Analysis of a low magnetic field TE m11 gyroklystron amplifier†

Large- and small-signal numerical calculations are presented for a two-cavity, low magnetic field gyroklystron amplifier operating in the TE _m11 whispering-gallery mode. The gyroklystron system modelled consists of a bunching cavity and an output cavity separated by a drift tube. For operation of both cavities at TE₅₁₁, gain and emission efficiency are studied for a high energy (γ = 1·6), low axial velocity (β_Verbar; = 0·1) electron beam. Prebunching prior to the output cavity in the gyroklystron leads to an increase of the maximum efficiency by more than a factor of two over that obtained from operating the output cavity as an oscillator. Model calculations are presented which show that magnetic field tapering greatly reduces the effects of any initial axial velocity spread in the electron beam, allowing high gain (?40 dB) and high efficiency (25-30%) to be achieved.     

Analyses of two-cavity gyroklystron with feedback

Making use of the linear kinetic theory and the perturbation approach, a gyroklystron with feedback, in which a low order (TE₀₁₁) mode is presented in the bunching cavity by the feedback of the output cavity, and the low order mode in the bunching cavity is coupled by the electron beam to a high order (TE₀₂₁) mode in the output cavity, has been investigated in detail. And electron beam to wave interaction in each cavity and electron bunching mechanism in the bunching cavity have been analysed. The formulas of electron beam to wave interaction power and starting current etc. have been derived. Then some calculations are carried out.     

Nonlinear analysis of a multi-cavity gyro-twystron

To preserve high gain, high efficiency and high power merits of gyroklystron, a gyro-twystron is designed using an electron beam with α(v_⊥/v_z) greater than unity. With a multi-cavity section of high gain, the length of the waveguide output section can be made shorter than the threshold length of the absolute instability without losing total system gain. Numerical simulations are carried out to analyze a ka-band gyro- twystron consisting of three TE₁₁₁ mode cavities and an output section of a TE₁₁ mode waveguide. Stability study is performed to ensure the tube without self-excited oscillations. With α=1.5, the 3-dB linear and saturated gain bandwidth in excess of 2 % can be obtained by stagger tuning for an 80 kV, 3 A electron beam with 5 % axial velocity spread. The maximum saturated gain is more than 55 dB at 33 % efficiency. By tapering the magnetic field of the last 2 cm of the interaction region, the efficiency can be increased to 43 % without degrading the bandwidth, which corresponds to an output power of 103 kW.     

Input coupler for Ka band TE021 circular electric mode operation in a gyroklystron amplifier

An input coupler for the TE₀₂₁ circular electric mode in a gyroklystron amplifier through using TE₈₁₁ as a transition coupling mode in a coaxial cavity has been proposed and numerically simulated in a frequency range of Ka-Band with HFSS code. The coupling efficiency of the TE₀₂₁ mode and the suppression for the TE₈₁₁ mode in the main cylindrical cavity are deliberately considered by adjusting the position and size of coupling holes as well as the radius of the drift tubes. The numerical results show that the TE₀₂₁ mode can successfully be excited, and rational coupling efficiency and high mode purity for the TE₀₂₁ mode in the cylindrical cavity can be reached for the engineering application of gyroklystron amplifier study.     

Efficient Output Mode Converter for 30 GHz Gyroklystron at IAP

An output mode converter for Ka-band multi-MW gyroklystron in the Institute of Applied Physics (IAP) operating in the TE₅₃ mode is suggested. Two variants of the converter, aimed for different applications, are presented: the TE₅₃ to TE₀₁ mode converter with power output along the device axis and the TE₅₃ mode to Gaussian wavebeam quasi-optical converter with a visor. The suggested designs include the built-in electron beam collector. The converters were designed using a new synthesis algorithm, which implies iterative improvement of the waveguide wall shape in order to achieve high efficiency. The calculation results were proven by HFSS simulation and low-power tests of one version of the converter.     

