108 results found
Walsh CA, Florido R, Bailly-Grandvaux M, et al., 2022, Exploring extreme magnetization phenomena in directly driven imploding cylindrical targets, Plasma Physics and Controlled Fusion, Vol: 64, Pages: 1-19, ISSN: 0741-3335
This paper uses extended-magnetohydrodynamics (MHD) simulations to explore an extreme magnetized plasma regime realizable by cylindrical implosions on the OMEGA laser facility. This regime is characterized by highly compressed magnetic fields (greater than 10 kT across the fuel), which contain a significant proportion of the implosion energy and induce large electrical currents in the plasma. Parameters governing the different magnetization processes such as Ohmic dissipation and suppression of instabilities by magnetic tension are presented, allowing for optimization of experiments to study specific phenomena. For instance, a dopant added to the target gas-fill can enhance magnetic flux compression while enabling spectroscopic diagnosis of the imploding core. In particular, the use of Ar K-shell spectroscopy is investigated by performing detailed non-LTE atomic kinetics and radiative transfer calculations on the MHD data. Direct measurement of the core electron density and temperature would be possible, allowing for both the impact of magnetization on the final temperature and thermal pressure to be obtained. By assuming the magnetic field is frozen into the plasma motion, which is shown to be a good approximation for highly magnetized implosions, spectroscopic diagnosis could be used to estimate which magnetization processes are ruling the implosion dynamics; for example, a relation is given for inferring whether thermally driven or current-driven transport is dominating.
Suzuki-Vidal F, Clayson T, Stehlé C, et al., 2021, First radiative shock experiments on the SG-II laser, High Power Laser Science and Engineering, Vol: 9, ISSN: 2095-4719
We report on the design and first results from experiments looking at theformation of radiative shocks on the Shenguang-II (SG-II) laser at the ShanghaiInstitute of Optics and Fine Mechanics in China. Laser-heating of a two-layerCH/CH-Br foil drives a $\sim$40 km/s shock inside a gas-cell filled with argonat an initial pressure of 1 bar. The use of gas-cell targets with large(several mm) lateral and axial extent allows the shock to propagate freelywithout any wall interactions, and permits a large field of view to imagesingle and colliding counter-propagating shocks with time resolved,point-projection X-ray backlighting ($\sim20$ $\mu$m source size, 4.3 keVphoton energy). Single shocks were imaged up to 100 ns after the onset of thelaser drive allowing to probe the growth of spatial non-uniformities in theshock apex. These results are compared with experiments looking atcounter-propagating shocks, showing a symmetric drive which leads to acollision and stagnation from $\sim$40 ns onward. We present a preliminarycomparison with numerical simulations with the radiation hydrodynamics codeARWEN, which provides expected plasma parameters for the design of futureexperiments in this facility.
Suzuki-Vidal F, Li Y, Kuranz C, et al., 2019, Editorial review of HPLSE special issue on laboratory astrophysics, High Power Laser Science and Engineering, Vol: 7, ISSN: 2095-4719
Garcia-Senz D, Velarde P, Suzuki-Vidal F, et al., 2019, Interaction of hemispherical blast waves with inhomogeneous spheres: Probing the collision of a supernova ejecta with a nearby companion star in the laboratory, Astrophysical Journal, Vol: 871, ISSN: 0004-637X
Past laboratory experiments at high energy density have provided insights into the physics of supernovae, supernova remnants, and the destruction of interstellar clouds. In a typical experimental setting, a laser-driven planar blast wave interacts with a compositionally homogeneous spherical or cylindrical target. In this work we propose a new laboratory platform that accounts for curvature of the impacting shock and density stratification of the target. Both characteristics reflect the conditions expected to exist shortly after a supernova explosion in a close binary system. We provide details of a proposed experimental design (laser drive, target configuration, diagnostic system), optimized to capture the key properties of recent ejecta–companion interaction models. Good qualitative agreement found between our experimental models and their astrophysical counterparts highlights the strong potential of the proposed design to probe details of the ejecta–companion interaction for broad classes of objects by means of laboratory experiments at high energy density.
