Experimental study of differentially rotating supersonic plasma flows produced by aluminium wire array Z-pinches

A novel approach to cylindrical wire array z-pinches has been developed in order to create a rotating plasma flow analogous to astrophysical accretion discs. The method involves subjecting the wire array to a cusp magnetic field (B_r) to create converging off axis ablation streams to form a rotating flow. The rotation is sustained by the ram pressure of the ablation streams in a quasi-equilibrium state for approximately 150 ns. This corresponds to one full rotation of the plasma about the axis. The rotating plasma is supersonic with Mach number ~2 and a radially constant rotation velocity between 60 and 75 km/s; the angular velocity therefore has an r^-1 dependence and the flow is differential. A Thomson scattering diagnostic is used to measure the electron and ion temperatures as Te ~30 eV and Ti >55 eV and the ionisation of the plasma (Z) between 6 and 8. These parameters are used to calculate the Reynolds number (10^5 to 10^6) and magnetic Reynolds numbers (20 to 100) which are large enough for viscous and resistive effects to be negligible on the large scale of the flow. These are of sufficient magnitude for the experiment to be scalable to astrophysical accretion discs. Further more the Reynolds number for the experiment is large enough for shear instabilities to manifest in the plasma. Some evidence for this can be seen in XUV images and Thomson spectra which indicate the development of perturbations and vorticity within the flow. Predictions for the growth rate of the Kelvin Helmholtz instability, 12 to 40 ns, agree reasonably well with the observed perturbation growth of ~30 ns. It is also possible that shear instabilities are driving hydrodynamic turbulence. Turbulent heating of the plasma could explain the approximately 500 eV increase in the ion temperature observed from some Thomson spectra. Further work is required however to prove the existence of shear flows and turbulence within the experiments.Open Acces

Large-scale ocean connectivity and planktonic body size

2018 Ocean Sciences Meeting, 11-16 February, in Portland, OregonGlobal patterns of planktonic species diversity are in large part determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size plays in determining spatial patterns of diversity remains unclear. Here, we analyzed the spatial community structure – β-diversity – for several planktonic and micro-nektonic organisms spanning a broad range of body sizes, from prokaryotes to small mesopelagic fishes, collected during the Malaspina 2010 Circumnavigation Expedition. Global patterns of β-diversity for these communities were then compared to surface ocean transit times, derived from a global circulation model. Our results reveal that for these communities, there is a significant negative relationship between β-diversity and surface ocean transit times, more so than with differences in environmental factors. We used these data to estimate dispersal scales of each biological group. We found a negative relationship between dispersal scale and body-size: less abundant large-bodied plankton and micro-nekton communities in near-surface epipelagic waters have shown significantly shorter dispersal scales and larger spatial species-turnover rates when compared to more abundant small-bodied plankton. These results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversityPeer Reviewe