Instability of Dust Particles in a Coulomb Crystal due to Delayed Charging

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Abstract
A self-excited vertical oscillation of dust particles in a Coulomb crystal has been observed. It occurs near the plasma-sheath boundary of a dc plasma operated at a low plasma density and gas pressure. The excitation of this spontaneous oscillation is attributed to the finite charging time of particles. As particles move, their charge varies with the local plasma conditions, but with a delay due to the charging time. As particles traverse the vertical electric potential gradient in the sheath, they gain energy. A model of this mechanism is developed to predict the instability threshold and the energy gain.