  Double layers (DLs) have been observed in a plasma reactor composed of a
source chamber attached to a larger expanding chamber. Positive ion beams
generated across the DL were characterized in the low plasma potential region
using retarding field energy analyzers. In electropositive gases, DLs were
formed at very low pressures between 0.1 and 1 mTorr with the plasma expansion
forced by a strongly diverging magnetic field. The DL remains static, robust to
changes in boundary conditions, and its position is related to the magnetic
field lines. The voltage drop across the DL increases with decreasing pressure,
i.e., with increasing electron temperature around 20 V at 0.17 mTorr. DLs were
also observed in electronegative gases without a magnetic field over a greater
range of pressure 0.5 to 10 mTorr. The actual profile of the electronegative DL
is very sensitive to external parameters and intrusive elements, and they
propagate at high negative ion fraction. Electrostatic probes measurements and
laser-induced photodetachment show discontinuities in all plasma parameters
electron density, electron temperature, negative ion fraction at the DL
position. The voltage drop across the electronegative DL is about 8 V, is
independent of the gas pressure and therefore of the electron temperature.
