摘要:

Experimental measurements of the microwave properties of co‐axial 2D Bragg structures and 3D PIC code MAGIC simulations of these same structures are reported. Two‐dimensional coaxial Bragg structures, which can be obtained by providing shallow bi‐periodic corrugation of either the inner or the outer conductor, or by coating the conductors with dielectric material having bi‐periodic refractive index, are recognized as having useful properties for applications in high power microwave electronics. The influence of distributed RF power losses and the parameters of the corrugation on the transmission coefficient and the field distribution inside the 2D Bragg structure were studied and the results obtained are presented and discussed. The simulations of the RF field evolution inside the 2D Bragg coaxial structure using the 3D PIC code MAGIC were undertaken. For the experimental studies several different co‐axial 2D Bragg structures were designed and constructed. A microwave vector network analyzer was used to measure the RF properties of the co‐axial 2D Bragg structures. The transmission coefficient and RF fluxes associated with the near cut‐off modes of a coaxial waveguide were measured and compared with the MAGIC simulations. Good agreement was obtained between the experimental measurements and the MAGIC simulations, which has enabled the suitability of 2D Bragg structures in different applications such as input and output mirrors of a cavity, narrow band filters, active RF sources and pulse compressors to be ascertained. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635146

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Electron beam generation from a multi‐gap pseudospark discharge was investigated. A pseudospark‐sourced electron beam has two phases, an initial hollow cathode phase (HCP) beam followed by a conductive phase (CP) beam. The beam brightness was measured by a field‐free collimator to be 109and 1011Am−2rad−2for the hollow cathode phase (HCP) beam and the conductive phase (CP) beam respectively. The initial HCP beam from an eight‐gap pseudospark discharge was applied in a Cherenkov interaction between the electron beam and the TM01mode of a 60‐cm long alumina‐lined waveguide. It was found experimentally that significant microwave radiation was generated only when the dielectric was present in the interaction space. If there was no dielectric in the cylindrical waveguide, then a very small background microwave output was detected even when the guide B‐field was absent. This demonstrated, in conjunction with the observation that the microwave output signal was independent of the guide magnetic field over the range 0.13 to 0.26 T, that the radiation from the experiment was due to the Cherenkov interaction mechanism. In addition, two components of the microwave pulse were observed corresponding to the two energy components of the electron beam during the pseudospark discharge breakdown. These results demonstrated that the microwave radiation was generated by Cherenkov amplification of the broadband emission from the pseudospark discharge itself. A background signal level of around 100 W was measured in the frequency range 20 – 50 GHz with a percentage of (2.7 ± 0.6)&percent; in the frequency range 25.5 – 28.6 GHz, when the dielectric lining was removed from the maser. The frequency of the microwave output after the Cherenkov maser interaction was measured to be mainly around 25.5 GHz and the dominating mode was identified as being TM01. The duration of the microwave pulse was approximately 80 ns, with a peak power of around 2 ± 0.2 kW. The gain of this amplifier was measured as 29 ± 3 dB. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635147

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The effect of the finite axial magnetic field on the excitation of a backward wave oscillator (BWO) is investigated. The driver beam is assumed to be mono‐energetic helical electron beam such that all the constituent electrons have their gyration centers on the axis of the slow wave structure (SWS). Such a beam supports negative energy fast and slow cyclotron modes (FCM and SCM) that can excite the structure modes in the SWS. This may contribute to the microwave generation in BWO. All the previous analyses on BWO have assumed the electrons without the initial perpendicular velocity component. The formulation is as follows: The thin annular large orbit beam is perturbed and the first order perturbations in velocity and density are obtained. Integrating radially across the beam, we derive the expression for surface current density. It can be expressed in terms of the azimuthal and axial components of the perturbed electric field. The boundary conditions on the beam surface are as follows. (a) Matching of the continuous axial and azimuthal components of electric field and (b) Matching of the discontinuous axial and azimuthal magnetic fields across the beam by the presence of surface current density. They are augmented by the requirement of Floquet periodicity on the RF fields and the boundary conditions that the tangential electric field should vanish on the metal SWS surface, 6(2N+1) × 6(2N+1) order determinant that must to be zero and this is the dispersion relation of the system. Here, 2N+1 is the number of Floquet harmonics involved. Numerical analysis is made assuming N=4 and appropriate practical experimental parameters. The excitations of the unstable non‐axisymmetric Q‐TE11and Q‐TM11modes caused by negative energy FCM in addition to unstable axisymmetric Q‐TM01mode caused by slow beam mode that is identical to conventional BWOs are observed. Defining &agr; as the ratio of the transverse to the longitudinal velocity components of the beam electrons, it is found that for &agr; < 0.065 the growth rate of the conventional axisymmetric Q‐TM01mode is dominant, while for &agr; ⩾ 0.065 the growth rate of non‐axisymmetric Q‐TE11mode dominates for the chosen set of parameters. The effect of slow cyclotron instability is comparatively small. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635148

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The use of a cluster of cavities in frequency multiplying gyro‐amplifiers is considered. An analytical theory has been developed to maximize the second harmonic current and optimize the drift section length for the case of a single low‐Q input cavity, operating at the fundamental cyclotron harmonic, and bunching clustered cavities operating at the second‐harmonic. MAGY simulations have been conducted to benchmark the theory and further study the detailed characteristics of cluster‐cavity gyro‐amplifiers. The theory and MAGY code simulation agree. In the small signal regime, the bandwidth of a cluster‐cavity device (with a pair of cavities in the cluster) is twice that of a single cavity device, while both have the same peak bunching. With a gyro‐TWT output section, a peak power of 247kW, efficiency of 24.2&percent; and bandwidth of 1.08&percent; have been simulated using a cluster of cavities as a buncher. Also, the power‐bandwidth product is 105kW×MHz, which is double that of the single cavity buncher case. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635149

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In a gyrotron, it is difficult to accurately predict in advance where the resonant interaction between the electrons and outgoing radiation stops. For accurately calculating the start currents for the interacting modes, the exit coordinate has to be fixed where the resonant interaction stops. This paper discusses the difficulty in fixing the exit coordinates for studying start currents in an overmoded gyrotron. Start currents are studied for the operating and the most dangerous parasitic mode of a 140 GHz gyrotron being developed by Communication and Power Industries (CPI). Calculations show that the start currents vary over considerably large values with varying exit coordinates that can cause difficulties in predicting which mode dominates the mode competition. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635150

出版商:AIP    

年代:1903

数据来源: AIP