Charging mechanisms of dust on a surface at different neutral pressures

We present experimental and modeling results of dust mobilization on a dielectric surface exposed to an electron beam with a background plasma in two pressure regimes. Our results reveal two charging mechanisms: charge pumping and ion lensing. These two charging mechanisms are advanced from the patched charge model, which describes charging within microcavities between dust and its substrate surface. At lower pressure, as the plasma density increases relative to the electron beam, the substrate surface potential changes from positive to negative, driving more secondary electrons induced by the electron beam to be pumped into microcavities (charge pumping), causing enhanced negative charges within the microcavities and subsequently stronger repulsive forces to mobilize dust. On the other hand, at a higher pressure, increased ion-neutral charge-exchange collisions result in slow ions within the plasma sheath, which are attracted into the negatively charged microcavities (ion lensing), leading to charge neutralization and reduced dust mobility.