Treatment unit for a facility for the surface treatment of a substrate in motion, this installation further comprising - a support for the substrate, - a counter-electrode, in particular coincident with the support; this treatment unit comprising - at least one electrode suitable for cooperating with said counter-electrode in order to create an electrical discharge; - at least one housing for receiving at least the electrode; - first injection means (20, 21, 22) for injecting a first gas, or treatment gas, towards said support, this first gas comprising at least one plasma-forming gas; characterized in that the first injection means comprise - a member for admission of the first gas, intended to be put in communication with a source supplying this first gas, this admission member having a principal axis defining a first direction of flow of the gas, - a member for injecting the first gas, emerging opposite the support, - an intermediate chamber connecting these two respective admission and injection members, this intermediate chamber comprising - an upstream region extending from the admission member, the intermediate chamber being configured in such a way that the first gas flows in service in a second direction in this upstream region, said second direction being oblique, in particular perpendicular, with respect to said first direction, and in such a way that the cross section of passage of the first gas increases from the inlet (25E, 26E) towards the outlet (25S, 26S) of the upstream region in longitudinal view and/or in transverse view, - as well as a downstream region extending between the upstream region and the injection member, the intermediate chamber being configured in such a way that the first gas flows in service in a third direction in this downstream region, said third direction being oblique, in particular perpendicular, with respect to said second direction, and in such a way that the cross section of passage of the first gas increases from the inlet (28E, 29E) towards the outlet (28S, 29S) of the downstream region in transverse view, but the cross section of passage of the first gas decreases in the vicinity of the outlet (28S, 29S) of the downstream region in longitudinal view. - a support for the substrate, - a counter-electrode, in particular coincident with the support; - at least one electrode suitable for cooperating with said counter-electrode in order to create an electrical discharge; - at least one housing for receiving at least the electrode; - first injection means (20, 21, 22) for injecting a first gas, or treatment gas, towards said support, this first gas comprising at least one plasma-forming gas; characterized in that the first injection means comprise - a member for admission of the first gas, intended to be put in communication with a source supplying this first gas, this admission member having a principal axis defining a first direction of flow of the gas, - a member for injecting the first gas, emerging opposite the support, - an intermediate chamber connecting these two respective admission and injection members, - an upstream region extending from the admission member, the intermediate chamber being configured in such a way that the first gas flows in service in a second direction in this upstream region, said second direction being oblique, in particular perpendicular, with respect to said first direction, and in such a way that the cross section of passage of the first gas increases from the inlet (25E, 26E) towards the outlet (25S, 26S) of the upstream region in longitudinal view and/or in transverse view, - as well as a downstream region extending between the upstream region and the injection member, the intermediate chamber being configured in such a way that the first gas flows in service in a third direction in this downstream region, said third direction being oblique, in particular perpendicular, with respect to said second direction, and in such a way that the cross section of passage of the first gas increases from the inlet (28E, 29E) towards the outlet (28S, 29S) of the downstream region in transverse view, but the cross section of passage of the first gas decreases in the vicinity of the outlet (28S, 29S) of the downstream region in longitudinal view.