N. Niasse

Oblique shock structures formed during the ablation phase of aluminium wire array z-pinches

A series of experiments has been conducted in order to investigate the azimuthal structures formed by the interactions of cylindrically converging plasma flows during the ablation phase of aluminium wire array Z pinch implosions. These experiments were carried out using the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. The main diagnostic used in this study was a two-colour, end-on, Mach-Zehnder imaging interferometer, sensitive to the axially integrated electron density of the plasma. The data collected in these experiments reveal the strongly collisional dynamics of the aluminium ablation streams. The structure of the flows is dominated by a dense network of oblique shock fronts, formed by supersonic collisions between adjacent ablation streams. An estimate for the range of the flow Mach number (M = 6.2-9.2) has been made based on an analysis of the observed shock geometry. Combining this measurement with previously published Thomson Scattering measurements of the plasma flow velocity by H...

The formation of reverse shocks in magnetized high energy density supersonic plasma flows

A new experimental platform was developed, based on the use of supersonic plasma flow from the ablation stage of an inverse wire array z-pinch, for studies of shocks in magnetized high energy density physics plasmas in a well-defined and diagnosable 1-D interaction geometry. The mechanism of flow generation ensures that the plasma flow (ReM ∼ 50, MS ∼ 5, MA ∼ 8, Vflow ≈ 100 km/s) has a frozen-in magnetic field at a level sufficient to affect shocks formed by its interaction with obstacles. It is found that in addition to the expected accumulation of stagnated plasma in a thin layer at the surface of a planar obstacle, the presence of the magnetic field leads to the formation of an additional detached density jump in the upstream plasma, at a distance of ∼c/ωpi from the obstacle. Analysis of the data obtained with Thomson scattering, interferometry, and local magnetic probes suggests that the sub-shock develops due to the pile-up of the magnetic flux advected by the plasma flow.

Study of micro-pinches in wire-array Z pinches

Bright and hot areas with a high plasma density and temperature are observed in all kinds of Z pinches. We studied bright radiating spots produced by micro-pinches in cylindrical and planar wire-arrays at the 1 MA Zebra pulsed power generator using an x-ray streak camera synchronized with laser diagnostics, x-ray time-gated pinhole camera, and spectroscopy. Hot spots with extremely dense and relatively hot plasma arise during the collapse of the micro-pinches. These hot spots radiate a continuum spectrum with energy >2.5 keV. Typical micro-pinches in Al wire arrays generate x-ray bursts with durations of 0.4–1 ns in the soft x-ray range and 0.1–0.4 ns in the keV range. UV two-frame shadowgraphy shows spatial correlation of hot spots with the collapse and explosion of micro-pinches. Micro-pinches typically occur at the necks of the Z pinch, but can demonstrate a variety of parameters and different dynamics. An analysis of x-ray streak images shows that micro-pinches can generate >20% of the x-ray energy in some types of wire-array Z pinches.

Signatures of asymmetry in neutron spectra and images predicted by three-dimensional radiation hydrodynamics simulations of indirect drive implosions

We present the results of 3D simulations of indirect drive inertial confinement fusion capsules driven by the “high-foot” radiation pulse on the National Ignition Facility. The results are post-processed using a semi-deterministic ray tracing model to generate synthetic deuterium-tritium (DT) and deuterium-deuterium (DD) neutronspectra as well as primary and down scatteredneutronimages. Results with low-mode asymmetries are used to estimate the magnitude of anisotropy in the neutronspectra shift, width, and shape. Comparisons of primary and down scatteredimages highlight the lack of alignment between the neutron sources,scatter sites, and detector plane, which limits the ability to infer the ρr of the fuel from a down scattered ratio. Further calculations use high bandwidth multi-mode perturbations to induce multiple short scale length flows in the hotspot. The results indicate that the effect of fluid velocity is to produce a DT neutronspectrum with an apparently higher temperature than that inferred from the DD spectrum and which is also higher than the temperature implied by the DT to DD yield ratio.

