Abstract: An inflator (10) for providing inflation fluid to inflate an inflatable vehicle occupant protection device includes a pressure vessel (12) defining a chamber (30) for inflation fluid. The pressure vessel (12) has a passage (40) through which inflation fluid flows from the chamber (30). A burst disk (52) blocks fluid flow through the passage (40). A member (80) is disposed adjacent to and on one side of the burst disk (52) prior to the inflation fluid entering the pressure vessel (12). The burst disk (52) deforms toward the one side due to the pressure of the inflation fluid when the inflation fluid is disposed in the chamber (30). The member (80) limits deformation of the burst disk (52) by contacting the burst disk. An actuatable igniter assembly (66), when actuated, moves the member toward the burst disk (52) to rupture the burst disk and enable the inflation fluid to flow from the chamber (30) through the passage (40).