PIERS ONLINE, VOL. 3, NO. 8, 2007 1228 Numerical Analysis of Cylindrical Cavities Used for Microwave Heating, Employing the Mode Matching Technique A. P. Orfanidis 1 , G. A. Kyriacou 1 , and J. N. Sahalos 2 1 Democritus University of Thrace, Xanthi, Greece 2 Aristotle University of Thessaloniki, Thessaloniki, Greece Abstract— The analysis and design of a cylindrical cavity for microwave heating applications, including the feeding mechanism is proposed in this paper. The cylindrical cavity design aims at the production of a uniform field distribution, avoiding non-uniform heating and thermal runaway. The analysis of the device is based on the Mode Matching Technique. This has been proved to be an efficient and robust technique for the analysis of multiple discontinuities. The feeding structures maybe a circular, rectangular or coaxial waveguide. All quantities involved in the analysis are evaluated analytically achieving a fast and accurate method. 1. INTRODUCTION The application of high power microwaves for thermal processing of dielectric materials, has received a great attention in the past. The benefits of using microwaves instead of conventional heating mechanisms are mainly due to the fact that microwave energy can penetrate the material achieving rapid internal heating. The main disadvantages are non-uniform heating and thermal runaway [1]. As rectangular cross-section cavities are mainly used, the amplitude field distribution depends on the cavity dimensions and the modes excited in the cavity. Even for a high order modes cavity there is a great fluctuation of the field distribution, resulting in non uniform heating of the material under process. Many techniques have been proposed to overcome this problem, such as the frequency variation and the field disturbance using a metallic blade. Frequency variation can be used only in relatively low power or small size devices, since high power microwave generators cannot alter their frequency. Moreover, the use of a metallic blade in a high power microwave cavity will produce high voltage arcs with unpredictable results. The main strength of rectangular and in particular cubic cavities is the possibility of high order mode degeneration. Namely, up to 12 modes can be made to resonate at the frequency of operation. The always challenging question is, what is the appropriate excitation which optimally excites all modes, in order to achieve homogeneous heating energy deposition. Instead of working toward this direction, the present work tries to examine the possibility of producing uniform fields using cylindrical cavities either ordinary or with corrugated walls. The corrugations aim at the establishment of a hybrid HE 11 mode which is expected to present a more homogeneous field distribution. The exact analysis of the cavity as well as the feeding mechanism will be performed using a closed-form mode matching technique. Since all the involved coupling integrals are evaluated analytically, this results in a very fast and compact technique without numerical instabilities. The dimensions of the cavity and the feeding source section will be designed aiming at the higher possible uniformity of the field amplitude. The feeding structure can be a circular, rectangular, coaxial waveguide or a combination of them. Its position will be optimized for the proper excitation of all necessary modes in the cavity. The material to be heated will be inserted to the cavity with the aid of a moving belt, since the device aims at industrial applications, as shown in Fig. 1. For this purpose, two openings will be included in the cavity, while λ/4 chokes will prevent microwave leakage [2]. 2. GEOMETRY OF THE MICROWAVE HEATING STRUCTURE A three dimensional view of the structure to be used for microwave heating is shown in Fig. 1(a). The structure is simplified for electromagnetic simulation convenience reasons. A vertical cross section of the simplified structure is shown in Fig. 1(b). In order to apply the Mode Matching technique the latter structure can be identified as comprised of waveguide sections as shown in Fig. 1(b). The purpose of the Mode Matching technique is to characterize each discontinuity between different waveguides through a generalized scattering matrix. In turn, all the discontinuity scattering matrices along with those of the waveguide section are combined together to yield a system matrix representing the whole structure.