Shi-Chang Zhang

Mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

Physics issues of mode competition in a large-orbit coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier are investigated. It is shown that the use of a coaxial waveguide brings about two peculiarities: possible enhancement of the transverse dimensional size and the rarefied spectrum and that the applications of a large-orbit electron beam and CARM mechanism can efficiently exclude the parasitic effect of the cyclotron harmonics and spurious oscillations of undesired modes. The dispersion diagram and numerical simulation demonstrate that mode competition and spurious gyrotron-like or CARM-like oscillations can be suppressed when a large-orbit coaxial-waveguide CARM amplifier operates in a higher-order mode.

Effect of ripple shape and taper on frequency response of reflectivity and transmission in a coaxial Bragg structure

By making use of the CST software, a comparative study is carried out for the effect of three ripple shapes (sinusoidal, rectangular and triangular ripples) on the frequency response in a coaxial Bragg structure. Results show that the rectangular ripple leads to the widest bandwidth while the triangular ripple narrows the bandwidth. Moreover, based on the mode-coupling model, numerical analysis is presented to demonstrate the influence of the ripple tapers. It is shown that the interval between the band gaps of the competing mode and the desired working mode is narrowed by the use of a positive taper, but is expanded if a negative taper is employed.

Nonlinear characteristics of a coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier

A detailed model of nonlinear self-consistent simulation for a coaxial-waveguide cyclotron autoresonance maser amplifier is presented to analyse the nonlinear characteristics of the device. Mode spectrums in cylindrical-waveguide and coaxial-waveguide are analysed. The effects of the electron-beam velocity spread and the misalignment between the electron-beam axis and the waveguide axis on the characteristics of the device are investigated and found to be detrimental to the power and efficiency. Specific simulations show that the output power and the efficiency drop from 493.4 kW and 10.97% to 382.33 kW and 8.5% when the axial velocity spread is 1% and decrease to 3.33% efficiency at 0.1 cm misalignment. However, the use of a tapering guide magnetic field with a slope e = −0.001 cm−1 and the starting position z = 25 cm may enhance the power and efficiency to 1.84 MW and 40.82%, respectively.

Multiwave interaction analysis of a coaxial Bragg structure with a localized defect introduced in sinusoidal corrugations

A multiwave interaction formulation is presented to investigate the effects of a localized defect on the reflective spectrum of a coaxial Bragg structure with sinusoidal corrugations. Good agreement has been achieved between the theoretical results obtained by the present formulation and those simulated by the software HFSS, which confirms the validity and the significance of the multiwave interaction formulation. It is found that, the localized defect creates defected eigenmodes within each reflective band gap of the initial standard Bragg structure, which the parameter can be controlled by the location of the localized defect.

Eigenfrequency analysis of a coaxial cavity with large eccentricity

A numerical approach is presented to analyze the eigenfrequency of a coaxial cavity with large eccentricity. An example is given to a lower mode TE/sub 111/ to compare with the software HFSS, where the eccentricity is up to 70%. Generally, this approach has the following peculiarities: the CPU time needed is much less than that needed by the software HFSS and especially, the calculation can be easily extended to higher-order mode such as TE/sub 28,16,11/as well as to high-frequency domain of hundreds giga hertz, which the software HFSS hardly reaches.

Effect of defect location and corrugation phase on transmission in a defected coaxial Bragg structure with two corrugated surfaces

Numerical simulations are carried out to investigate the effect of defect location and corrugation phase on transmission in a defected coaxial Bragg structure with two corrugated surfaces. It is found that, the desired narrow passband can be generated only if the outer-wall and inner-rod ripples are both applied with a defect in the middle-section and their initial corrugation phase difference is set to π. A slight misalignment between the outer-wall and the inner-rod defect can be used to perform a fine adjustment on the characters of pass-band; however, if these defects are separated with a lager gap, several passbands can be generated, which provides a potential approach to construct multi-channel filters.

Suppression of the velocity-spread influence on a coaxial-waveguide cyclotron autoresonance maser

The influence of the electron-beam velocity spread on the coaxial-waveguide cyclotron autoresonance maser (CARM) is investigated by nonlinear simulation. It is found that the velocity spread substantially decreases the power indeed, but it may be suppressed efficiently by a tapered magnetic field.

An outer-inner-slotted-waveguide cyclotron autoresonance maser (CARM) amplifier

Structure of cyclotron autoresonance maser (CARM) amplifier with outer-inner-slotted waveguide is proposed. The attractive features are large waveguide radii and low magnetic field. A linear theory for outer-inner-slotted-waveguide CARM amplifier is developed. It is shown using nonlinear simulation code that the proposed device may be well suited for operate at both high power and high gain in millimeter wave and far infrared regions. Example is presented for an outer-inner-slotted waveguide optimized CARM amplifier designed to operated in the TE/sub 28,16/ mode at 138 GHz, yielding a power of 540.33 kW on using a 90 kV and 50 A electron beam controlled by a 43.24 kG magnetic field. The influence of waveguide structures including slot width ratio and the ratio of inner radii to outer radii of outer slot on the output power are investigated. The mode competition is discussed in conclusions.

Calculation approach of the RF-field distribution in an eccentrically coaxial cavity

A calculation approach for evaluating RF-field distribution in an eccentrically coaxial cavity is proposed, where the theoretical model is based on the solution of scalar Helmholtz equation. Numerical analysis is carried out to compare with the software HFSS that is based on the finite element method. It is found that, although the results obtained by these two methods are in close agreement for lower-order modes, both the calculation time and memories used by the present approach are much less than that needed by the software HFSS, and especially, the present approach is still valid for the higher-order mode such as TE/sub 28,16,1/and higher frequency up to hundreds giga hertz which the software HFSS can hardly process.

Study of frequency of a coaxial-cavity gyrotron oscillator

The eigenvalues of a coaxial cavity must be modified by the structural eccentricity (for instance, by the misalignment of the inner rod), which causes an eigenfrequency shift. In this paper the eigenfrequency shift of the higher-order modes is numerically investigated in terms of the eigenvalue equation. Taking the TE/sub 31,17,1/, TE/sub 32,17,1/,TE/sub 33,17,1/ and TE/sub 34,17,1/ modes as examples, calculations show that the eigenfrequency of a mode may have a down-shift or tip-shift, which depends on the ratio of the outer conductor radius to the inner rod radius R/sub out//R/sub in/. For a higher-order mode, the greater the value of R/sub out//R/sub in/, the smaller the influence of the structural eccentricity on the eigenfrequency shift. Moreover, the structural eccentricity may have a weaker influence if the azimuthal index of the mode is higher.