Plasma Physics (physics.plasm-ph)

Abstract: A new workflow in the OMFIT integrated modelling framework (STEP-0D) has been developed to make theory-based prediction of tokamak scenarios starting from zero-dimensional (0D) quantities. The workflow starts with the PRO-create (profiles creator) module, which generates physically plausible plasma profiles and a consistent equilibrium from the same 0D quantities as the IPB98(y,2) confinement scaling. These results form the starting point for the STEP (Stability, Transport, Equilibrium, and Pedestal) module, which then iterates between state-of-the-art theory-based physics models for the equilibrium, core transport, and pedestal to obtain a self-consistent solution. A systematic validation against the International Tokamak Physics Activity (ITPA) global H-mode confinement database demonstrated that on average STEP-0D is capable of predicting the energy confinement time with a mean relative error (MRE) <19%. The validated workflow was used to predict proposed fusion reactor plasmas finding moderate H-factors between 0.9 and 1.2 its further use will be instrumental for the prediction of the plasma performance of a viable fusion power plant and is being utilized in design studies.

Abstract: We study the effect of an inhomogeneous gas density on positive streamer discharges in air using a 3D fluid model with stochastic photoionization, generalizing earlier work with a 2D axisymmetric model by Starikovskiy and Aleksandrov (2019 Plasma Sources Sci. Technol. 28 095022). We consider various types of planar and (hemi)spherical gas density gradients, focusing on the case in which streamers propagate from a region of density n0 towards a region of higher gas density, where n0 corresponds to 300 K and 1 bar. We observe streamer branching at the density gradient, with branches growing in a flower-like pattern over the gradient surface. Depending on the gas density ratio, the gradient width and other factors, narrow branches are able to propagate into the higher-density gas. In a planar geometry, we find that such propagation is possible up to a gas density slope of 3.5n0/mm, although this value depends on a number of conditions, such as the gradient angle. Surprisingly, a higher applied voltage makes it more difficult for streamers to penetrate into the high-density region, due to an increase of the primary streamer's radius.