Suzuki Vidal F, Clayson T, Lebedev S, et al., 2018, Inverse liner z-pinch: an experimental pulsed power platform for studying radiative shocks, IEEE Transactions on Plasma Science, Vol: 46, Pages: 3734-3740, ISSN: 0093-3813
We present a new experimental platform for studying radiative shocks using an ``inverse liner z-pinch'' configuration. This platform was tested on the MAGPIE pulsed power facility (~1 MA with a rise time of ~240 ns) at Imperial College London, U.K. Current is discharged through a thin-walled metal tube (a liner) embedded in a low-density gas-fill and returned through a central post. The resulting magnetic pressure inside the liner launched a cylindrically symmetric, expanding radiative shock into the gas-fill at ~10 km/s. This experimental platform provides good diagnostic access, allowing multiframe optical self-emission imaging, laser interferometry, and optical emission spectrography to be fielded. Results from experiments with an Argon gas-fill initially at 0.04 mg/cm³are presented, demonstrating the successful production of cylindrically symmetric, expanding shocks that exhibit radiative effects such as the formation of a radiative precursor.
Rodriguez R, Espinosa G, Gil JM, et al., 2018, Analysis of microscopic properties of radiative shock experiments performed at the Orion laser facility, High Power Laser Science and Engineering, Vol: 6, Pages: e36-e36, ISSN: 2095-4719
In this work we have conducted a study of the radiative and spectroscopic properties of the radiative precursor and the post-shock region from experiments with radiative shocks in xenon performed at the Orion laser facility. The study is based on post-processing of radiation hydrodynamics simulations of the experiment. In particular, we have analyzed the thermodynamic regime of the plasma, the charge state distributions, the monochromatic opacities and emissivities, and the specific intensities for plasma conditions of both regions. The study of the intensities is a useful tool to estimate ranges of electron temperatures present in the xenon plasma in these2experiments and the analysis performed of the microscopic properties commented above helps to better understand the intensity spectra. Finally, a theoretical analysis of the possibility of the onset of isobaric thermal instabilities in the post-shock has been made, concluding that the instabilities obtained in the radiative hydrodynamic simulations could be thermal ones due to strong radiative cooling.
Rose SJ, Santos JJ, Bailly-Grandvaux M, et al., 2018, Laser-driven strong magnetostatic fields with applications to charged beam transport and magnetized high energy-density physics, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
Powerful laser-plasma processes are explored to generate discharge currents of a few 100 kA in coil targets,yielding magnetostatic fields (B-fields) in excess of 0.5 kT. The quasi-static currents are provided from hotelectron ejection from the laser-irradiated surface. According to our model, which describes the evolution ofthe discharge current, the major control parameter is the laser irradianceIlasλ2las. The space-time evolutionof the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and by proton-deflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targetsthrough resistive diffusion. We applied it in experiments of laser-generated relativistic electron transportthrough solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at60μm depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetizedhigh-energy density physics investigations, related to laser-generated secondary sources of radiation and/orhigh-energy particles and their transport, to high-gain fusion energy schemes and to laboratory astrophysics.
Suttle LG, Hare JD, Lebedev SV, et al., 2018, Ion heating and magnetic flux pile-up in a magnetic reconnection experiment with super-Alfvenic plasma inflows, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
This work presents a magnetic reconnection experiment in which the kinetic, magnetic, and thermal properties of the plasma each play an important role in the overall energy balance and structure of the generated reconnection layer. Magnetic reconnection occurs during the interaction of continuous and steady flows of super-Alfvénic, magnetized, aluminum plasma, which collide in a geometry with two-dimensional symmetry, producing a stable and long-lasting reconnection layer. Optical Thomson scattering measurements show that when the layer forms, ions inside the layer are more strongly heated than electrons, reaching temperatures of Ti∼Z⎯⎯⎯Te≳300 eV—much greater than can be expected from strong shock and viscous heating alone. Later in time, as the plasma density in the layer increases, the electron and ion temperatures are found to equilibrate, and a constant plasma temperature is achieved through a balance of the heating mechanisms and radiative losses of the plasma. Measurements from Faraday rotation polarimetry also indicate the presence of significant magnetic field pile-up occurring at the boundary of the reconnection region, which is consistent with the super-Alfvénic velocity of the inflows.
