  Double-layers (DLs) were observed in the expanding region of an inductively
coupled plasma with $\text{Ar}/\text{SF}\_6$ gas mixtures. No DL was observed
in pure argon or $\text{SF}\_6$ fractions below few percent. They exist over a
wide range of power and pressure although they are only stable for a small
window of electronegativity (typically between 8\% and 13\% of $\text{SF}\_6$
at 1mTorr), becoming unstable at higher electronegativity. They seem to be
formed at the boundary between the source tube and the diffusion chamber and
act as an internal boundary (the amplitude being roughly
1.5$\frac{kT\_e}{e}$)between a high electron density, high electron
temperature, low electronegativity plasma upstream (in the source), and a low
electron density, low electron temperature, high electronegativity plasma
downstream.
