Abstract: A reaction vessel used in industrial applications is heated by a multiple mode microwave beam that is directed to an interior wall of the reaction vessel. The beam is in an inclined-angular fashion and the wall is arranged so the beam is absorbed and reflected from it many times to provide a helical-like reflection and absorption pattern within the vessel interior to uniformly heat the vessel wall and the material. A microwave isolator connected between a source of the microwave energy and the reaction vessel includes a quartz plate and a seal for compensating disparities in thermal expansion coefficients between the plate and a housing for the plate.