Hare J, Suttle L, Lebedev S, et al., 2018, An experimental platform for pulsed-power driven magnetic reconnection, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
We describe a versatile pulsed-power driven platform for magnetic reconnection experiments, based on the exploding wire arrays driven in parallel [Suttle et al., Phys. Rev. Lett. 116, 225001 (2016)]. This platform produces inherently magnetised plasma flows for the duration of the generator current pulse (250 ns), resulting in a long-lasting reconnection layer. The layer exists for long enough to allow the evolution of complex processes such as plasmoid formation and movement to be diagnosed by a suite of high spatial and temporal resolution laser-based diagnostics. We can access a wide range of magnetic reconnection regimes by changing the wire material or moving the electrodes inside the wire arrays. We present results with aluminium and carbon wires, in which the parameters of the inflows and the layer that forms are significantly different. By moving the electrodes inside the wire arrays, we change how strongly the inflows are driven. This enables us to study both symmetric reconnection in a range of different regimes and asymmetric reconnection.
Spindloe C, Wyatt D, Astbury S, et al., 2017, Design and fabrication of gas cell targets for laboratory astrophysics experiments on the Orion high-power laser facility, High Power Laser Science and Engineering, Vol: 5, ISSN: 2095-4719
This paper describes the design and fabrication of a range of ‘gas cell’ microtargets produced by the Target FabricationGroup in the Central Laser Facility (CLF) for academic access experiments on the Orion laser facility at the AtomicWeapons Establishment (AWE). The experiments were carried out by an academic consortium led by Imperial CollegeLondon. The underlying target methodology was an evolution of a range of targets used for experiments on radiativeshocks and involved the fabrication of a precision machined cell containing a number of apertures for interaction foilsor diagnostic windows. The interior of the cell was gas-filled before laser irradiation. This paper details the assemblyprocesses, thin film requirements and micro-machining processes needed to produce the targets. Also described is theimplementation of a gas-fill system to produce targets that are filled to a pressure of 0.1–1 bar. The paper discusses thechallenges that are posed by such a target.
Hare JD, Lebedev SV, Suttle LG, et al., 2017, Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments, Physics of Plasmas, Vol: 24, ISSN: 1070-664X
We describe magnetic reconnection experiments using a new, pulsed-powerdriven experimental platform in which the inflows are super-sonic butsub-Alfv\'enic.The intrinsically magnetised plasma flows are long lasting,producing a well-defined reconnection layer that persists over manyhydrodynamic time scales.The layer is diagnosed using a suite of highresolution laser based diagnostics which provide measurements of the electrondensity, reconnecting magnetic field, inflow and outflow velocities and theelectron and ion temperatures.Using these measurements we observe a balancebetween the power flow into and out of the layer, and we find that the heatingrates for the electrons and ions are significantly in excess of the classicalpredictions. The formation of plasmoids is observed in laser interferometry andoptical self-emission, and the magnetic O-point structure of these plasmoids isconfirmed using magnetic probes.
Suzuki Vidal F, Clayson T, Swadling GF, et al., 2017, Counter-propagating radiative shock experiments on the Orion laser, Physical Review Letters, Vol: 119, ISSN: 1079-7114
We present new experiments to study the formation of radiative shocks and the interaction between two counterpropagating radiative shocks. The experiments are performed at the Orion laser facility, which is used to drive shocks in xenon inside large aspect ratio gas cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently three-dimensional shocks, provides a novel platform particularly suited for the benchmarking of numerical codes. The dynamics of the shocks before and after the collision are investigated using point-projection x-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modeling of the experiments using the 2D radiation hydrodynamic codes nym and petra shows very good agreement with the experimental results.
