Messung der Ionenreibungskraft an frei fallenden Partikeln in einem Plasma

The ion drag force on micrometer sized particles in a plasma was investigated. Acting on these particles was the gravitational force and the drag force of an ion beam with ion energies ranging from 3...45 eV, and an ion drift with ion velocities of approx. 500 m/s. As both ion flows are in a horizontal direction, the ion drag force could be measured from the deflection of the free falling microspheres from the vertical direction. By a thorough diagnostics of the background plasma parameters and the ion density and energy, a comparison with theoretical models was possible. At high ion energies, a good agreement was found with the standard model (Barnes 1992), when using the Debye length as a cut-off parameter. At low energies however, a better agreement is found with recent numerical models (Khrapak 2003). In particular, it is found, that the standard model overestimates the collection radius for low ion energies and particle sizes comparable to the screening length (Debye length), due to a neglection of the shielding properties of the plasma.