Abstract
The problem of calculating the ion drag force acting on a dust grain immersed in a weakly ionized collisional plasma is studied using an approach based on the direct numerical solution of the VlasovBhatnagarGrossKrook kinetic equations. A uniform subthermal flow of argon plasma past a spherical dust grain is considered. The numerical computations are performed for a wide range of plasma pressures. On the basis of the obtained results, the effect of ionneutral collisions on the ion drag force is analyzed in a wide range of ion collisionality. In the collisionless limit, our results are shown to be in good agreement with the results obtained by the binary collision approach. As the ion collisionality increases, the ion drag force is found to decrease sharply and even become negative, i.e., directed oppositely to the plasma flow. A qualitative explanation of this effect is presented and a comparison of our results with those obtained using the drift diffusion approach is discussed. The velocity dependence of the ion drag force in the highly collisional regime is examined. The relationship between the ion and the neutral drag forces in the highly collisional limit is analyzed and the possibility of a superfluidlike behavior of dust grains is discussed.
The work was supported by the joint grant of the National Academy of Sciences of Ukraine and the Russian Fund of Fundamental Researches.
I. INTRODUCTION
II. FORMULATION OF THE PROBLEM
A. Problem and basic equations
B. Modelcollision integrals
C. Boundary conditions
III. NUMERICAL METHOD
IV. RESULTS AND DISCUSSIONS
V. CONCLUDING REMARKS
Key Topics
 Plasma flows
 33.0
 Plasma kinetic equations
 21.0
 Plasma collisions
 20.0
 Boundary value problems
 16.0
 Collision theories
 14.0
Figures
The dependence of the ion drag force on the flow velocity in the collisionless limit ( ) for and . Here, the solid lines (—) indicate the present results for (1) and (2a) . Symbols show the results obtained using the binary collision approach ^{ 12 } for and .
The dependence of the ion drag force on the flow velocity in the collisionless limit ( ) for and . Here, thesolid lines (—) indicate the present results for (1) and (2a) . Symbols show the results obtained using the binary collision approach ^{ 12 } for and .
The dependence of the ion drag force on the ionneutral Knudsen number for , and . Here, the solid line (—) indicates the present results. Symbols ( ) show the results obtained using the driftdiffusion approach. ^{ 15 } The values of the relative effective charge are (1), 0.390 (2), 0.405 (3), 0.420 (4), 0.450 (5), 0.478 (6), 0.495 (7), and 0.522 (8). The dashed line (– – –) indicates the zero level.
The dependence of the ion drag force on the ionneutral Knudsen number for , and . Here, the solid line (—) indicates the present results. Symbols ( ) show the results obtained using the driftdiffusion approach. ^{ 15 } The values of the relative effective charge are (1), 0.390 (2), 0.405 (3), 0.420 (4), 0.450 (5), 0.478 (6), 0.495 (7), and 0.522 (8). The dashed line (– – –) indicates the zero level.
The distributions of the normalized ion and electron concentrations (a) and distributions of the normalized charge density (b) along the axis of symmetry for different values of the ionneutral Knudsen number in the case of , and . The arrows show the flow direction. In (a), the solid lines (—) indicate the normalized concentration of ions , and the dashed lines (– – –) indicate the normalized concentration of electrons . In (b), the normalized charge is given by , where and . Note that far from the grain the quasineutrality condition holds .
The distributions of the normalized ion and electron concentrations (a) and distributions of the normalized charge density (b) along the axis of symmetry for different values of the ionneutral Knudsen number in the case of , and . The arrows show the flow direction. In (a), the solid lines (—) indicate the normalized concentration of ions , and the dashed lines (– – –) indicate the normalized concentration of electrons . In (b), the normalized charge is given by , where and . Note that far from the grain the quasineutrality condition holds .
The dependence of the ion drag force on the flow velocity in the highly collisional regime. The curves are shown for two different values of the ionneutral Knudsen number in the case of and . Here, the solid lines (—) indicate the present results for (1) and (2a) . The dashed line (– – –) indicates the value of the ion drag force obtained using the formula derived in Ref. ^{ 47 } [see Eq. (48) ].
The dependence of the ion drag force on the flow velocity in the highly collisional regime. The curves are shown for two different values of the ionneutral Knudsen number in the case of and . Here, the solid lines (—) indicate the present results for (1) and (2a) . The dashed line (– – –) indicates the value of the ion drag force obtained using the formula derived in Ref. ^{ 47 } [see Eq. (48) ].
The distributions of the normalized ion and electron concentrations along the axis of symmetry for two values of the normalized flow velocity in the case of , and . The arrows show the flow direction. The solid lines (—) indicate the normalized concentration of ions , and the dashed lines (– – –) indicate the normalized concentration of electrons .
The distributions of the normalized ion and electron concentrations along the axis of symmetry for two values of the normalized flow velocity in the case of , and . The arrows show the flow direction. The solid lines (—) indicate the normalized concentration of ions , and the dashed lines (– – –) indicate the normalized concentration of electrons .
The ratio of the normalized ion drag force to the normalized neutral drag force . The results are shown for different values of the normalized flow velocity and electrontoion temperature ratio in the case of . Here, the solid line (—) indicates the velocity dependence of the ratio in the case of . Symbols ( ) show the values of the ratio at for different values of the electrontoion temperature ratio: (1), 3 (2), and 5 (3).
The ratio of the normalized ion drag force to the normalized neutral drag force . The results are shown for different values of the normalized flow velocity and electrontoion temperature ratio in the case of . Here, the solid line (—) indicates the velocity dependence of the ratio in the case of . Symbols ( ) show the values of the ratio at for different values of the electrontoion temperature ratio: (1), 3 (2), and 5 (3).
Tables
The plasma parameters used in our calculations.
The plasma parameters used in our calculations.
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