Laboratory measurements of the expansion characteristics of dust impact plasmas

Abstract

The impact ionization process provides a method for the sensitive detection and analysis of dust particles in space. Dust particles impacting solid surfaces at high velocity evaporate and partially ionize. The characterization of the generated transient impact plasma plume is important for the understanding of dust impact signals detected in space by antenna instruments, or for the optimization of dust instrumentation. The angular and velocity distributions of the ions emerging from the dust impact plasma are measured experimentally using a Delay-Line Detector (DLD) setup. A small tungsten target is exposed to the impacts of micron- and submicron-sized iron dust particles using the dust accelerator facility operated at the University of Colorado. The ions from the impact plasma expand in a field-free region and are detected by the DLD. The angular and velocity distributions are calculated from the spatial and temporal distributions of the recorded ions. The velocity distribution measurements are performed for dust impact velocity ranges of $2-5$, $10-15$, and >$20\mathrm{km}/s$. The ion velocity distribution is relatively narrow (≤ 10 km/s) for the lowest dust velocity range, but significantly wider (exceeding 50 km/s) for the higher dust impact speeds. The velocity distribution can be decomposed into contributions from the most prevalent ion species in the impact plasma (Fe⁺, Na⁺, C⁺, and H⁺). The measurements suggest that for dust impact speeds greater than 10 km/s the translational energy of the ions is in the range of about 3–8 eV, while the superimposed thermal energy is about 1 eV, with only a weak variation with impact speed and ion species. The ion angular distribution was measured by averaging the signals from 110 dust impacts from a 10–15 km/s dust impact speed and is consistent with a cosine distribution.