Ion drag and thermophoretic forces acting on free falling charged particles in an rf-driven complex plasma

Abstract

Monodisperse plastic microspheres have been dropped through a long radio-frequency discharge column. The trajectories of the falling particles have been measured. It was observed that the particles are driven out of the plasma. From the trajectory analysis and plasma measurements the forces on the particles have been derived. Special attention has been paid to the thermophoresis and ion drag forces which are also considered to be responsible for the void formation in microgravity experiments. Two experimental situations have been considered here: first a plasma characterized by its natural symmetric electric potential and temperature distribution and second, a plasma with an asymmetric temperature and electric potential profile. For both cases a good agreement has been found between the measured “trajectory force” obtained from the particle trajectory analysis and the sum of the ion drag, thermophoretic and electric field force.

Response to “Comment on ‘Measurement of the ion drag force on falling dust particles and its relation to the void formation in complex (dusty) plasmas’ ” [Phys. Plasmas 10, 4579 (2003)]

“…We have already discussed the applicability of collisionless models to our experiment to some extent in our previous papers. 7,8 For our experiments at lower gas pressure ͑20 Pa͒ 8 the ion mean free path is significantly larger than the characteristic impact parameter for 90 degree collisions b /2 , but only of the order of the electron Debye length De . It is therefore reasonable to apply the standard approach of Barnes et al 9 with a Coulomb logarithm cutoff at the Debye length.…”

“…1-6 Although the origin of the ion drag force is undisputed, a reliable quantitative description, especially with ion-neutral collisions present, is still lacking. Hence, our experiments on the ion drag force in an rf plasma tube 7,8 provide, to our knowledge, the first quantitative and direct measurements of this force. These measurements set a benchmark for the theoretical description of the ion drag force in the strongly and weakly collisional case.…”

“…In our analysis 7,8 we have used the electron Debye length, whereas Khrapak et al 10,11 argue that the linearized Debye length would be more appropriate.…”

Micro‐Particles as Electrostatic Probes for Plasma Sheath Diagnostics

“…The system of differential Equation (6) and (11) together with boundary conditions (1,3,7,8) and algebraic Equation (4) and (9) is complete, giving U(t, x), n e (t, x), n i (x), v i (x) and L.…”

“…The ion drag force, depending on the mechanism of momentum transfer from ions to the dust particle, consists of two components, [8,14] …”