Seite - 28 - in Adaptive and Intelligent Temperature Control of Microwave Heating Systems with Multiple Sources

2. IntroductionofHEPHAISTOS tagon [LLHG14] and others. The shape of a microwave cavity signifi- cantly influencestheelectromagneticfieldmodesthatcanexistwithin the cavity. According to the number of excited modes, the microwave cavitycanbeclassifiedinto the followingtwotypes. • Singlemodecavity The single mode cavity is designed to operate at a single resonant mode, which means the amplitude of this resonant frequency is significantly larger than that of the others. For a single mode cav- ity with the resonant frequency at f0, its size is normally limited by the half wavelengthλ0/2 (λ0 is the wavelength), e.g. 6.1 cm at 2.45 GHz. Because of the small size, single mode cavities are in- appropriate to heat large-scale products, and not widely used in industrial microwave heating. But on the other hand, for the same power applied, the electric field strength and the corresponding microwaveheatingpowerdensity inasinglemodecavity ismuch higher than a multimode cavity [MM83], therefore it is suitable to heat low loss dielectric materials. In addition, due to the narrow frequency band and simple geometry, the EM field distribution within a single mode cavity can be determined analytically, which givesdirectguidesofwhereandhowtoput theheated loadtoget thedesiredheatingcharacteristics [FBLI+95]. •Multimodecavity Compared with the single mode cavity, the multimode cavity is much more complex. In general, the size of a multimode cavity is muchlarger thanasinglemodecavity,whichgivesthemultimode cavity the ability to handle a wide range of workpieces with dif- ferent sizes. More important, a large number of different modes can be excited within a multimode cavity, which leads to a more uniform EM field distribution and heating pattern than in a sin- gle mode cavity [HB01]. Due to these reasons, multimode cavities have been dominantly used in industrial microwave heating ap- plications[Mer98]. Althoughmultimodecavitiesareconvenientto use,bothdesignandanalysisofmultimodecavitiesaredifficult. Averyimportantparameterforbothsinglemodeandmultimodecav- ities is the quality factorQ, which indicates how much energy is dis- sipated compared to the total stored energy referring to the resonant 28