Publications
12 results found
Strucka J, Halliday JWD, Gheorghiu T, et al., 2022, A portable X-pinch design for x-ray diagnostics of warm dense matter, Matter and Radiation at Extremes, Vol: 7, Pages: 1-11, ISSN: 2468-080X
We describe the design and x-ray emission properties (temporal, spatial, and spectral) of Dry Pinch I, a portable X-pinch driver developed at Imperial College London. Dry Pinch I is a direct capacitor discharge device, 300 × 300 × 700 mm3 in size and ∼50 kg in mass, that can be used as an external driver for x-ray diagnostics in high-energy-density physics experiments. Among key findings, the device is shown to reliably produce 1.1 ± 0.3 ns long x-ray bursts that couple ∼50 mJ of energy into photon energies from 1 to 10 keV. The average shot-to-shot jitter of these bursts is found to be 10 ± 4.6 ns using a combination of x-ray and current diagnostics. The spatial extent of the x-ray hot spot from which the radiation emanates agrees with previously published results for X-pinches—suggesting a spot size of 10 ± 6 µm in the soft energy region (1–10 keV) and 190 ± 100 µm in the hard energy region (>10 keV). These characteristics mean that Dry Pinch I is ideally suited for use as a probe in experiments driven in the laboratory or at external facilities when more conventional sources of probing radiation are not available. At the same time, this is also the first detailed investigation of an X-pinch operating reliably at current rise rates of less than 1 kA/ns.
Suttle LG, Hare JD, Halliday JWD, et al., 2021, Collective optical Thomson scattering in pulsed-power driven high energy density physics experiments (invited), Review of Scientific Instruments, Vol: 92, Pages: 033542-1-033542-8, ISSN: 0034-6748
Optical collective Thomson scattering (TS) is used to diagnose magnetized high energy density physics experiments at the Magpie pulsedpower generator at Imperial College London. The system uses an amplified pulse from the second harmonic of a Nd:YAG laser (3 J, 8 ns, 532 nm) to probe a wide diversity of high-temperature plasma objects, with densities in the range of 1017–1019 cm−3 and temperatures between 10 eV and a few keV. The scattered light is collected from 100 μm-scale volumes within the plasmas, which are imaged onto optical fiber arrays. Multiple collection systems observe these volumes from different directions, providing simultaneous probing with different scattering K-vectors (and different associated α-parameters, typically in the range of 0.5–3), allowing independent measurements of separate velocity components of the bulk plasma flow. The fiber arrays are coupled to an imaging spectrometer with a gated intensified charge coupled device. The spectrometer is configured to view the ion-acoustic waves of the collective Thomson scattered spectrum. Fits to the spectra with the theoretical spectral density function S(K, ω) yield measurements of the local plasma temperatures and velocities. Fitting is constrained by independent measurements of the electron density from laser interferometry and the corresponding spectra for different scattering vectors.This TS diagnostic has been successfully implemented on a wide range of experiments, revealing temperature and flow velocity transitions across magnetized shocks, inside rotating plasma jets and imploding wire arrays, as well as providing direct measurements of drift velocitiesinside a magnetic reconnection current sheet.
Hare J, Burdiak G, Merlini S, et al., 2021, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Review of Scientific Instruments, Vol: 92, ISSN: 0034-6748
We report on a recently developed laser-probing diagnostic which allows direct measurements of ray-deflection anglesin one axis, whilst retaining imaging capabilities in the other axis. This allows us to measure the spectrum of angulardeflections from a laser beam which passes though a turbulent high-energy-density plasma. This spectrum containsinformation about the density fluctuations within the plasma, which deflect the probing laser over a range of angles. Wecreate synthetic diagnostics using ray-tracing to compare this new diagnostic with standard shadowgraphy and schlierenimaging approaches, which demonstrates the enhanced sensitivity of this new diagnostic over standard techniques. Wepresent experimental data from turbulence behind a reverse shock in a plasma and demonstrate that this technique canmeasure angular deflections between 0.06 and 34 mrad, corresponding to a dynamic range of over 500.
