Laboratory study of dust impact ion free expansion

This study presents the investigation of the angular and velocity distribution of ions expanding freely from a dust impact generated plasma plume. The characteristic angular and velocity distributions are relevant for the design of dust detector and analyzer instrument, or for the interpretation of electric field antenna signals generated by dust impacts on the spacecraft body.  Iron dust particles of micron and sub-micron size are accelerated to velocities 2–40 km/s using the electrostatic dust accelerator operated at the University of Colorado. A unique experimental setup with a delay line detector (DLD) is used to measure the properties of the expanding ion cloud. The DLD provides the position and the time of impact for individual ions originating from the tungsten impact plate. The angular distribution of ions with respect to target normal is calculated from these positions. The velocity distribution is determined from arrival times of the ions on the detector. The experimental results show that the impact-generated ions expand in the form of a plume with angular distribution following a cosine law and half angle 25°. The velocity distribution consists from different parts which correspond to expansion of a distinct plasma components. The slowest component of the distribution has the most probably speed around 5 km/s and the fastest component around 30 km/s. The contribution of the components changes according to velocity of dust.