We present experimental results on the formation of supersonic, radiatively cooled jets driven by the toroidal magnetic field generated by the 1.5 MA, 250 ns current from the MAGPIE generator. The morphology of the jet produced in the experiments is relevant to astrophysical jet scenarios in which the jet on the axis of a magnetic cavity expanding into an ambient medium is collimated by a toroidal magnetic field. The jets in our experiments have similar Mach number, plasma beta and cooling parameter to those in protostellar jets and additionally the Reynolds, magnetic Reynolds and Peclet numbers are much larger than unity, allowing the experiments to be scaled to astrophysical flows. The experimental configuration generates episodic magnetic cavities, suggesting that periodic formation of jets in astrophysical situations could be responsible for some of the variability observed in astrophysical jets. The dynamics of the formation of laboratory jets are presented, together with new results including preliminary measurements of magnetic, kinetic and Poynting energy of the outflows. In addition first estimates of jet temperature and trapped toroidal magnetic field are presented and discussed.

We present data from experiments using nested wire-array Z -pinches where the wires of one of the arrays (the outer or inner array) are inclined to produce a cone. The use of these nested conical arrays can potentially provide a valuable tool in X-ray radiation pulse shaping for Z-pinch-driven high-yield inertial-confinement-fusion schemes. Conical nested arrays can produce a zippered implosion which broadens the main radiation pulse and, possibly, the pulse associated with the interaction of the two arrays. Results from experiments at current levels of 1 MA (240 ns) and 18 MA (100 ns) are presented and compared. Experiments at 1 MA with a conical outer array indicate broadening of the full-width at half-maximum of the main stagnation radiation pulse, with results at 20 MA showing a similar result. Conical inner arrays do not broaden the main radiation pulse because a longer inner array ablation time offsets the earlier interaction time of the outer array with the inner array. Although array dynamics data suggest that a conical inner or outer array can potentially provide control of the interaction radiation pulse, this was not observed. Observations are consistent with a wire-array trajectory model incorporating outer and inner array ablation, snowplow physics, and a simplified array interaction model.

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.

We report the results of experiments where a quantitative analysis of the behavior of ablation modulations in X-pinches (including wavelength and contrast between streams) was carried out as a function of distance from the cross-point, and consequently, as a function of the global to local magnetic field ratio. Experiments were performed using two tungsten wires of 10 μm diameter in an X-pinch configuration on a pulsed power generator capable of producing an 80 kA current over a 50 ns rise time. We compare these findings directly to three-dimensional magneto-hydrodynamic (MHD) simulations using the MHD resistive code GORGON. Here, we demonstrate a dependence between the magnetic field ratio and ablation properties, suggesting a transition to a classical m = 0 instability far from the cross-point. Additionally, we examine the ablation properties of similar Z number nickel and copper wire X-pinches.

Bow shock structures are observed in a nested wire array z-pinch as ablation streams from the outer array pass the inner array. The jump in plasma conditions across these shocks results in an enhancement of snowplow emission from the imploding plasma piston. Results from a snowplow model modified to account for the shock jumps are discussed and compared to experimental data from MAGPIE. Magnetohydrodynamic simulations indicate that this is the primary heating mechanism responsible for the interaction pulse recorded on the Z generator, which is required for pulse shaping for inertial confinement fusion.

Summary form only given. Accurately determining magnetic diffusion parameters in plasmas is important for benchmarking 3D MHD codes used for the design and interpretation of Z pinch and other plasma experiments. In this work, two planar arrays consisting of four wires each are at a fixed inter-plane spacing. The inter-wire spacing is then varied to alter the ratio of local to global magnetic field. This in turn determines the location of local precursor column structures, the current carrying regions, and the rate at which plasma may travel across the magnetic field toward the axis. Experimental work is carried out on UCSD's GenASIS LTD pulsed power driver. For this experiment, 200kA is delivered in an approximately sin2 waveform with a 150ns rise time. A variety of diagnostics are employed to diagnose parameters such as density, temperature, and magnetic field, both spatially and temporally. Simulation of the system is performed using the 3D resistive MHD code, GORGON. Computations are carried out using a new 304 processor cluster, PLEIADES, installed at the San Diego Super Computer Center at UCSD. The experimental results and comparison with simulation will be presented.

The aim of this work is to model the jets produced by conical wire arrays on the MAGPIE generator, and to design and test new setups to strengthen the link between laboratory and astrophysical jets. We performed the modelling with direct three-dimensional magneto-hydro-dynamic numerical simulations using the code GORGON. We applied our code to the typical MAGPIE setup and we successfully reproduced the experiments. We found that a minimum resolution of ∼100 μm is required to retrieve the unstable character of the jet. We investigated the effect of changing the number of wires and found that arrays with less wires produce more unstable jets, and that this effect has magnetic origin. Finally, we studied the behaviour of the conical array together with a conical shield on top of it to reduce the presence of unwanted low density plasma flows. The resulting jet is shorter and less dense.

