It has previously been shown Tuszewski et al., Plasma Sources Sci. Technol. 12, 396 2003 that inductive discharges in electronegative gases are subject to two types of instability: the source instability related to the E to H transition and a transport instability, occurring downstream when an expanding chamber is present. These two types of instability are observed in our "helicon" reactor operated without a static magnetic field in low-pressure Ar/ SF 6 mixtures. Temporally and spatially resolved measurements show that, in our experiment, the downstream instability is a periodic formation and propagation of a double layer. The double layer is born at the end of the source tube and propagates slowly to the end of the expansion region with a velocity of 150 m s −1 . © 2005 American Institute of Physics.