UV Laser-Probing Diagnostics for the Dense Z Pinch

Laser diagnostics at 266 nm were developed for the investigation of dense Z-pinch plasma at the 1 MA Zebra generator. A three-channel diagnostic can be configured as shadowgraphy and interferometry with two temporal frames or as a Faraday rotation polarimeter. Absorption and refraction of ultraviolet (UV) radiation in dense plasma is significantly smaller compared with regular diagnostics at the wavelength of 532 nm. Therefore, UV diagnostics allow direct investigation of the fine structure of the dense Z-pinch, development of instabilities, and a distribution of magnetic fields in Z-pinch plasma. Micropinches and instabilities with characteristic scales of 15-200 μm were observed in 1 MA wire-array Z pinches. Development of instabilities in wire-array Z pinches is in agreement with the magnetohydrodynamic simulations. Interferometry at the wavelength of 266 nm allows measurement of plasma density in the range (1-2)×1020 cm-3 in the ablating wires, imploding plasma, stagnating pinch, and trailing material. A fast plasma motion was observed at the stagnation stage with two-frame shadowgraphy. Plasma motion at stagnation and prolonged implosion of trailing mass can provide the additional kinetic energy in the Z pinch and can be a source of enhanced X-ray radiation. A Faraday rotation diagnostic reveals a distribution of magnetic fields in the pinch and trailing material. The magnetic field strength and current were reconstructed from the rotation angles and phase shifts in plasma using the Abel transform. Current in the pinch can switch from the high-inductance neck and redistribute to the trailing material when resistance of peripheral plasma drops owing to heating by X-ray radiation. Further development of UV diagnostics to short wavelengths can help to apply well-established optical methods to Z-pinch plasma in multiMA pulsed power facilities.

Magneto‐Hydrodynamic Modeling in the Design and Interpretation of Wire Array Z‐pinches

Magneto‐hydrodynamic simulations provide a powerful tool for improving our understanding of the complex physical processes underlying the behavior of wire array Z‐pinches. We show how, by using large scale parallel 3D simulations of the array as a whole, it is possible to encompass all of the important features of the wire ablation, implosion and stagnation phases and to observe how these phenomena control the X‐ray pulse that is achieved. Comparison of code results with experimental data from the ‘Z’ and MAGPIE pulsed power generators is shown to provide a detailed benchmark test for the models. The simulation results are also used to highlight key areas for future research.

Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments

We describe magnetic reconnection experiments using a new, pulsed-power driven experimental platform in which the inflows are super-sonic but sub-Alfvenic. The intrinsically magnetised plasma flows are long lasting, producing a well-defined reconnection layer that persists over many hydrodynamic time scales. The layer is diagnosed using a suite of high resolution laser based diagnostics, which provide measurements of the electron density, reconnecting magnetic field, inflow and outflow velocities, and the electron and ion temperatures. Using these measurements, we observe a balance between the power flow into and out of the layer, and we find that the heating rates for the electrons and ions are significantly in excess of the classical predictions. The formation of plasmoids is observed in laser interferometry and optical self-emission, and the magnetic O-point structure of these plasmoids is confirmed using magnetic probes.

Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

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 (V_{in}=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 (T_{e}=100  eV, T_{i}=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.

Structure of a magnetic flux annihilation layer formed by the collision of supersonic, magnetized plasma flows

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 (T_{i}∼Z[over ¯]T_{e}, with average ionization Z[over ¯]=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.

Cylindrical liner Z-pinch experiments for fusion research and high-energy-density physics

A gas-filled cylindrical liner z-pinch configuration has been used to drive convergent radiative shock waves into different gases at velocities of 20–50 km s −1 . On application of the 1.4 MA, 240 ns rise-time current pulse produced by the Magpie generator at Imperial College London, a series of cylindrically convergent shock waves are sequentially launched into the gas-fill from the inner wall of the liner. This occurs without any bulk motion of the liner wall itself. The timing and trajectories of the shocks are used as a diagnostic tool for understanding the response of the liner z-pinch wall to a large pulsed current. This analysis provides useful data on the liner resistivity, and a means to test equation of state (EOS) and material strength models within MHD simulation codes. In addition to providing information on liner response, the convergent shocks are interesting to study in their own right. The shocks are strong enough for radiation transport to influence the shock wave structure. In particular, we see evidence for both radiative preheating of material ahead of the shockwaves and radiative cooling instabilities in the shocked gas. Some preliminary results from initial gas-filled liner experiments with an applied axial magnetic field are also discussed.