A device for forming an ion sheath in a plasma to deposit coatings on a non-conducting substrate. The device comprises a tubular reaction chamber having an outer surface wound helically with a first electrode having a first width. Helical winding of the first electrode provides a plurality of first wraps around the outer surface of the tubular reaction chamber. The device further includes a second electrode having a second width that is larger than the first width. Helical winding of the second electrode provides a plurality of second wraps alternating with the first wraps around the outer surface of the tubular reaction chamber. An ion sheath in a plasma forms to a thickness extending at least to the longitudinal axis of the tubular reaction chamber when the first electrode has a connection to a source of radio-frequency power and the second electrode provides a path to ground.