Observations of supersonic jet propagation in low-current x-pinches are reported. X-pinches comprising of four 7.5 μm diameter tungsten wires were driven by an 80 kA, 50 ns current pulse from a compact pulser. Coronal plasma surrounding the wire cores was accelerated perpendicular to their surface due to the global J × B force, and traveled toward the axis of the x-pinch to form an axially propagating jet. These jets moved towards the electrodes and, late in time (∼150 ns), were observed to propagate well above the anode with a velocity of 3.3±0.6×10 4 m/s. Tungsten jets remained collimated at distances of up to 16 mm from the cross point, and an estimate of the local sound speed gives a Mach number of ∼6. This is the first demonstration that supersonic plasma jets can be produced using x-pinches with such a small, low current pulser. Experimental data compares well to three-dimensional simulations using the GORGON resistive MHD code, and possible scaling to astrophysical jets is discussed.

Previous experiments on the 1-MA “Zebra” generator at UNR studied precursor plasmas with Ni-60 alloy (96% Cu) wire arrays1. Those precursor plasmas were shown to consistently have electron temperatures > 400 eV 1. New recent experiments performed on Zebra using cylindrical wire arrays (CWA) continue to emphasize stronger radiating precursor plasma but from other mid-atomic number wire arrays. Time-integrated spatially-resolved (TISR) and time-gated spatially-integrated (TGSI) x-ray spectral data, time-integrated and time-gated pinhole x-ray images, shadowgraphy, as well as optical streak camera images were obtained during the recent mid-atomic number (Cu, brass, and alumel) CWA experiments. Preliminary results on these CWA are consistent with previous Ni-60 results. Non-LTE kinetic models of Cu, Zn, and Ni have been applied to account for the L-shell radiation from precursor and main x-ray burst plasmas. The resulting plasma parameters from modeling of TGSI and TISR spectra together with analysis of corresponding images allow for the study of precursor plasma formation in time and in space, respectively.

We present the application of a short rise (similar to 150 ns) 250 kA linear transformer driver (LTD) to wire array z-pinch loads for the first time. The generator is a modification of a previous driver in which a new conical power feed provides a low inductance coupling to wire loads. Performance of the new design using both short circuit and plasma loads is presented and discussed. The final design delivers similar to 200 kA to a wire array load which is in good agreement with SCREAMER calculations using a simplified representative circuit. Example results demonstrate successful experiments using cylindrical, conical, and inverse wire arrays as well as previously published work on x-pinch loads.

Summary form only given. Whilst the dynamical evolution of wire arrays is well understood, and multi-dimensional Magneto-Hydrodynamic (MHD) modeling has demonstrated significant progress, a comprehensive predictive capability has not been realized to date. Recent experimental investigations have highlighted the need to more closely examine the ablation structure and its dependence on the initial parameters of the array. In particular, the range over which the ablated plasma is accelerated, and hence extent to which magnetic flux is convected into the array, is often a disputed point in the comparison simulation and analytical work. Recent work at UC San Diego [ 1 ] uses a modification of the Lebedev rocket model of wire ablation to fit the range of ablation velocities which are observed in experiments. This analysis can be extended to infer the 2D distribution of effective velocities which redistribute the mass as a function of time. From this data it may be possible to direct observe the acceleration region in a wire array. An analysis of the effect of array geometry on the determined acceleration region will be attempted using interferographic and radiographic data from experiments at 200 kA to 1 MA. Conclusions and possible extensions to this work will be presented and discussed.

Summary form only given. Determination of B-field structures in pulsed power driven exploding wire experiments is vital to recover detailed information about the evolution, driving mechanisms of ablation, and subsequent instability development, but is complicated by the presence of large volumes of hot, dense plasma. Optical and electrical probe diagnostics typically fail early in the experiment. We present progress on a new project, which examines the use of proton deflectometry to measure magnetic fields in pulsed power plasmas. Experimental work is carried out at the Nevada Terawatt Facility (NTF) using both 10 J, 0.3 ps Leopard laser and the 1.6 MA Zebra pulsed power driver. Leopard provided focused intensities of ~5×1019 W cm-2, and generated up to 8 MeV protons from thin metallic targets with good reproducibility and low divergence, even when placed within the ZEBRA chamber. Interpretation of experimental results can only be achieved by performing many iterations of simulation, which may then be compared to actual data. Quantitative data from experiments detailing the conditions of the plasma, such as density and temperature, unique to each experimental setup, is used to constrain the simulation of wire plasmas performed using the 3D resistive MHD code, GORGON. Protons are then injected and tracked through the plasma using the 3D PIC (Particle-In-Cell) Large Scale Plasma code in order to produce possible proton image plane data. All computational work is carried out using a new 304-processor cluster, PLEIADES, installed at the San Diego Super Computer Center at UCSD. The first simulations of protons through single wire plasmas in pulsed power experiments, along with comparison to recent experimental results will be presented.

