as a point source and can be used for point-projection imaging. Three new modes of X pinch imaging on COBRA have been implemented. One of these techniques involves coupling the point-projection imaging with an MCP camera. This method is in development in the hopes that it can be used to take high resolution images of imploding (i.e. strongly radiating) wire-array Z pinches by time-gating out the radiation from the Z pinch. Previous attempts resulted in low contrast, blurry images. Presented here are new results which show an improvement in image quality. Another recent development involves a new large format imaging camera which allows point-projection imaging with high magnification. This new camera can be used to take backlighting images of wire arrays with ~34times magnification. Presented here are recent images of dense wire cores and coronal plasma, both showing significant axial modulation. Finally, for the first time on COBRA, a technique is in development for end-on X pinch backlighting. Preliminary results from this new method will be presented.

Results of experimental studies of the spatial and temporal characteristics of the hard X-rays generated by high-current electron beams in X-pinch experiments on the XP (Cornell University) and BIN (LPI RAS) pulsed power generators are presented. Some issues concerning high-current electron beam transport from the X pinch to the diagnostic system and measurements of the beam current by Faraday cups with different geometries are discussed. Generation of electron beams is an unavoidable property of X-pinches and other pulsed-power-driven pinches of different geometry. Results of computer simulation of electron beam propagation from X-pinch to Faraday cup give the limits of measured current for beams having different energy spread. We emphasize the partially neutralized nature of the beam propagating from the X-pinch to a diagnostic system. Different schemes for beam space-charge neutralization are discussed. Longitudinal neutralization is thought to be the most important method because it enables transport of beams with close to the Alfven current. Experimental and computer simulation results on electron beam generation are compared.

Summary form only given. Wire-array Z pinches have been studied on the 1.2 MA peak current, 100 ns rise time COBRA generator at Cornell University using a wide range of diagnostics. In addition to the X-ray and electrical diagnostics, two framing cameras with 5 ns temporal resolution, a three frame laser shadowgraphy system with 170 ps temporal resolution and optical and X-ray streak cameras have been used in the experiments. Wire-arrays made with Al, W and Invar wires with 16, 8 and 4 mm array diameters and interwire gaps from 4.6 mm to 0.8 mm have been studied. The investigations include dynamics of the wire-array implosion and hot spot formation in the on-axis stagnation plasma. The experiments showed that the wire-arrays with 4 mm diameter have an X-ray energy yield 1.5-2 times higher than larger diameter arrays for all wire materials studied. The study also shows a complicated structure of the hot spots in space and in time. Measurements with spatial and temporal resolution using the X-ray streak- camera have shown the beginning of the hot spot formation starts near the time of maximum compression of the arrays. Hot spots that last 1-5 ns are observed within a time interval of 30 - 40 ns. Energy measurements using filtered PCDs show that the hot spots radiate a major part of the wire-array energy yield for all wire materials and array diameters.

Summary form only given. It is well known that X pinches produce very small, dense plasma micropinches that emit short bursts of 1.5-8 keV radiation. Immediately after the softer X-ray burst, higher energy emission in the range 8-100 keV is seen and attributed to energetic electrons accelerated in the gaps that are visible in the X pinch structure in the images recorded using X pinch point projection radiography. Electron beam parameters in the X pinches (current, average energy, and divergence) were not previously measured. Such measurements on symmetric X pinches are compromised by strong absorption and electron scattering that occurs between the X pinch crossing point and the anode. Cold, relatively high density plasma fills this space as a result of the ablation of the wire cores. An asymmetric X pinch scheme that decreases plasma density and size along the path of propagation of the electron beam has been tested. In this configuration, the anode side of the X pinch was made 4-7 times shorter than the cathode side and the angle between wires was almost 180 deg. Electron beam parameters were determined using two low impedance coaxial Faraday cups (with the filter of the inner cup working as the collector for the outer one). The filters on these cups enabled simultaneous measurements of the beam current in two energv bands. For two-wire. 17 mum, asymmetric, tungsten X pinches the electron current reached 5-10 kA with a pulse duration of 3-5 ns and a peak electron energy of more than 80 keV. These electrons were emitted in a solid angle of about 10-2 steradians. In symmetrically configured X pinches the measured electron beam current is much less and has much higher divergence. The asymmetric configuration may also be useful for ion beam studies by reversing the pulse polarity.

