Gas convection caused by electron pressure drop in the afterglow of a pulsed ICP discharge

Neutral depletion is an important phenomenon in a CW high-density plasmas. Under typical conditions used for material processing (medium density), it is mostly caused by the gas heating. However, we show that in \textit{pulsed} discharges, the neutral depletion caused by the electron pressure P$_{e}$ plays an important role on radical transport. In the afterglow, P$_{e}$ drops in 10 $\mu $s due to the electron cooling. The conservation of the total (neutral plus electron) pressure through the reactor volume, imposes thus a neutral pressure gradient $\Delta $P$_{N}$ between the plasma bulk and the reactor walls. In turn, this forces the cold surrounding gas to move rapidly towards the reactor center. Measured drift velocity of Al atoms in the early afterglow of Cl$_{2}$/Ar discharge by time-resolved LIF is as high as 250 m.s$^{-1}$. This is accompanied by a rapid gas cooling. The opposite phenomenon is expected to take place during the plasma ignition when the electron pressure rises. The transfer of pressure between electrons and neutrals is expected to take place through ambipolar diffusion: ions, accelerated by the ambipolar field exerts a friction force on the neutrals.