Burdiak GC, Lebedev SV, Bland SN, et al., 2017, The structure of bow shocks formed by the interaction of pulsed-power driven magnetised plasma flows with conducting obstacles, PHYSICS OF PLASMAS, Vol: 24, ISSN: 1070-664X
We present an experimental study of the development and structure of bow shocks produced by the interaction of a magnetised, collisional, super-Alfvénic plasma flow with conducting cylindrical obstacles. The plasma flow with an embedded, frozen-in magnetic field (ReM ∼ 20) is produced by the current-driven ablation of fine aluminium wires in an inverse, exploding wire array z-pinch. We show that the orientation of the embedded field with respect to the obstacles has a dramatic effect on the bow shock structure. When the field is aligned with the obstacle, a sharp bow shock is formed with a global structure that is determined simply by the fast magneto-sonic Mach number. When the field is orthogonal to the obstacle, magnetic draping occurs. This leads to the growth of a magnetic precursor and the subsequent development of a magnetised bow shock that is mediated by two-fluid effects, with an opening angle and a stand-off distance, that are both many times larger than in the parallel geometry. By changing the field orientation, we change the fluid regime and physical mechanisms that are responsible for the development of the bow shocks. MHD simulations show good agreement with the structure of well-developed bow shocks. However, collisionless, two-fluid effects will need to be included within models to accurately reproduce the development of the shock with an orthogonal B-field.
Singh RL, Stehle C, Suzuki-Vidal F, et al., 2017, Experimental study of the interaction of two laser-driven radiative shocks at the PALS laser, HIGH ENERGY DENSITY PHYSICS, Vol: 23, Pages: 20-30, ISSN: 1574-1818
Clayson T, Suzuki-Vidal F, Lebedev SV, et al., 2017, Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors, High Energy Density Physics, Vol: 23, Pages: 60-72, ISSN: 1878-0563
We present results from new experiments to study the dynamics of radiative shocks, reverse shocks and radiative precursors. Laser ablation of a solid piston by the Orion high-power laser at AWE Aldermaston UK was used to drive radiative shocks into a gas cell initially pressurised between 0.1 and 1.0 bar with different noble gases. Shocks propagated at 80 ± 10 km/s and experienced strong radiative cooling resulting in post-shock compressions of ×25 ± 2. A combination of X-ray backlighting, optical self-emission streak imaging and interferometry (multi-frame and streak imaging) were used to simultaneously study both the shock front and the radiative precursor. These experiments present a new configuration to produce counter-propagating radiative shocks, allowing for the study of reverse shocks and providing a unique platform for numerical validation. In addition, the radiative shocks were able to expand freely into a large gas volume without being confined by the walls of the gas cell. This allows for 3-D effects of the shocks to be studied which, in principle, could lead to a more direct comparison to astrophysical phenomena. By maintaining a constant mass density between different gas fills the shocks evolved with similar hydrodynamics but the radiative precursor was found to extend significantly further in higher atomic number gases (∼4 times further in xenon than neon). Finally, 1-D and 2-D radiative-hydrodynamic simulations are presented showing good agreement with the experimental data.
Espinosa G, Rodriguez R, Gil JM, et al., 2017, Influence of atomic kinetics in the simulation of plasma microscopic properties and thermal instabilities for radiative bow shock experiments, PHYSICAL REVIEW E, Vol: 95, ISSN: 2470-0045
Hare JD, Suttle L, Lebedev SV, et al., 2017, Anomalous heating and plasmoid formation in a driven magnetic reconnection experiment, Physical Review Letters, Vol: 118, ISSN: 0031-9007
We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B=3 T), advected by supersonic, sub-Alfvénic carbon plasma flows (Vin=50 km/s), are brought together and mutually annihilate inside a thin current layer (δ=0.6 mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (Te=100 eV, Ti=600 eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.