Summary form only given. X pinches are often arranged in parallel. Parallel X pinches allow one to get point-projection radiograph images of a single object either at different times, different perspectives, or both. Also, when X pinches are in parallel, each one can make a point-projection radiograph of the other. During the formation of the X pinch neck and hot spots, current is divided between the parallel X pinches according to their relative inductance. The self-adjusting nature of the current distribution usually results in the parallel X pinches producing better sources than single X pinches. Parallel X pinches more often develop only one hot spot and generate electron beams of less intensity than single X pinches. These two characteristics are very important for point-projection radiography. Presented here are the results of experiments to characterize the behavior of parallel X pinches. Experimental parameters included wire material, wire diameter, number of wires, and polarity of the radial electric field. Of particular interest was the current measured individually through each X pinch. By keeping the return current path of each X pinch separate, we were able to monitor any changes in or interactions between the individual currents. The relative time between the occurrences of individual pinches on the same shot was observed using PCDs. Also of interest was the effect of polarity on the time evolution of four-wire X pinches. This information was obtained from a series of point-projection radiographs taken at different times on different shots. It was shown that the diameters of the wire cores at the time of the pinch were significantly larger in positive polarity than compared to negative polarity.

We present an experimental study of plasma initiation of a solid metal liner at the 1 MA level. In contrast to previous work, we introduce a vacuum gap at one of the liner connections to the power feed to investigate how this affects plasma initiation and to infer how this may affect the symmetry of the liner in compression experiments. We observed that the vacuum gap causes non-uniform plasma initiation both azimuthally and axially in liners, diagnosed by gated optical imaging. Using magnetic field probes external to the liner, we also determined that the optical emission is strongly linked to the current distribution in the liner. The apparent persistent of azimuthal non-uniformities may have implications for fusion-scale liner experiments.

Summary form only given. We present here the results of a series of experiments comparing the operation of an X-pinch in positive and negative polarity. Of particular interest are the X-ray yield, timing, radiating spot size, and the number of radiating spots in a given X-pinch. Conventionally, an X-pinch is operated in negative polarity resulting in electric field lines that terminate on the wires. This choice of polarity for the generator is due to the fact that breakdown in these machines is easier to prevent when they are operated in negative polarity. Previous experiments involving single wire explosions at low current (1-5 kA) have shown that more energy can be deposited in the wire cores when they are driven in positive as opposed to negative polarity. These results raised the question of what happens when pinch-type experiments are driven in positive polarity. Recently, it has become possible to use the 450 kA XP pulser at Cornell to drive X-pinches in positive polarity. The positive polarity condition is established by inserting a convolute before the load and this results in electric field lines that originate on the wires of the X-pinch. This arrangement added complexity and inductance to the positive polarity shots, making it difficult to directly compare results from positive and negative shots. More recently, the entire pulser was switched to positive polarity. This allows a more direct comparison between positive and negative polarity X-pinch operation

Summary form only given. The X pinch has proven itself to be both an extraordinary X-ray back lighting source and an interesting high energy density plasma for plasma dynamics and X-ray spectroscopy studies. However, some of the most interesting physics of an X pinch happens on a few ns, or even subnanosecond, time scale. The X pinch has been estimated to approach solid density on a size scale of 1 micron and achieve temperatures greater than 1 keV for a period of about 0.1 ns during which there is an intense X-ray burst. The final stages of implosion leading to the formation of the X-ray burst last only a few nanoseconds. Therefore, images of the X pinch taken with an apparatus that integrates over a few ns are not sufficient to resolve the dynamics of the X pinch during the last critical stages of implosion. Here, we report experiments using a sub-200 ps laser pulse to record several frames of each X pinch pulse with both shadow and interference images. We expect to show images that are of interest both as observations of X pinch dynamics for materials such as Al, Mo, and W and for comparison with code calculations used to model the X pinch

In this paper, the shapes of the ablation streams in non-imploding cylindrical wire-array Z-pinches are investigated. Experimental observations using axial X pinch imaging show an azimuthal pinching of the streams that appear to depend on the topology of the global magnetic field. With fewer wires and increased interwire spacing, the radial component of the global field is increased; resulting in a stronger pinching of the streams. Computer simulations are used to model the magnetic field development and show that the sparser array has a significantly stronger azimuthal J→×B→ force.