Analysis and Numerical Calculations of the Beam-Wave Interaction for Gyroklystron Amplifiers

A four-cavity gyroklystron was designed and optimized after analysis and calculation of RF system and magnetron injection gun, numerical simulations showed that the TE₀₁₁ mode gyroklystron achieved 280kW peak output power, 38% efficiency, 35dB saturated gain with 250Mhz bandwidth centered at 34GHz for a 68 kV, 11A electron beam. The numerical simulation results were used to build a Ka band high power gyroklystron amplifier. In this paper, analysis and numerical calculation results of the beam-wave interaction are presented. The influences of electron beam, RF system parameters, magnetic field, and input RF signal on output power, efficiency, bandwidth and gain are discussed.     

Passive gyrotron cavity loading and frequency shift

Measurements of change in Q and frequency shift are presented for the penultimate cavity in a 3-cavity gyroklystron under conditions where the cavity does not oscillate. This allowed detailed comparisons to be made with linearized theory, over a wide range of beam currents and DC magnetic fields. The theory was formulated to take into account the effects of cut-off beam tunnels on the effective length of the cavity, the geometrical beam filling factor, the effect of a spread in beam pitch angle, and the effect of the cavity RF magnetic fields. Best fits between the data and the theoretical curves for change in Q and frequency shift were obtained for beam values in good agreement with those predicted from a modified Herrmannsfeldt code. Calculations of oscillation starting currents for TE₀₁, TE₀₁₁, TE₀₁₂, and TE rectangular₀₁₃ cavities as a function of the cavity detuning parameter show that serious errors can occur if one neglects the cavity RF magnetic fields.     

Method for excitation of a high-order spatial mode in the input cavity of gyroklystron

Microwave-power input devices for a multimegawatt pulsed gyroklystron that works in the centimeter-wavelength range using a relativistic electron beam are considered. A device representing a supply waveguide with rectangular cross section that is tangentially interfaced with the side wall of a hollow cylindrical cavity is proposed for the excitation of a high-order rotating spatial mode as the working mode of the input cavity. The efficiency of the device is numerically optimized and measured to be 70% at a low power level using a broadband Bragg reflector for the input variant that provides the excitation of the TE_6,1,1 mode.     

Experimental study of a multimegawatt pulsed gyroklystron

A pulsed gyroklystron with the TE_5,3 output mode is studied. A gain of 30 dB is obtained at a frequency 30 GHz, an output power of 15 MW, an efficiency of 40%, a pulse duration of 0.35 μs, and a gain bandwidth of 50 MHz.     

Experimental investigation of a 140 GHz gyrotron-backward wave oscillator

We report the experimental operation of a voltage tunable gyrotron backward wave oscillator (gyro-BWO) in the frequency range near 140 GHz. Voltage tunability is an important feature of the gyro-BWO for application as a fast tuning source for driving high power free electron lasers or cyclotron autoresonance maser amplifiers. The gyro-BWO operated in an overmoded cylindrical waveguide structure in the TE_1,2 mode. The electron beam source was a Pierce-wiggler gun producing an 80 kV, 6.2 A beam. Frequency tuning with voltage between 134 and 147 GHz was achieved in the TE_1,2 mode with constant magnetic field. However, this tuning was found to be discontinuous. Output powers of up to 2 kW and 2% efficiency were found, significantly below theoretical predictions for a cold beam. The theoretical beam velocity spread was modeled by a 3D beam transport code. The code results show that space charge forces, coupled with the wiggler-induced helical motion and the short cyclotron wavelength of the beam, produce large increases in velocity spread in the magnetic compression region. A beam with smaller velocity spread would be needed to make the gyro-BWO operate at the desired efficiency.     

W波段四腔回旋速调管放大器

介绍了W波段四腔回旋速调管放大器的设计.放大器工作在基膜TE01圆电模式,电子束工作电压70 kV,工作电流6A,设计的双阳极磁控式注入电子枪,电子束纵横速度比1.5,速度零散小于4％.采用粒子模拟方法分析了各种参数对器件性能的影响.模拟结果显示,设计的放大器在电子束速度零散4％的情况下,增益35 dB,带宽800 MHz,输出功率100 kW,效率为23.8％.     

