A reaction chamber apparatus includes a vertically movable heater- susceptor with an attached annular attached flow ring that performs as a gas conduit. The outlet port of the flow ring extends below the bottom of a wafer transport slot valve when the susceptor is in its process (higher) position, while the gas conduit formed by the flow ring has an external surface at its edge that isolates the outer space of the reactor above the wafer from the confined reaction space. In some cases, the outer edge of the gas conduit is in proximity to a ring attached to the reactor lid and, together, the ring and conduit act as a tongue-in-groove (TIG) configuration. In some cases, the TIG design may have a staircase contour, thereby limiting diffusion-backflow of downstream gases to the outer space of the reactor.