Guy Burdiak

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Ablation Flow Interactions in Wire Array Z-Pinches on the MAGPIE generator

We present the results of experiments investigating the interactions of ablations streams in alum... more We present the results of experiments investigating the interactions of ablations streams in aluminium and tungsten wire array z-pinches. These experiments were carried out on the 1.4MA, 240ns MAGPIE generator at Imperial College London. The primary diagnostics used for these studies were an optical Thomson scattering diagnostic and an end-on aligned, two colour, Mach-Zehnder imaging interferometer. In aluminum arrays, the interactions of the ablation flows produces a dense network of oblique shocks. Measurements of the geometry of these shocks allows us to place limits on the plasma parameters of the flows. In tungsten arrays the data shows a prolonged period of collisionless flow. No shock structures were observed, the flow densities varied smoothly between the ablation streams and the inter-wire regions. The region about the axis appears azimuthally isotropic, and Thomson scattering measurements indicate significant interpenetration of the flows in this region.

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Cylindrically converging radiative shocks in noble gases driven by the MAGPIE pulsed-power device

tal data from gas-filled cylindrical liner z-pinch experiments are presented. The current dischar... more tal data from gas-filled cylindrical liner z-pinch experiments are presented. The current discharge from the MAGPIE pulsed-power device at Imperial College London (1.4MA,240ns) is applied to a thin walled (80µm) Al tube with a static gas-fill inside (initial gas density 10-5 g/cc). The system is used to drive cylindrically converging strong shock waves (U s = 20km/s) into different gases. Axial diagnostics include interferometry, optical streak photography and time gated, spatially resolved optical spectroscopy. The experimental geometry is nominally uniform along the diagnostic line of sight and in addition the shock waves show a high degree of azimuthal symmetry. This allows determination of the radial dependence of axially averaged plasma parameters (n e ,T e ). The spectroscopy diagnostic is used to determine the temperature profile across the shock (in the precursor and post-shock regions) in different noble gases. Comparisons are made between experimental temperature and electron density profiles and the 1D radiation-MHD code HELIOS-CR. In addition, varying degrees of shock stability are seen in different noble gases. These observations will be briefly compared to cooling function calculations and analytical stability models.

The effect of advected magnetic fields in jet propagation experiments

Astrophysical jets are a common feature during star formation. It is believed that magnetic field... more Astrophysical jets are a common feature during star formation. It is believed that magnetic fields play a key role in the formation of these jets, however the interaction of the jet with the ambient medium far from the source is dominated by purely hydrodynamical processes [1]. Appropriately scaled laboratory experiments, in which the amount of advected magnetic field can be controlled, can aid our understanding of the observed complex structure of these jets. The use of pulsed power-driven ablation of conical wire arrays [2] has allowed the production of supersonic, radiatively cooled jets scalable to astrophysical systems. In some of these experiments, the advected magnetic field was dynamically significant, which was most evident in the interaction of the jet with an ambient medium [3]. To enable the full range of jet dynamics to be studied, it is desirable to be able to modify the jet magnetisation within the same experimental setup. Here we present experimental results from a n...

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Ion heating and magnetic flux pile-up in a magnetic reconnection experiment with super-Alfvénic plasma inflows

Physics of Plasmas, 2018

This work presents a magnetic reconnection experiment in which the kinetic, magnetic, and thermal... more 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 ...

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Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors

High Energy Density Physics, 2017

We present results from new experiments to study the dynamics of radiative shocks, reverse shocks... more 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.

Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

Physical Review Letters, 2017

We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driv... more 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 semi-collisional plasmoid theory.

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The effect of magnetic field orientation on the structure and interaction of magnetised bow shocks in pulsed-power driven experiments

2016 IEEE International Conference on Plasma Science (ICOPS), 2016

Summary form only given. We present results from magnetised bow shock experiments performed on th... more Summary form only given. We present results from magnetised bow shock experiments performed on the Magpie (~1 MA, 250 ns) pulsed-power facility. Bow shocks are formed around cylindrical conducting obstacles (diameter ~0.5 mm) positioned in a supersonic, super-Alfenic, plasma flow (MS > 3, MA > 2.5, Vflow ~70 km/s). This radially diverging flow is produced by an exploding wire array z-pinch, and carries an embedded azimuthal magnetic field provided by the driving current. The bow shock structure is dramatically affected by the orientation of the obstacle with respect to the advected field. When the field lines are aligned parallel to the obstacle, the bow shock structure resembles that of a hydrodynamic bow shock. However, when the field lines lie perpendicular to the obstacle, we observe an additional bow-shaped sub-shock. This sub-shock has a large opening angle and a large stand-off distance (~1.5 mm) that is several times the obstacle radius. This sub-shock is mediated by a pileup and bending of magnetic field lines about the conducting obstacle, and appears to form indirectly via the preferential stopping of magnetised electrons in the region where the magnetic field piles up.We also discuss the interaction of multiple magnetised bow shock limbs at oblique angles. The limbs have oppositely oriented magnetic fields, and the observed geometry suggests a dramatic loss of pressure in the interaction region. This is consistent with magnetic flux annihilation lowering the magnetic pressure.

