homogeneity of the field and temperature distribution within a microwave cavity
Author: langfeng1 Time: 2017-12-20
In the early days, the researches were mainly done in a singe mode resonant cavity made of rectangular waveguide at the frequency of 2.45 GHz or 915 MHz. Usually, Q factor of this cavity is high, which results in that microwave energy concentrates on a small area, so the field around the heated sample is not uniform. Although the rotation and moving of the sample can appropriately improve the heating homogeneity with time average, the method can only used for the experimental study of small size samples. To improve the heating homogeneity in a larger area for satisfying the requirement of industrial applications, scientists enhance the cavity size. For a specific operating frequency, the increase of the cavity size means that more modes can exist in the cavity. So mode stirrer and some special geometrical transition structures have been used to excite these modes in the cavity through changing the wave vector direction of input microwave to promote the generation of different modes. To get sufficient homogeneity of the field and temperature distribution within samples in a multimode cavity, Patterson et al. demand the characteristic size of the oven of about 12 meter if a frequency of 2.45 GHz is used. This is not only a too large size for manufacture, but also needs too high power for keeping enough microwave power density to heat the samples. So too large and complicated microwave system becomes a barrier for most industrial application. With the development of high power gyrotron, millimeter wave radiation has been utilized to improve the homogeneity of the field and temperature distribution inside the heated samples. Because the wavelength of millimeter wave is very short, the same homogeneity of the field and temperature distribution can be realized in a much smaller cavity, compared with that at the frequency of 2.45 GHz. In addition, the rotation and moving of the samples can also be used in the multimode cavity for further improving the homogeneity of the field and temperature distribution inside the heated samples with time average.
Based on above-mentioned description, increasing frequency, enlarging cavity size, rotating and moving the samples, and exciting multimode in the cavity can all improve the homogeneity of the field and temperature distribution. Except for the rotation and moving of the samples, the other three kinds of methods are mutually correlative. The increase of frequency is dependent on the state of production and research of microwave power source. The enlargement of cavity size is determined by mechanical technology. The multimode excitation is affected by the frequency of microwave source, the size and structure of heating cavity, the design of mode stirrer, the wave vector direction of input wave beam, and so on. So the excitation of multimode is much moreimportant than the other two kinds of methods for improving the homogeneity. One knows that,the higher the frequency, the more modes there are in a size-specified cavity, and the larger the size of a cavity, the more modes there are in the cavity for a specified frequency. Obviously, the effects of the design of heating cavity and mode stirrer as well as the wave vector direction of input wave on the multimode generation are very complicated and important from the point of technical development. In this paper, we will discuss how above-mentioned factors affect multimode generation in detail from structure design of cavity and mode stirrer, methods of mode excitation,and variation of operating frequency in order to better understand multimode forming process.