We present experimental results on the formation of supersonic, radiatively cooled jets driven by pressure due to the toroidal magnetic field generated by the 1.5 MA, 250 ns current from the MAGPIE generator. The morphology of the jet produced in the experiments is relevant to astrophysical jet scenarios in which a jet on the axis of a magnetic cavity is collimated by a toroidal magnetic field as it expands into the ambient medium. The jets in the experiments have similar Mach number, plasma beta and cooling parameter to those in protostellar jets. Additionally the Reynolds, magnetic Reynolds and Peclet numbers are much larger than unity, allowing the experiments to be scaled to astrophysical flows. The experimental configuration allows for the generation of episodic magnetic cavities, suggesting that periodic fluctuations near the source may be responsible for some of the variability observed in astrophysical jets. Preliminary measurements of kinetic, magnetic and Poynting energy of the jets in our experiments are presented and discussed, together with estimates of their temperature and trapped toroidal magnetic field.

Astrophysical observations have demonstrated many examples of bow shocks, for example, the head of protostellar jets or supernova remnants passing through the interstellar medium or between discrete clumps in jets. For such systems where supersonic and super-Alfvénic flows and radiative cooling are all important, carefully scaled laboratory experiments can add insight into the physical processes involved. The early stage of a wire array z-pinch implosion consists of the steady ablation of material from fine metallic wires. Ablated material is accelerated toward the array axis by the J×B force. This flow is highly supersonic (M>5) and becomes super-Alfvénic (MA>2). Radiative cooling is significant in this flow and can be controlled by varying the material in the ablated plasma. The introduction of wires as obstructions in this steady flow leads to the formation of bow shocks, which can be used as a laboratory testbed for astrophysical bow shocks. The magnetic field associated with this obstruction wire can be controlled by varying the current through it. Differences in the shock for different cooling rates and different magnetic fields associated with the obstruction will be discussed, along with comparisons of dimensionless parameters in the experiments to astrophysical systems.

Summary form only given. The ablation phase of exploding multi-wire experiments driven by fast-rising currents in which both global and local magnetic fields are dynamically significant is poorly understood at present. In particular, a quasi-periodic modulation in the plasma flow accelerated from the wire cores following initiation appears at all current levels, and is not fully explained. The lack of a complete description of the physical process which drives this ablation structure leads to uncertainties in the scaling of the plasma parameters with drive current. Performance at very high current levels cannot be predicted with a high degree of confidence. Such scaling is particularly important for wire array z-pinches which show promise as a driver for high yield Inertial Confinement Fusion (ICF), as well as the generation of novel High Energy Density Physics (FTEDP) states which may be achievable using exploding wire systems.We present a quantitative investigation of the ablated plasma from a wire system in which magnetic field local to the wire is fixed and the global field varies, namely the x-pinch. Twoframe laser interferometry is used to examine 2 wire xpinches formed from W, Al, Cu and Ni wires at 80 kA, along with gated XUV imaging and time integrated soft x-ray imaging. Two-dimensional areal electron density maps of the ablation structure are recovered as a function of both space and time with spatial resolution -50 μm. Results show a change in the plasma characteristics from regions where the global field plays a significant role, to where the local field dominates, far from the cross-point, producing classical m=0 MHD instabilities. This is demonstrated through measurements of the periodicity of the plasma structure and the density contrast parallel to the wire as a function of distance from the cross-point. Data compare well to 3D MHD simulations using the Gorgon code which closely reproduce much of the experimental behavior observed.

Summary form only given. The development of shocks in plasma flows occurs in a wide range of environments, including fusion schemes and astrophysical objects. In wire array experiments, the plasma accelerated from the wire via the Lorentz force rapidly exceeds both the local sound and Alfven speeds, providing an interesting source for shock studies. Recently, the plasma flow in a 1 MA wire array z-pinch demonstrated both the formation of bow shocks around an obstacle in the plasma, and the feasibility of testing the effect of magnetic fields and radiation cooling on the shock formation in these systems.In this work we present examination of bow shock formation in wire array plasma flows driven by the 250 kA GenASIS Linear Transformer Driver (LTD). The plasma densities produced are lower than on the 1 MA device used in Ref 1, and this allows continuous 2-dimensional quantitative measurements of the electron density, which was not previously possible. A closer examination of the shock structure is therefore possible for comparison for both analytical theory and simulation work. Shock formation is examined as a function of obstacle geometry and size, and preliminary results from both standard and inverse wire arrays will be presented.

Highly collimated outflows or jets are produced by a number of astrophysical objects including protostars. The morphology and collimation of these jets is thought to be strongly influenced by the effects of radiative cooling, angular momentum and the interstellar medium surrounding the jet. Astrophysically relevant experiments are performed with conical wire array z‐pinches investigating each of these effects. It is possible in each case to enter the appropriate parameter regime, leading the way towards future experiments where these different techniques can be more fully combined.

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, and x-ray diodes are used to compare the dynamical behavior on the two generators, and X-pinch radiography and XUV spectroscopy provide density evolution and temperature measurements respectively. Results are compared to predictions from an analytical scaling model developed previously from MAGPIE data, based on a fixed ablation velocity. For COBRA the column formation time occurs at 116±5 ns and for Al arrays and 146±5 ns for W arrays, with Al column temperature in the range of 70–165 eV. These values lie close to model predictions, inferring only a small change in the ablation velocity is induced by the factor of 2.5 change in current rise time. Estimations suggest the effective ablation velocities for MAGPIE and COBRA experiments vary by a maximum of 30%.