Conical Wire Arrays Research Articles

Multi frame radiography of supersonic water jets interacting with a foil target

Pulsed-power-driven underwater electrical explosion of cylindrical or conical wire arrays produces supersonic water jets that emerge from a bath, propagating through the air above it. Interaction of these jets with solid targets may represent a new platform for attaining materials at high pressure (>1010 Pa) conditions in a university-scale laboratory. However, measurements of the internal structure of such jets and how they interact with targets are difficult optically due to large densities and density contrasts involved. We utilized multi-frame x-ray radiographic imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility to explore the water jet and its interaction with a 50 μm thick copper foil placed a few mm from the surface of water. The jet was generated with a ∼130 kA-amplitude current pulse of ∼450 ns rise time applied to a conical wire array. X-ray imaging revealed a droplet-type structure of the jet with an average density of <400 kg/m3 propagating with a velocity of ∼1400 m/s. Measurements of deformation and subsequent perforation of the target by the jet suggested pressures at the jet–target interface of ∼5 × 109 Pa. The results were compared to hydrodynamic simulations for better understanding of the jet parameters and their interaction with the foil target. These results can be used in future research to optimize the platform, and extend it to larger jet velocities in the case of higher driving currents supplied to the wire array.

Supersonic water jets as point-like sources of extremely high pressure

Two interacting supersonic water jets and collisions of a water jet with an aluminum target are studied experimentally and by hydrodynamic simulations. Supersonic water jets form, when shocks generated by underwater electrical explosions of conical wire arrays converge. The arrays are supplied by a ∼250 kA, ∼1 μs rise time current pulse. Underwater explosion of two conical arrays placed face to face produces jets propagating in air with velocities of ∼2.5×103 m/s leading to hot plasma formation at a temperature of ∼2200–3000 K, pressure ∼1.7×1010 Pa, and density >1029 m−3. When a single array explodes underwater in front of an aluminum target, the collision of the jet with the target produces a local pressure of ∼3×1010 Pa on the surface of the target.

Shock interactions between plasma jets from conical wire array Z-pinches and laser-produced plasma plume

The interaction of a supersonic plasma jet outflow with a background laser-produced plasma plume is experimentally studied. The aim of these studies is to provide an experimental platform to study jet-ambient interactions in an environment relevant to high energy density physics scenarios. The jet outflow is produced by a conical wire array Z-pinch composed of 16 aluminum wires (40 μm diameter each) acting as load of the Llampudken generator (∼400kA, ∼350 ns). The laser plasma plume is produced by focusing a 2 × 1010 W/cm2 laser pulse onto an aluminum target. Our experimental results show that in the absence of laser plasma plume, there is no significant photoionized plasma from the target that interacts with the jet outflow. On the contrary, when combining both plasma sources, a new structure appears at the interaction region. The thickness of this structure is of the order of the inter-particle ion-ion mean free path, which was calculated for a wide range of parameters for each plasma. These results indicate the presence of a collisional shock layer created after the interaction. Further details and potential applications are shown and discussed.

Supersonic jet generation by underwater sub-microsecond electrical explosions of wire arrays

Experiments in which supersonic water jets are generated by underwater sub-μs timescale electrical explosions of cylindrical and conical wire arrays are presented. These are compared with previous experiments [Maler et al., Phys. Plasmas 28, 063509 (2021)] in which the generation of supersonic water jets was demonstrated using a μs timescale generator. Although in the present experiments less energy is deposited into the wire arrays, the water jets acquire higher velocities compared to when the deposited energy is higher but the timescale is slower. That is, with a higher energy density deposition rate, faster radial wire expansion is induced resulting in a stronger converging shockwave and a faster waterflow behind its front. In addition, two dimensional hydrodynamic numerical simulations show that the formation of the water jet is the result of extremely high pressure at the axis of the shockwave implosion and the cumulative edge effect realized at the array output.

Generation of supersonic jets from underwater electrical explosions of wire arrays

Underwater electrical explosion experiments of cylindrical or conical wire arrays accompanied by the generation of fast (up to ∼4500 m/s) water jets are presented. In these experiments, a pulse generator with a stored energy of up to ∼5.7 kJ, current amplitude of up to ∼340 kA, and rise time of ∼0.85 μs was used to electrically explode copper and aluminum wire arrays underwater. Streak and fast framing shadow imaging was used to extract the space–time resolved velocity of the ejected jet from the array while it propagates in air. The jet generation occurs due to high pressure and density of water formed in the vicinity of the array axis by the imploding shockwave. It was shown that the velocity of the jet ejected from the array depends on the array geometry and the thickness of the water layer above the array. The results suggest that ≥50% of the energy deposited into the array is transferred to the kinetic energy of this jet and the axial waterflow.