High energy density (HED) plasmas, produced by diverse techniques, such as lasers or pulsed power generators, can help scientists to better understand extreme states of matter as well as astrophysical phenomena. While fast Z-pinches are the most common approach to generating HED plasmas in the pulsed power community, radial foil configurations can yield plasma pressures on the order of 0.5 Mbar on a 1 MA, 100 ns current rise time generator, similar to wire array configurations producing Z-pinches. In this experimental setup, a thin metallic foil stretched onto a circular anode connects to a very small 'pin' cathode at the center. Radial currents flow in the foil then down the pin cathode, thereby generating an axi-symmetric toroidal magnetic field under the foil. Initial experimental results (Gourdain et al 2010 Phys. Plasmas 17 012706) showed that the foil current interacts with this field and the resulting J × B force lifts the foil upward. Very rapidly the foil plasma turns into a bubble-shaped cavity above the central pin, and that bubble expands at 300 km s−1, until instabilities destroy the axial symmetry, bursting the bubble open. This paper complements these initial results by using time-integrated x-ray pin-hole cameras and a focusing spectrometer with spatial resolution. In addition to laser interferometry, these new data help to provide a better estimate of the plasma electron density inside the bubble, above 1020 cm−3, and of the electron temperature, between 300 and 400 eV inside the central plasma column. We also discovered the presence of 'bright' spots in the plasma, with densities larger than 5 × 1021 electrons cm−3 and temperatures above 1 keV. Finally laser interferometry gives a precise mapping of the initial plasma jet and bubble areal densities.

For the first time, a geometry has been developed to allow for an axial imaging system for wire-array Z-pinch experiments that produce high-resolution x-ray images. The new geometry required a significant redesign of the electrode hardware. Calibrated areal density measurements of the Z-pinch plasma including wire cores, coronal plasma, streaming plasma, and the precursor were obtained. The system used eight-wire molybdenum (Mo) X pinches in series with and directly below the Z-pinch axis to provide micron-scale x-rays sources for point-projection radiography. The images formed on the x-ray sensitive film had a 15 mm diameter field of view at the center height of the array and a magnification of about 7.5:1. Titanium (Ti) filters in front of the film transmitted radiation in the spectral range of 3-5 keV. For calibration, a separate film with the same thickness Ti filter was placed the same distance from the X pinch. This film had an unobstructed path that bypasses the Z-pinch but included step wedges for calibration of the Z-pinch plasma. The step wedges had thicknesses of tungsten (W) ranging from 0.015 to 1.1 microm to obtain areal density measurements of the W plasma from the wire-array. Images had subnanosecond temporal resolution and about 10 microm spatial resolution.

Azimuthally correlated wire core ablation was compared for closely spaced versus widely spaced wires in a 1 MA Z-pinch. X-ray point-projection diagnostics revealed that 240 μm spaced wires exhibited a correlation coefficient approaching unity in both real space and in k-space. This correlated ablation between wires at a fixed axial location is believed to occur due to an enhanced, localized Joule heating. Wires separated by 2.47 mm or greater were uncorrelated in real space, but correlated in k-space, indicating the ablation structure between wires was shifted in phase.

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-r...

Wire core and coronal plasma formation and expansion in wire-array Z pinches with small numbers of wires have been studied on a 1MA, 100ns rise time pulsed power generator and a 500kA, 50ns generator. Two-frame point-projection x-ray imaging and three-frame laser optical imaging and interferometry were the principal diagnostic methods used for these studies. The x-ray images show that dense coronal plasma forms and is maintained close to each dense wire core in the array. A less dense, rapidly expanding (∼10μm∕ns) coronal plasma, best seen in the laser images, surrounds the ∼100μm radius dense corona. These results are in agreement with computer simulations and modeling carried out by Yu et al. [Phys. Plasmas 14, 022705 (2007)]. Results are also presented for the dependence of the wire core and coronal plasma expansion rates on the wire diameter, number of wires and current through individual wires and the overall configuration for Al, Cu, and W wire arrays. For example, the W wire dense core expansion ra...