Speaker
Andriy Momot
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.3009.pdf
Negative drag force on finite-size dust grain in strongly collisional plasma
A.I. Momot
Faculty of Physics, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
A finite-size charged conductive spherical 1.0
0.1
dust grain in strongly collisional weakly ionized
Normalized force
0.8
0.5
plasma is considered. It is assumed that the grain 1
0.6
is charged due to collection of encountered elec- ak = 2
D
trons and ions. The stationary plasma flow or the 0.4
movement of the grain with a constant velocity 0.2
v breaks the spherical symmetry of the electric
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
field and the plasma particle distribution around
the grain, and the force on the charged grain ap- Figure 1: Absolute value of normalized nega-
pears. The nonlinear problem for the drag force tive drag force Fe2 /(Te akD )2 vs ṽ in isothermal
is solved numerically within the drift-diffusion plasma. Solid lines with dots are results of nu-
approximation. merical calculations. Solid line corresponds to
The analytical expression for the drag force in (1), dashed line is given by formula (98) from [1]
strongly collisional plasma is presented in [1] by and dotted line is q2 kD2 ṽ/24.
Eq. (98). It was obtained in the linear approximation for point-like grain. Considering the ratio
of diffusion coefficients Di /De as a small parameter this expression can be expanded to
∞
2q2 kD 2 Z x2 x(A + τ)
Aṽ
F =− dx 2 arctan − 1 , A = x2 τ + x2 + 1, (1)
π ṽ A Aṽ x(A + τ)
0
where τ = Te /Ti , ṽ = vλD /Di , kD = 1/λD , q – grain charge, a – radius. Sign "−" in (1) means
that the drag is negative. For small velocity ṽ 1 Eq. (1) gives F = −q2 kD
2 ṽ/24, which coincide
up to the designations with Eq. (11) from [2].
The force acting on a charged grain is directed along its velocity, i.e. the negative drag is take
place. This force depends nonmonotonically on grain velocity (see Fig. 1) and is approximately
proportional to the square of grain radius. Formula (1) is applicable for quantitative estimates
of the drag force on small particles a λD in both non- and isothermal plasmas. It gives the
upper boundary of the negative drag force on finite-size grains [3].
References
[1] A.V. Filippov, A.G. Zagorodny, A.I. Momot et al., J. Exp. Theor. Phys. 108, 497 (2009)
[2] S.A. Khrapak, S.K. Zhdanov, A.V. Ivlev, and G.E. Morfill, J. Appl. Phys. 101, 033307 (2007)
[3] A.I. Momot, Phys. Plasmas 24, 103704 (2017)