Axial outflows from conical wire array z-pinches as a tool for surface modifications

A novel application for conical wire array Z-pinches to surface science is presented. The axial outflows emitted by a tungsten conical wire array in the form of both a dense plasma jet and energetic ions are used to produce morphological modification of substrate surfaces without using any kind of chemical treatment. In particular, surface modifications of (100) oriented Si substrates in the form of micropores and stripes-like formations after a single interaction of the Si to the axial outflows are obtained. It is found that the prevalence of each kind of formation, either micropores or stripes, on the treated substrate depends on the relative position of the target with respect to the wire array. The results indicate that at distances of 11 cm over the array there is still effect of the dense plasma jet outflow since both micropores and stripes are found. On the contrary, solely stripe-like patterns are found at greater distances where the effect of the dense plasma outflow is minimal or null, and only the ion outflow irradiates the surface. The description of the method and future perspectives of this novel application of surface treatment are shown and discussed.

Role of collisionality and radiative cooling in supersonic plasma jet collisions of different materials

Currently there is considerable interest in creating scalable laboratory plasmas to study the mechanisms behind the formation and evolution of astrophysical phenomena such as Herbig-Haro objects and supernova remnants. Laboratory-scaled experiments can provide a well diagnosed and repeatable supplement to direct observations of these extraterrestrial objects if they meet similarity criteria demonstrating that the same physics govern both systems. Here, we present a study on the role of collision and cooling rates on shock formation using colliding jets from opposed conical wire arrays on a compact pulsed-power driver. These diverse conditions were achieved by changing the wire material feeding the jets, since the ion-ion mean free path (λ_{mfp-ii}) and radiative cooling rates (P_{rad}) increase with atomic number. Low Z carbon flows produced smooth, temporally stable shocks. Weakly collisional, moderately cooled aluminum flows produced strong shocks that developed signs of thermal condensation instabilities and turbulence. Weakly collisional, strongly cooled copper flows collided to form thin shocks that developed inconsistently and fragmented. Effectively collisionless, strongly cooled tungsten flows interpenetrated, producing long axial density perturbations.

Emission of fast ions from conical wire array Z-pinches studied at different background pressures

A series of experimental measurements of fast ions generated in a tungsten conical wire array are presented. The ions are detected using a set of negatively biased Faraday cups located on axis above the array at distances between 10 and 67 cm. Significant differences in the signals from the probes are found on varying the background atmospheric pressure between 10−2 and 10−5 Torr. At 10−5 Torr, a pulse of ions moving at 106 m/s is collected by the cups. In contrast, at intermediate (10−4 Torr) and high (10−2 Torr) pressures, both electrons and ions are detected with a velocity of order 106 m/s. The differences with pressure are ascribed to the ionization of the background gas by the tungsten ions. Time-of-flight measurements show the emission of fast ions during the ablation phase of wire array dynamics. Comparing the temporal behavior of the ions and their velocity of propagation with the dynamics observed using UltraViolet / eXtreme UltraViolet imaging allows us to conclude that the ions and the dense plasma jet are different phenomena. Moreover, the application of a transverse electric field establishes the presence of massive ions in the outflow, suggesting also the presence of MeV tungsten ions with ionization of five or less. The observation of a fast particle outflow from a conical wire array and a possible ejection mechanism is discussed.

Generation of ultra-fast cumulative water jets by sub-microsecond underwater electrical explosion of conical wire arrays

The results of experiments with underwater electrical explosion of modified conical arrays of copper and aluminum wires are presented. A pulsed generator producing a 550 kA-amplitude current with a 400 ns rise time was used in the explosion of the arrays. The array explosion generates water flows converging at the axis of the cone. This flow generates a fast-moving water jet with a velocity exceeding 1.8 × 105 cm/s, which was observed being ejected from the surface of the water covering the array. The positions of the water jet were measured by multiple-exposure fast framing imaging. In experiments, the apex angle of the array, the thickness of the water layer above the arrays, or the material of the wires was altered, which changed the resulting velocities and shapes of the emitted jets. A model that considers the converging stationary flow of a slightly compressible fluid is suggested. The velocities and shapes of the jets obtained by this model agree well with the experimentally measured jet velocities.

