Page - 29 - in Adaptive and Intelligent Temperature Control of Microwave Heating Systems with Multiple Sources

2.2. StructureofHEPHAISTOS frequencyf0,orequivalently, the3dBbandwidth[HB01]withinacav- ity regarding to this frequency. The quality factorQof the moden is definedby[Mer98] Qn= 2pi totalenergystored inmoden energydissipatedpercycle = 2pi totalenergystored inmoden×resonant frequencyfn powerdissipated inmoden , (2.8) or Qn≈ fn ∆f , (2.9) where ∆f is the 3 dB bandwidth defined as the frequency range with halfpowerof theresonant frequencyfn. Based on equation 2.8, the total quality factorQof the whole cavity canbefurtherdividedas [Mer98], 1 Q = 1 QL + 1 QW + 1 QD . (2.10) whereQL corresponds to the loss absorbed by the heated load,QW correspondstothelossduetothedielectricpropertiesof thewall,and QD consists of the diffraction loss leaking through unclosed parts of thecavity. HEPHAISTOS is a giant multimode microwave cavity. From figure 2.5, it is clear that it has a very unique hexagonal shape. According to the simulation results in [Feh09], the electric energy density in the hexagonal shape applicator is much more homogeneous than densi- ties of other shapes (including octagon, heptagon, cylindric and cir- cular). In other words, the unique hexagonal design is able to reduce the EM field fluctuations and increase the EM field homogeneity over mostof thecavityvolume,whichisalsohelpful togetamoreuniform temperaturedistributionontheheated load[Feh09]. Unlike most of other microwave heating systems with only one source, in HEPHAISTOS there are more than one microwave heating sources per module. There are several advantages for using multiple feedingsourcesatdifferent locations,e.g. the improvementof theEM 29