Experimental investigations of ablation stream interaction dynamics in tungsten wire arrays: Interpenetration, magnetic field advection, and ion deflection

Physics of Plasmas, 2016

Experiments have been carried out to investigate the collisional dynamics of ablation streams pro... more 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 rotation imaging 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 imaging measurements 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...

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Formation of radiatively cooled, supersonically rotating, plasma flows in Z-pinch experiments: Towards the development of an experimental platform to study accretion disk physics in the laboratory

High Energy Density Physics, 2015

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Interpenetration, Deflection, and Stagnation of Cylindrically Convergent Magnetized Supersonic Tungsten Plasma Flows

Physical Review Letters, 2014

The interpenetration and interaction of supersonic, magnetized tungsten plasma flows has been dir... more The interpenetration and interaction of supersonic, magnetized tungsten plasma flows has been directly observed via spatially and temporally resolved measurements of the Thomson scattering ion feature. A novel scattering geometry allows independent measurements of the axial and radial velocity components of the ions. The plasma flows are produced via the pulsed power driven ablation of fine tungsten wires in a cylindrical wire array z pinch. Fits of the data reveal the variations in radial velocity, axial velocity, and temperature of the ion streams as they interpenetrate and interact. A previously unobserved increase in axial velocity is measured near the array axis. This may be the result of ṽ × B bending of the ion streams by a toroidal magnetic field, advected to and accumulated about the axis by the streams.

End-on laser probing of the ablation phase of wire array z-pinch implosions on the magpie generator

2011 Abstracts IEEE International Conference on Plasma Science, 2011

Abstract New end-on measurements have taken of the areal electron density distribution of wire ar... more

Development of laser based diagnostics for the investigation of wire array Z-pinch dynamics on the MAGPIE generator

2010 Abstracts IEEE International Conference on Plasma Science, 2010

Summary form only given. End on Interferometric imaging is a useful technique for diagnosing the ... more Summary form only given. End on Interferometric imaging is a useful technique for diagnosing the electron density distribution in the interior of wire array z-pinches during their ablation phase. These measurements are limited as there is often no known density reference point in the image. By using a time resolved, CW line integrated interferometry system, we can measure the electron

Cylindrical liner z-pinch experiments on the magpie generator

2012 Abstracts IEEE International Conference on Plasma Science, 2012

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Interaction of radiatively cooled plasma jets with neutral gases for laboratory astrophysics studies

High Energy Density Physics, 2013

ABSTRACT A supersonic (Mach∼2–3), radiatively cooled plasma jet is produced by the ablation of al... more ABSTRACT A supersonic (Mach∼2–3), radiatively cooled plasma jet is produced by the ablation of aluminium plasma from a radial foil, a disc subjected to a ∼1.4 MA, 250 ns current from the MAGPIE pulsed-power generator. The ablated plasma converges on axis, producing a steady and collimated jet with axial velocities reaching ∼100 km/s. The study of jet-ambient interactions is achieved by introducing a neutral gas above the foil using a fast valve with a supersonic gas nozzle. The system has flexibility to study different interaction geometries in order to vary critical dimensionless parameters for astrophysical studies. In particular the effects of radiative cooling on the working surface of the jet are strongly affected by varying the gas composition. Experimental results are compared to numerical simulations using the 3-D MHD code GORGON.

Radiative precursors driven by converging blast waves in noble gases

Physics of Plasmas, 2014

A detailed study of the radiative precursor that develops ahead of converging blast waves in gas-... more A detailed study of the radiative precursor that develops ahead of converging blast waves in gas-filled cylindrical liner z-pinch experiments is presented. The experiment is capable of magnetically driving 20 km s−1 blast waves through gases of densities of the order 10−5 g cm−3 (see Burdiak et al. [High Energy Density Phys. 9(1), 52–62 (2013)] for a thorough description). Data were collected for Ne, Ar, and Xe gas-fills. The geometry of the setup allows a determination of the plasma parameters both in the precursor and across the shock, along a nominally uniform line of sight that is perpendicular to the propagation of the shock waves. Radiation from the shock was able to excite NeI, ArII, and XeII/XeIII precursor spectral features. It is shown that the combination of interferometry and optical spectroscopy data is inconsistent with upstream plasmas being in LTE. Specifically, electron density gradients do not correspond to any apparent temperature change in the emission spectra. E...

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