Generation of fast cumulative water jets by underwater electrical explosion of conical wire arrays

The results of experiments with underwater electrical explosion of conical arrays of copper wires are presented. A pulsed generator producing a 300 kA-amplitude current with a 1.2 μs rise time was used in the explosion of the arrays. As a result of the explosion, fast-moving water jets, with velocities of up to 1200 m/s, were observed being ejected from the surface of the water covering the wire array. The position of the water jets was measured by multiple-exposure fast framing imaging. The apex angle of the array or the thickness of the water layer above the arrays was altered from shot to shot, which changed the resulting velocities and shapes of the emitted jets. A numerical model, based on the models of cumulation and penetration of a jet through material of similar density, is suggested. The velocities of jets obtained by this model agree well with the experimentally observed jet velocities.

Counter-propagating plasma jet collision and shock formation on a compact current driver

In this paper we report on the ability of a compact current driver yielding 250 kA in 150 ns to produce counter-propagating plasma flows. The flows were produced by two vertically-opposed conical wire arrays separated by 1 cm, each comprised of 8 wires. With this array configuration, we were able to produce two supersonic plasma jets with velocities on the order of 100–200 km/s that propagate towards each other and collide. Aluminum wires were tested first; we observed a shock wave forming at the collision region that remained stationary for an extended period of time (∼50 ns) using optical probing diagnostics and Extreme Ultraviolet imaging. After this period, a bow shock is formed that propagates at 20 km/s towards the cathode of the array, likely due to small differences in the density and/or speed of the jets. The inter-jet ion mean free path was estimated to be larger than the shock scale length for aluminum, indicating that the shock is not mediated by collisions, but possibly by a magnetic field, whose potential sources are also discussed. Radiative cooling and density contrast between the jets were found to be important in the shock wave dynamics. We studied the importance of these effects by colliding jets of two different materials, using aluminum in one and copper in the other. In this configuration, the bow shock was observed to collapse into a thin shell and then to fragment, forming clumpy features. Simultaneously, the tip of the bow shock is seen to narrow as the bow shock moves at a similar speed observed in the Al–Al case. We discuss the similarity criteria for scaling astrophysical objects to the laboratory, finding that the dimensionless numbers are promising.

Interferometric Characterization of Laboratory Plasma Astrophysical Jets Produced by a 1-<inline-formula> <tex-math notation="TeX">\(\mu \) </tex-math></inline-formula>s Pulsed Power Driver

A high current driver based on microsecond LTD technology has been used to perform laboratory plasma astrophysics studies using a conical wire array load coupled a 950 kA, 1.2-μs pulsed power generator. A plasma jet is generated as a result of the on-axis shock formed by the ablation streams from the wires of a conical tungsten wire-array load together with conservation of the axial momentum. The aim of this paper is to produce a scaled-down laboratory simulation of astrophysical Herbig-Haro plasma jets occurring during star formation along with some of their interactions with the interstellar medium, such as a crosswind. Due to the relatively long duration of the current pulse delivered by the driver, the jet develops on a 2-μs timescale and grows up to 100 mm. A time-resolved laser interferometer has been fielded to measure the plasma areal electron density as a function of time in and around the plasma jets. The setup consists of a continuous diode-pumped solid state laser (5 W-532 nm), a Mach-Zehnder interferometer and fast gated visible multiframe camera.

Study of the implosion characteristics of quasi-spherical wire arrays on the Angara-5-1 facility at currents of up to 4 MA

Results are presented from experimental studies of the spatial distribution of the density of matter in the central part of the discharge gap and the formation of the temporal profile of the X-ray power in the course of implosion of quasi-spherical wire arrays at discharge currents of up to 4 MA. The spatial distribution of the X-ray intensity in the central part of the discharge gap and the temporal profile of the X-ray power are used as implosion characteristics of quasi-spherical wire arrays. The quasi-spherical arrays were formed by the radial stretching of unstrained wires of initially cylindrical and conical wire arrays under the action of the electrostatic field. The temporal profile of the output X-ray pulse in the photon energy range of 0.1–1 keV is shown to depend on both the geometrical parameters of the quasi-spherical array and the longitudinal distribution of its mass. It is found that a 40% increase in the wire mass due to deposition of an additional mass in the equatorial region of a quasi-spherical array leads to a 15% increase in the average current radius of the pinch and a 30% decrease in the X-ray yield. Experiments with quasi-spherical arrays made of kapron fibers with deposited Al and Bi conducting layers were also carried out. It is demonstrated that application of such arrays makes it possible to control the profile and duration of the generated X-ray pulse by varying the mass, material, and location of the deposited layer. It is found that deposition of an additional mass in the form of a thin Bi stripe on tungsten wires near the cathode end of the array allows one to mitigate the influence of the cathode zipper effect on the pinch compression and formation of the X-ray pulse in tungsten arrays.