Suttle LG, Hare JD, Lebedev SV, et al., 2016, Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We present experiments characterizing the detailed structure of a current layer, generated by the collision of two counterstreaming, supersonic and magnetized aluminum plasma flows. The antiparallel magnetic fields advected by the flows are found to be mutually annihilated inside the layer, giving rise to a bifurcated current structure—two narrow current sheets running along the outside surfaces of the layer. Measurements with Thomson scattering show a fast outflow of plasma along the layer and a high ion temperature (Ti∼Z¯Te, with average ionization Z¯=7). Analysis of the spatially resolved plasma parameters indicates that the advection and subsequent annihilation of the inflowing magnetic flux determines the structure of the layer, while the ion heating could be due to the development of kinetic, current-driven instabilities.
Swadling GF, Lebedev SV, Hall GN, et al., 2016, Experimental investigations of ablation stream interaction dynamics in tungsten wire arrays: interpenetration, magnetic field advection, and ion deflection, Physics of Plasmas, Vol: 23, ISSN: 1089-7674
Experiments have been carried out to investigate the collisional dynamics of ablation streams produced by cylindrical wire array z-pinches. A combination of laser interferometric imaging, Thomson scattering, and Faraday rotationimaging has been used to make a range of measurements of the temporal evolution of various plasma and flow parameters. This paper presents a summary of previously published data, drawing together a range of different measurements in order to give an overview of the key results. The paper focuses mainly on the results of experiments with tungsten wire arrays. Early interferometric imagingmeasurements are reviewed, then more recent Thomson scattering measurements are discussed; these measurements provided the first direct evidence of ablation stream interpenetration in a wire array experiment. Combining the data from these experiments gives a view of the temporal evolution of the tungsten stream collisional dynamics. In the final part of the paper, we present new experimental measurements made using an imagingFaraday rotationdiagnostic. These experiments investigated the structure of magnetic fields near the array axis directly; the presence of a magnetic field has previously been inferred based on Thomson scattering measurements of ion deflection near the array axis. Although the Thomson and Faradaymeasurements are not in full quantitative agreement, the Faraday data do qualitatively supports the conjecture that the observed deflections are induced by a static toroidal magnetic field, which has been advected to the array axis by the ablation streams. It is likely that detailed modeling will be needed in order to fully understand the dynamics observed in the experiment.
Haerendel G, Suttle L, Lebedev SV, et al., 2016, Stop layer: a flow braking mechanism in space and support from a lab experiment, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587
The paper presents short summaries and a synopsis of two completely independent discoveries of a fast flow braking process, one realized by a laboratory experiment (Lebedev et al 2014 Phys. Plasmas 21 056305), the other by theoretical reasoning stimulated by auroral observation (Haerendel 2015a J. Geophys. Res. Space Phys. 120 1697–714). The first has been described as a magnetically mediated sub-shock forming when a supersonic plasma flow meets a wall. The second tried to describe what happens when a high-beta plasma flow from the central magnetic tail meets the strong near-dipolar field of the magnetosphere. The term stop layer signals that flow momentum and energy are directly coupled to a magnetic perturbation field generated by a Hall current within a layer of the width of c/ω pi and immediately propagated out of the layer by kinetic Alfvén waves. As the laboratory situation is not completely collision-free, energy transfer from ions to electrons and subsequent radiative losses are likely to contribute. A synopsis of the two situations identifies and discusses six points of commonality between the two situations. It is pointed out that the stop layer mechanism can be regarded as a direct reversal of the reconnection process.
Burdiak GC, Lebedev SV, Clayson T, et al., 2016, THE EFFECT OF MAGNETIC FIELD ORIENTATION ON THE STRUCTURE AND INTERACTION OF MAGNETISED BOW SHOCKS IN PULSED-POWER DRIVEN EXPERIMENTS, 43rd IEEE International Conference on Plasma Science (ICOPS), Publisher: IEEE
Burdiak GC, Lebedev SV, Clayson T, et al., 2016, LABORATORY ASTROPHYSICS WITH SUPERSONIC MAGNETISED PLASMAS: EXPERIMENTS ON THE MAGPIE PULSED-POWER FACILITY, 43rd IEEE International Conference on Plasma Science (ICOPS), Publisher: IEEE
Suzuki-Vidal F, Lebedev SV, Ciardi A, et al., 2015, BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS, Astrophysical Journal, Vol: 815, ISSN: 1538-4357
The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counterstreaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame, and the experiments are driven over many times the characteristic cooling timescale. The initially smooth bow shock rapidly develops small-scale nonuniformities over temporal and spatial scales that are consistent with a thermal instability triggered by strong radiative cooling in the shock. The growth of these perturbations eventually results in a global fragmentation of the bow shock front. The formation of a thermal instability is supported by analysis of the plasma cooling function calculated for the experimental conditions with the radiative packages ABAKO/RAPCAL.
