Abstract: An apparatus is disclosed for electromagnetically and thermally treating polymeric materials, including PAN and other carbon fiber precursors at large scale at atmospheric pressure, while measuring the temperature in the closed environment of the process chamber. The apparatus is designed for continuous processing, and to be compatible with other stages of existing carbon fiber production lines. It provides direct electromagnetic coupling to the fiber tow(s) in the near-field region of one or more microwave waveguide launchers and also provides direct radiative or IR heating from susceptor plates located on the opposite side of the tow from the waveguide opening for processing a band of multiple tows of fiber. It produces low-temperature-carbonized (LTC) fiber with shorter residence time and higher density compared to the conventional process. Its design is inherently scalable to larger production. A related method is also disclosed.