Many methods, for example, coating attenuation materials in a open cavity, making some slots along longitudinal or azimuthal direction on the cavity wall, or using some complex cavities [1], have been used for increasing the mode selectivity, improving the stability of operating mode and enhancing the output power and interaction efficiency in gyrotron applications. In this paper, a new type of complex cavity structure is used to improve the selectivity of modes. The resonant characteristics of the cavity as well as the effects of thickening the mutual wall and constructing a drift tube in it on the operating mode stability are numerically analyzed and adjusted with Ansoft HFSS code [2]. The cavity has been designed and tested. Fig. 1 is a model of the complex cavity, consisting of inner-and outer-cavity, the two cavities are mutually coupled by rectangular holes. The wall of the outer cavity is closed. If R2-R1=O and RI/R3=Xmn/Xmn' (Xmn and Xmn' are respectively the roots of nth and n'th zero points of Jm (x) ), one can adjust the sizes of the radii (R1 and R3) of the inner-and outer-cavity for enhancing operating mode and suppressing competing modes. An example of the complex cavity is discussed for the TE021/TE031 as operating mode in the complex cavity. With eigenvalue calculation manner or driving excitation manner, the resonant characteristics of the modes in the complex cavity at Ka frequency band have been computed and analyzed with Ansoft HFSS code at RI=10.26mm, R2=R3=14.88mm, and 1=12mm. Figure 2(a) is the electric field distribution of the TE031 mode on the crossed-section in the complex cavity. It can been seen from the figure that the symmetry of the electric field is very good and the zero point and maximum value of the mode field in r direction can obviously be identified, which shows mode purity is good. If R2-R1.O, operating mode is TE021/TE031, and R1/R3 still equal to X02/XO3, the boundary condition that inter cavity wall do not affect the TE031 mode in the complex cavity is broken because of the increase of R2. Fig. 2(b) gives the electric field profile for R2-R=0.3mm. The electric field profile in the complex cavity departs from the ideal structure of the TE021/TE031 mode, the effect of mode competition rises. In order to alleviate the effect, we analyzed the variation of E-field in the cavity with Ansoft HFSS code by enlarging R3 from 14.88 to 15.14mm at R2-R=0.25mm and R1=10.26mm. Fig. 3 shows the azimuthal variation of E-field peak value in radial direction for different R3, which shows that the E-field profile has good uniformity among R3=15.08-15.14mm. In addition, the wall of the inner cavity is opened along axial direction for gyrotron applications (Fig.4), which makes the resonant frequency decrease and affects the match between inter-and outer-cavity. The effect can also be modified by further enlarging R3 as 15.20-15.26mm at R4=8.7mm. Fig.5 is the E-field profile of the TE021/TE031 mode at resonant frequency 34.56 GHz for R3=15.20. Compared with the E-field profile in Fig.2 (a), th...
In plasma simulations, where the speed of light divided by a characteristic length is at a much higher frequency than other relevant parameters in the underlying system, such as the plasma frequency, implicit methods begin to play an important role in generating efficient solutions in these multi-scale problems. Under conditions of scale separation, one can rescale Maxwell's equations in such a way as to give a magneto static limit known as the Darwin approximation of electromagnetics.In this work, we present a new approach to solve Maxwell's equations based on a Method of Lines Transpose (MOL T ) formulation, combined with a fast summation method with computational complexity O(N log N ), where N is the number of grid points (particles). Under appropriate scaling, we show that the proposed schemes result in asymptotic preserving methods that can recover the Darwin limit of electrodynamics.
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