Abstract: A drivable member has an opening therethrough and is positioned within a charging port of a chamber. The drivable member has suspended therefrom an inner cover. The drivable member is axially movable with respect to the charging port between a raised position, whereat the inner cover is moved into a sealing position against an inner surface of the charging port, and a lowered position, whereat the inner cover is moved away from the inner surface. An intermediate seal is formed between the exterior of the charging port and an exterior portion of the drivable member, and the intermediate seal is maintained during all positions of movement of the drivable member. An outer cover is selectively movable into sealing engagement with an upper portion of the drivable member to form an outer seal. The outer cover may be removed and replaced by a material supply container which also forms an outer seal with the upper portion of the drivable member.