Hare JD, Burdiak GC, Merlini S, et al., 2020, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Publisher: arXiv
We report on a recently developed laser-based diagnostic which allows directmeasurements of ray-deflection angles in one axis, whilst retaining imagingcapabilities in the other axis. This allows us to measure the spectrum ofangular deflections from a laser beam which passes though a turbulenthigh-energy-density plasma. This spectrum contains information about thedensity fluctuations within the plasma, which deflect the probing laser over arange of angles. The principle of this diagnostic is described, along with ourspecific experimental realisation. We create synthetic diagnostics usingray-tracing to compare this new diagnostic with standard shadowgraphy andschlieren imaging approaches, which demonstrates the enhanced sensitivity ofthis new diagnostic over standard techniques. We present experimental data fromturbulence behind a reverse shock in a plasma and demonstrate that thistechnique can measure angular deflections between 0.05 and 34 mrad,corresponding to a dynamic range of over 500.
Suttle LG, Burdiak GC, Cheung CL, et al., 2020, Interactions of magnetized plasma flows in pulsed-power driven experiments, Plasma Physics and Controlled Fusion, Vol: 62, ISSN: 0741-3335
A supersonic flow of magnetized plasma is produced by the application of a 1 MA-peak, 500 ns current pulse to a cylindrical arrangement of parallel wires, known as an inverse wire array. The plasma flow is produced by the J × B acceleration of the ablated wire material, and a magnetic field of several Tesla is embedded at source by the driving current. This setup has been used for a variety of experiments investigating the interactions of magnetized plasma flows. In experiments designed to investigate magnetic reconnection, the collision of counter-streaming flows, carrying oppositely directed magnetic fields, leads to the formation of a reconnection layer in which we observe ions reaching temperatures much greater than predicted by classical heating mechanisms. The breakup of this layer under the plasmoid instability is dependent on the properties of the inflowing plasma, which can be controlled by the choice of the wire array material. In other experiments, magnetized shocks were formed by placing obstacles in the path of the magnetized plasma flow. The pile-up of magnetic flux in front of a conducting obstacle produces a magnetic precursor acting on upstream electrons at the distance of the ion inertial length. This precursor subsequently develops into a steep density transition via ion-electron fluid decoupling. Obstacles which possess a strong private magnetic field affect the upstream flow over a much greater distance, providing an extended bow shock structure. In the region surrounding the obstacle the magnetic pressure holds off the flow, forming a void of plasma material, analogous to the magnetopause around planetary bodies with self-generated magnetic fields.
Hare JD, MacDonald J, Bland SN, et al., 2019, Two-colour interferometry and Thomson scattering measurements of a plasma gun, Publisher: IOP PUBLISHING LTD
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Hare JD, MacDonald J, Bland S, et al., 2019, Two-colour interferometry and Thomson scattering measurements of a plasma gun, Plasma Physics and Controlled Fusion, Vol: 61, ISSN: 0741-3335
We present experimental measurements of a pulsed plasma gun, using two-colour imaging laser interferometry and spatially resolved Thomson scattering. Interferometry measurements give an electron density ne ≈ 2.7 × 1017 cm−3 at the centre of the plasma plume, at 5 mm from the plasma gun nozzle. The Thomson scattered light is collected from two probing angles allowed us to simultaneously measure the collective and non-collective spectrum of the electron feature from the same spatial locations. The inferred electron densities from the location of the electron plasma waves is in agreement with interferometry. The electron temperatures inferred from the two spectra are not consistent, with Te ≈ 10 eV for non-collective scattering and Te ≈ 30 eV for collective scattering. We discuss various broadening mechanisms such as finite aperture effects, density gradients within the collective volume and collisional broadening to account for some of this discrepancy. We also note the significant red/blue asymmetry of the electron plasma waves in the collective scattering spectra, which could relate to kinetic effects distorting the distribution function of the electrons.
Palmer CAJ, Campbell PT, Ma Y, et al., 2019, Field reconstruction from proton radiography of intense laser driven magnetic reconnection, Publisher: AIP Publishing, ISSN: 1070-664X
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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.
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.
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.
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