Particle-induced oscillations in inductively coupled plasmas

The feedback between particles and the heating of an inductively coupled plasma (ICP) from argon is investigated. The ICP is heated by a pancake coil antenna, which is coupled to the plasma via a dielectric window. Particles are generated by injecting a pulse of acetylene for a few seconds into an argon plasma. Afterwards, the emission of the ICP starts to oscillate with a frequency of the order of several tens of hertz. This effect differs from an attachment instability, because the oscillation persists in the argon plasma even though argon is not an electronegative gas. It is proposed that the oscillation is linked to a periodic change of particle location in the discharge volume: if particles are close to the dielectric window, the local electron density is reduced and the inductive heating is inefficient; if the particles are located at the centre of the plasma, the electron density in front of the dielectric window can build up and the inductive heating is efficient. This is an unstable situation, because when the inductive heating is efficient the ion drag force expels particles from the centre and confines them again in front of the dielectric window. This reduces the efficiency of the inductive heating. An oscillation between two states results: (i) particles in front of the dielectric window and an inefficient inductive heating; (ii) particles at the centre of the discharge and an efficient inductive heating. The feasibility of this oscillation as a tool to monitor and to control particle sizes and densities is discussed