Cotelo M, Velarde P, de la Varga AG, et al., 2015, Simulation of radiative shock waves in Xe of last PALS experiments, HIGH ENERGY DENSITY PHYSICS, Vol: 17, Pages: 68-73, ISSN: 1574-1818
Larour J, Singh RL, Stehle C, et al., 2015, Optimization of an electromagnetic generator for strong shocks in low pressure gas, HIGH ENERGY DENSITY PHYSICS, Vol: 17, Pages: 129-134, ISSN: 1574-1818
Burdiak GC, Lebedev SV, Harvey-Thompson AJ, et al., 2015, Characterisation of the current switch mechanism in two-stage wire array Z-pinches, Physics of Plasmas, Vol: 22, ISSN: 1089-7674
In this paper, we describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA,240 ns rise-time Magpie pulsed-power device at Imperial College London. In this setup, an inversewire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wirearray, before rapidly switching the majority of the generator current into the load after a100–150 ns dwell time. A detailed analysis of the evolution of the load array during the pre-pulse ispresented. Measurements of the load resistivity and energy deposition suggest significant bulk heatingof the array mass occurs. The 5 kA pre-pulse delivers 0.8 J of energy to the load, leaving itin a mixed, predominantly liquid-vapour state. The main current switch occurs as the inverse arraybegins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicatethat the main current switch may evolve in part as a plasma flow switch, driven by the expansionof a magnetic cavity and plasma bubble along the length of the load array. Analysis ofimplosion trajectories suggests that approximately 1 MA switches into the load in 100 ns, correspondingto a doubling of the generator dI/dt. Potential scaling of the device to higher currentmachines is discussed. V
Ampleford DJ, Bland SN, Jennings CA, et al., 2015, Investigating Radial Wire Array Z-Pinches as a Compact X-Ray Source on the Saturn Generator, IEEE TRANSACTIONS ON PLASMA SCIENCE, Vol: 43, Pages: 3344-3352, ISSN: 0093-3813
Swadling GF, Hall GN, Lebedev SV, et al., 2015, Commissioning of a Rotated Wire Array Configuration for Improved Diagnostic Access, IEEE TRANSACTIONS ON PLASMA SCIENCE, Vol: 43, Pages: 2503-2508, ISSN: 0093-3813
Veloso F, Munoz-Cordovez G, Donoso-Tapia L, et al., 2015, Ablation dynamics in wire array Z-pinches under modifications on global magnetic field topology, Physics of Plasmas, Vol: 22, ISSN: 1089-7674
The dynamics of ablation streams and precursor plasma in cylindrical wire array Z-pinches under temporal variations of the global magnetic field topology is investigated through experiments and numerical simulations. The wire arrays in these experiments are modified by replacing a pair of consecutive wires with wires of a larger diameter. This modification leads to two separate effects, both of which impact the dynamics of the precursor plasma; firstly, current is unevenly distributed between the wires and secondly, the thicker wires take longer to fully ablate. The uneven distribution of current is evidenced in the experiments by the drift of the precursor off axis due to a variation in the global magnetic field topology which modifies the direction of the ablation streams tracking the precursor position. The variation of the global magnetic field due to the presence of thick wires is studied with three-dimensional magnetohydrodynamic (MHD) simulations, showing that the global field changes from the expected toroidal field to a temporally variable topology after breakages appear in the thin wires. This leads to an observed acceleration of the precursor column towards the region closer to the thick wires and later, when thick wires also present breakages, it continues moving away from the original array position as a complicated and disperse object subject to MHD instabilities.
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