Implosion characteristics of conical wirearray Z pinches on Qiangguang1 facility

Implosion and radiation characteristics of conical wire array in different regimes of implosion are investigated at the 1 MA Qiangguang-1 facility by X-ray self-emission diagnostics. Experimental results show that the shape of precursor plasma is also conical. The initial implosion takes place near the cathode by a stronger Lorenz force, forming a triangle-like structure that produces harder X ray. The current crossing the bubble in the front of the wire array near the anode persists in persist ablating, thereby leading to a zippered stagnation. The initial implosion of a bubble near the cathode is followed by the implosion zippering upwards as the zipper implosion speed is 2.59 107 cm/s. The results shows that the conical array has a broader X-ray power pulse than cylinderical wire array due to the zippered stagnation and zippered implosion of the array. The study of implosion characteristics of specialized axis symmetric wire array Z-pinch-like conical wire array can provide a valuable test bed for understanding the regime of implosion as well as theoretical model and magneto-hydrodynamic (MHD) codes, both of which primarily develop for the modeling of cylindrical wire array Z pinches.

250 kA compact linear transformer driver for wire arrayz-pinch loads

We present the application of a short rise ($\ensuremath{\sim}150\text{ }\text{ }\mathrm{ns}$) 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 $\ensuremath{\sim}200\text{ }\text{ }\mathrm{kA}$ 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.

Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator

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.

Generation of Ultrafast Jets Through the Implosion of a Nested Conical Wire Array

It is shown that in contrast to the electric pulse power driven implosion of a single conical wire array, the implosion of a nested conical wire array with opposite alternate opening angles can lead to the generation of fast jets, with velocities of the order 108 cm/s. This technique can be applied for the supersonic shear flow stabilization of a dense z-pinch, but possibly also for the fast ignition of a pre-compressed dense deuterium–tritium target.

Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator

AbstractWith the aim to model jets produced by conical wire arrays on the MAGPIE generator, and to strengthen the link between laboratory and astrophysical jets, we performed three-dimensional magneto-hydro-dynamic numerical simulations using the code GORGON and successfully reproduced the experiments. We found that a minimum resolution of ~100 μm is required to retrieve the unstable character of the jet. Moreover, arrays with less wires produce more unstable jets with stronger magnetic fields around them.

Radiation from wire arrays imploding on an axial target

Adding thin wires on the axis of cylindrical and conical wire arrays significantly affected the x-ray emission of the conical wire arrays, while having practically no effect on that of the cylindrical ones. The experimental study was performed using Al wire arrays with central Al and Cu wires on the 1 MA Zebra Z-pinch generator at the Nevada Terawatt Facility. Adding wires on the axis of cylindrical wire arrays led to a decrease in plasma temperature and produced practically no change in the total radiation yield. In contrast, for conical wire arrays, both the plasma temperature and the total radiation yield increased approximately proportional with the central wire thickness. Both effects were stronger for Cu center wires than for Al center wires. When using Cu, radiative cooling played a substantial role in the implosion dynamics, leading to a more compact pinch than for Al wires. Adding Al wires on the axis of conical arrays produced a cocoon-shaped emissivity distribution, with a minimum on axis. For the case of Cu wires, a short-lived, straight, and narrow filament emitting photons with energy greater than 2.5 keV formed on the axis of the cocoon. This hot filament is produced by a transient heating mechanism intense enough to fully ionize Al, and the cocoon represents plasma heated by a conical shock formed in the conical array.

Reduction of instabilities in wire arrays Z-pinches

Sheared flows have been predicted to stabilize the kink instability that disrupts the Z-pinch. Experiments conducted at the Nevada Terawatt Facility used conical wire arrays with the addition of a wire on axis to create a sheared flow. Multiframe laser images and time integrated pinhole images suggest that the sheared flow generated in this configuration is sufficient to suppress the kink instability.