2019, Wang, H. Q., Stepanenko, A. A., Степаненко, Александр Александрович

© 2019, Pleiades Publishing, Ltd.Abstract: Recent experimental observations of divertor plasma turbulence at the ASDEX Upgrade tokamak have demonstrated the onset of strong fluctuations of plasma radiation intensity in the near X-point region within the inner divertor leg, when the machine was operating with the pronounced asymmetry in detachment of plasma between the inner and outer divertors. As been recently shown, one of the possible mechanisms responsible for these fluctuations can be related to the current-convective instability (CCI). In this contribution, we assess whether the CCI can drive turbulence with spatial and temporal parameters similar to those observed lately in DIII-D experiments. It is demonstrated that the CCI drives saturated plasma fluctuations, dominated by low-frequency modes with toroidal wavelengths on the order of the machine circumference in the inner divertor region—features that are similar to turbulence characteristics found in experiments.

2020, Stepanenko, A. A., Степаненко, Александр Александрович

The impact of electromagnetic effects on macroscopic dynamics of blobs in hot low-beta ( beta < m e / m i) edge plasma of fusion devices is analyzed. The physical model governing filament dynamics is presented. The qualitative analysis of the blob motion in electrostatic and electromagnetic regimes is performed. The scalings for the plasma potential and filament advection velocity are obtained along with the estimates of plasma parameters, for which the physics of Alfven waves and skin effect can have an impact on the filament motion. It is shown that required edge/scrape-off layer conditions can be found in modern tokamaks. Analytical expressions for the distributions of the parallel current and electrostatic potential in a blob, containing electrostatic and electromagnetic contributions, are derived. It is demonstrated that in sufficiently hot, weakly resistive plasma the electromagnetic effects can lead to noticeable variations of the potential along the magnetic field lines distorting the blob propagation pattern in contrast to the electrostatic case. Results of the theoretical analysis are compared with results of 3D BOUT++ modeling, which show that in the electromagnetic limit, the macroscopic motion of blobs can exhibit ballooning features due to the emission of Alfven waves, absent in electrostatic simulations. It is concluded that, at least in the near scrape-off layer region of fusion devices, electromagnetic models of plasma dynamics may be appropriate for the analysis of blob motion instead of electrostatic ones.

2021, Zhdanov, V. M., Stepanenko, A. A., Степаненко, Александр Александрович

© 2021, Springer Science+Business Media, LLC, part of Springer Nature.The authors have studied the influence of the adsorption and surface diffusion of one component of a binary gas mixture on pore walls on the effect of separation of the gas mixture during its flow through a nanosize porous membrane in a free molecular regime. A capillary porous-medium model obtained by the momentum-balance method was taken as a basis for analysis. Expressions have been determined for the separation factor and the concentration jump of the adsorbed component for an assigned pressure difference on the membrane, which make it possible to analyze the influence of the initial concentration of the adsorbable component and of the temperature on the separation effect. Calculations have been performed of the concentration jump for a number of commercial gas mixtures (CO2/H2, CO/H2, CH4/H2, and N2/He). It has been shown that taking account of surface diffusion may lead to appreciable variations (of several tens of percent) of the obtained values from Knudsen values of the effect in the absence of adsorption.

2019, Zaznoba, V. A., Stepanenko, A. A., Zhdanov, V. M., Степаненко, Александр Александрович

© 2019 Author(s).The slip phenomena in multicomponent gas mixtures are studied. The moment equations, derived from the linearized Boltzmann equation by using the 13-moment approximation of the Grad's method, are used to obtain the full (containing the contribution from the Knudsen layer) and asymptotic (valid far from the plane physical boundary of the domain) expressions for the nonequilibrium macroscopic parameters of the mixture species. The latter relations are employed to deduce the expressions for the mixture slip velocity and the viscous, thermal, and diffusion slip coefficients by using the modified Maxwell method and the diffuse-specular model of molecule scattering on the wall. The derived relations for the slip coefficients are given in the convenient form expressed in terms of basic transport coefficients, such as partial viscosity and thermal conductivity coefficients, diffusion and thermal diffusion coefficients, and coefficients of molecule momentum accommodation at the wall. The expressions found are used to calculate the slip coefficients for binary (He-Ar) and ternary (He-Ar-Xe) gas mixtures. The numerical results are in good agreement with the data calculated by using other methods.

2021, Stepanenko, A. A., Степаненко, Александр Александрович

© 2021, Pleiades Publishing, Ltd.Abstract: The cumulative impact of the inhomogeneous distribution of the plasma temperature and electromagnetic effects on dynamics of plasma filaments in the hot edge plasma of a tokamak, characterized by small values of the parameter (Formula presented.), is analyzed. The physical model describing the filament dynamics is presented. Estimates of the scrape-off-layer plasma parameters, for which electromagnetic and temperature effects can have a substantial impact on the filament dynamics in conditions of the low ratio of the plasma pressure to the pressure of the magnetic field, are obtained. The model equations are used for three-dimensional simulations of the macroscopic motion of filaments in plasma, the parameters of which are similar to the parameters of the peripheral plasma of the tokamak T-15MD, for different types of the spatial distribution of the electron temperature inside the filaments. It is shown that simultaneously taking electromagnetic and temperature effects into account can lead to a considerable departure of the filament trajectories of motion from those in the electrostatic case.

2020, Wang, H. Q., Watkins, J. G., Guo, H. Y., Leonard, A. W., Stepanenko, A. A., Степаненко, Александр Александрович

© 2020 Author(s).An MHD mode with a frequency of <10 kHz has been identified near the inner strike point from various diagnostics, i.e., divertor Langmuir probes, magnetics sensors, and interferometers, but does not appear in the upstream and core diagnostics. This MHD mode is associated with magnetic oscillations of ≳5 G, has a long wavelength in the toroidal direction with toroidal mode number n = 1, but is localized in a narrow radial region of a few cm. The mode appears when the outer strike point remains attached and the inner strike point nearly detaches, grows with increasing density, and eventually weakens and vanishes as the outer strike point detaches. This mode results in particle flux with an order of magnitude higher than the background plasmas near the inner strike point. The mode characteristics are consistent with the Current-Convective-Instability theory prediction. Initial simulations based on experimental input have found oscillations with similar frequencies but weaker amplitude.

2021, Zaznoba, V. A., Zhdanov, V. M., Stepanenko, A. A., Степаненко, Александр Александрович

© 2021 Author(s).The binary gas mixture flow, diffusion, and heat transfer through a long tube in the near-continuum regime (moderately small Knudsen numbers) are analyzed. The system of linearized third-order moment equations, obtained by Grad's method, is used. An expression for the total mass flux of a binary gas mixture is deduced by using the extension of the procedure, proposed in the work by Zhdanov ["Slip and barodiffusion phenomena in slow flows of a gas mixture,"Phys. Rev. E 95, 033106 (2017)] for the study of slip phenomena in slow flows of a gas mixture. Relations for diffusion and heat fluxes are determined from the initial system of moment equations, averaged over the channel cross section, and supplied with several moments of the distribution function at the channel wall found with the modified Maxwell method. Analytical formulas for kinetic coefficients of the Onsager matrix, which connect averaged fluxes and gradients of the corresponding thermodynamic quantities, are obtained. It is shown that the employed approach automatically ensures the validity of the reciprocal relations for the cross terms in the Onsager matrix. The results of calculations of the derived kinetic coefficients for several binary mixtures of noble gases (He-Ar, Ne-Ar, and He-Xe) are presented and compared with numerical data found by the discrete velocity method on the basis of the linearized Boltzmann equation with the McCormack model.