A chemical vapor deposition (CVD) reactor includes a double wall vacuum processing chamber with an inner wall and an outer wall and fluid passages between the walls. A layer of thermal insulation covers the outer wall. A layer of high temperature thermal insulation covers the inner wall. Heating elements are positioned in the interior of the processing chamber to heat a substrate mounted in the chamber. A gas inlet structure is positioned through the inner and outer walls of the chamber and oriented to direct a flow of reactant gas against the substrate to form a CVD coating on the substrate. A gas outlet structure connected to a vacuum and effluent management system is positioned through the inner and outer walls of the chamber. Fluid inlet and outlet structures positioned to circulate heated thermal transfer fluid through the passages between the inner and outer walls maintain a controlled isothermal inner wall temperature above a condensation temperature of reactant gas and effluent reacted gas byproducts from condensing on the inner walls and insulation in the chamber.