A replaceable upper chamber section of a plasma reaction chamber in which semiconductor substrates can be processed comprises a monolithic metal cylinder having a conical inner surface which is widest at a lower end thereof, an upper flange extending horizontally outward away from the conical inner surface and a lower flange extending horizontally away from the conical inner surface. The cylinder includes an upper annular vacuum sealing surface adapted to seal against a dielectric window of the plasma chamber and a lower annular vacuum sealing surface adapted to seal against a bottom section of the plasma chamber. A thermal mass at an upper portion of the cylinder is effective to provide azimuthal temperature uniformity of the conical inner surface. A thermal choke is located at a lower portion of the cylinder and is effective to minimize transfer of heat across the lower vacuum sealing surface.