Computational model of the interaction of a helium atmospheric-pressure jet with a dielectric surface

Using a time-dependent two-dimensional axisymmetric fluid model the interaction of a plasma jet with a dielectric surface has been studied. The model is solved for two consecutive periods of a positive unipolar pulsed waveform. The study concentrates on determining the fluxes of the main oxygen ion species, $\text{O}_{2}^{+}$ , $\text{O}_{2}^{-}$ and the total accumulated charge on the surface. Approaching the dielectric surface, the streamer head is seen to divert its direction of propagation, spreading out radially approximately 0.2 mm above the dielectric surface. For $\text{O}_{2}^{+}$ generated near the streamer head, this leads to a maximum in their flux to the surface which moves radially outwards with the streamer propagation, driven by the applied electric field in pulse on-time. In the off-time, the flux of $\text{O}_{2}^{+}$ drops by at least two orders of magnitude. As a result, the total number of $\text{O}_{2}^{+}$ ions arriving at the surface over one entire pulse period (fluence) has an annular shape limited by the effective contact area of the streamer on the surface. In contrast $\text{O}_{2}^{-}$ ions generated in the pulse on-time do not reach the surface due to the direction of the applied electric field. In the off-time, $\text{O}_{2}^{-}$ ions generated at the edges of the deformed streamer are pushed by the accumulated surface charge outwards. As a result, the $\text{O}_{2}^{-}$ fluence has an annular structure with its maximum being outside the area of the dielectric surface covered by the plasma channel. Solving for the second pulse period shows small changes in the predicted fluences, with largest difference seen with $\text{O}_{2}^{-}$ . We see that increasing the flow rate (by a factor of three) shifts the position of the maximum fluence of $\text{O}_{2}^{+}$ outwards, and decreasing the $\text{O}_{2}^{-}$ fluence in the second pulse period

Evolution of Atmospheric O2 Through the Phanerozoic, Revisited

An oxygen-rich atmosphere is essential for complex animals. The early Earth had an anoxic atmosphere, and understanding the rise and maintenance of high O2 levels is critical for investigating what drove our own evolution and for assessing the likely habitability of exoplanets. A growing number of techniques aim to reproduce changes in O2 levels over the Phanerozoic Eon (the past 539 million years). We assess these methods and attempt to draw the reliable techniques together to form a consensus Phanerozoic O2 curve. We conclude that O2 probably made up around 5–10% of the atmosphere during the Cambrian and rose in pulses to ∼15–20% in the Devonian, reaching a further peak of greater than 25% in the Permo-Carboniferous before declining toward the present day. Evolutionary radiations in the Cambrian and Ordovician appear consistent with an oxygen driver, and the Devonian “Age of the Fishes” coincides with oxygen rising above 15% atm. ▪ An oxygen-rich atmosphere is essential for complex animals such as humans. ▪ We review the methods for reconstructing past variation in oxygen levels over the past 539 million years (the Phanerozoic Eon). ▪ We produce a consensus plot of the most likely evolution of atmospheric oxygen levels. ▪ Evolutionary radiations in the Cambrian, Ordovician, and Devonian periods may be linked to rises in oxygen concentration

Triple probe interrogation of spokes in a HiPIMS discharge

Using a triple probe situated above the racetrack and inside the magnetic trap of a magnetron, rotating spoke-like structures have been clearly identified in a single HiPIMS pulse as periodic modulations of the electron temperature T e, electron density n e, ion saturation current I isat, floating potential V f and plasma potential V p. The spokes rotate in the E  ×  B direction with a velocity of ~8.8 km s−1. Defining the spoke shape from the footprint of the ion current, they deliver to flush-mounted probes embedded in the target, each spoke can be characterised by a dense but cool leading edge (n e ~ 2.0  ×  1019 m−3, T e ~ 2.1 eV) and a relatively hotter but more rarefied trailing edge (n e ~ 1  ×  1019 m−3, T e ~ 3.9 eV). Measurements of V p show a potential hump towards the rear of the spoke, separated from regions of the highest density, with plasma potentials up to 8 V more positive than the inter-spoke regions. Azimuthal electric fields of ~1 kV m−1 associated with these structures are calculated. Transforming the triple probe time-traces to functions of the azimuthal angle θ and assuming a Gaussian radial profile for the plasma parameters, 2D spatial maps of n e, T e and V p have been constructed as well as the target ion current density J p from the embedded probes. The phase relationship between T e, V p and n e can be clearly seen using this representation with n e leading T e and V p with a phase shift between them of ~50°. Regions of maximum ion current to the target, delivered by individual spokes, coincide with the overlap of regions of high n e and T e measured above the target at a height of 15 mm. Ions created at elevated positions above the target in the observed dense region will take several micro-seconds to reach that surface, so contributing to the target ion current in the following spokes