RF Behavior of Cylindrical Cavity Based 240 GHz, 1 MW Gyrotron for Future Tokamak System

In this paper, we present the RF behavior of conventional cylindrical interaction cavity for 240 GHz, 1 MW gyrotron for futuristic plasma fusion reactors. Very high-order TE mode is searched for this gyrotron to minimize the Ohmic wall loading at the interaction cavity. The mode selection process is carried out rigorously to analyze the mode competition and design feasibility. The cold cavity analysis and beam-wave interaction computation are carried out to finalize the cavity design. The detail parametric analyses for interaction cavity are performed in terms of mode stability, interaction efficiency and frequency. In addition, the design of triode type magnetron injection gun is also discussed. The electron beam parameters such as velocity ratio and velocity spread are optimized as per the requirement at interaction cavity. The design studies presented here confirm the realization of CW, 1 MW power at 240 GHz frequency at TE_46,17 mode.     

Numerical Study of Processes in the Cavity of the 170 GHz Gyrotron for ITER Operating at the TE25.10 Mode

The opportunity of use of a TE_25.10 operating mode for CW 170 GHz/1 MW gyrotron for ITER is estimated. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. The influence of unwanted mode conversion at the output of the resonator, mode competition, electron beam potential depression, ion compensation of the space charge and beam energy recovery is investigated.     

MAGICTRAC,a novel method for conversion of whispering-gallery modes into a free-space Gaussian-like beam

A novel device, MAGICTRAC, is described for efficient conversion at millimeter wavelengths of the TE _m,n whispering-gallery mode into a linearly polarized, free-space Gaussian-like beam. MAGICTRAC uses a mode-converting waveguide taper and three mirror optics, one of which incorporates a twist reflector to linearly polarize the output beam. An example design is presented for the TE_15,2 mode at 140 GHz with a calculated efficiency of 96%. Related possible applications include (1) installation of the MAGICTRAC within the vacuum envelope of a gyrotron to separarate the spent e-beam from the generated rf, (2) generation of a whispering-gallery mode by injection of a Gaussian-like beam into the output end, and (3) conversion of TE _m,n modes into TE_0n modes for low-loss transmission in smooth-wall waveguide.     

A W-band Third Harmonic Gyrotron with an Iris Cavity

The design and experimental results of a W-band gyrotron operating at the third cyclotron harmonic are presented. The gyrotron is designed to operate at the TE₆₁ mode, which is significantly distinct from competing modes. An iris cavity is employed for the purpose of trapping the third harmonic mode more effectively and lowering its start current. In the experiment, the gyrotron is drived by a triode magnetron injection gun (MIG) which can produce a 45 kV, 3 A electron beam. When maximum axial magnetic field is 1.22 T, a single mode third harmonic gyrotron radiation is observed with the frequency of 94.86 GHz. The maximum output power is 5.5 kW, corresponding to an efficiency of 4%. Another third harmonic mode TE₀₂ is also detected at 88.8 GHz, with maximum output power of 1.5 kW.     

Design considerations for the gyro-peniotron oscillator

The gyro-peniotron shows great promise as a source of microwave radiation. In this paper we investigate the stability and the non-linear saturated efficiency of a gyropeniotron oscillatro with an axis encircling beam operating at the TE₂₁₁ and the TE₃₁₁ cylindrical cavity modes. Beam and cavity parameters for stable high efficiency operation are presented from a linear theory analysis of the start-oscillation condition. Using design parameters taken from the linear analysis, the strong field non-linear emission efficiencies are studied numerically. The sensitivity of the saturated efficiency to beam axial velocity and guiding center spread is also considered. The results of our modeling indicate that high efficiency (70–90%) operation of the gyro-peniotron oscillator is possible, even with large guiding center \((\delta r_{gc_ \circ } \mathop< \limits_ \sim 0.2 - 0.4r_{L_ \circ } )\) and large axial velocity \((\delta \beta _{||} \mathop< \limits_ \sim 0.3\beta _{||_ \circ } )\) spread.     

Linear and nonlinear analyses on gyropeniotron in a rectangular waveguide

The dispersion equation of the gyropeniotron using TE_n0 mode in a rectangular waveguide is derived in this paper. Through detailed numerical calculations of the dispersion equation using the TE₂₀ mode, dispersion curves of the peniotron are obtained. Some clear viewpoints are proposed here. In the nonlinear analysis, the model is assumed to be a rectangular cavity with two close ends, constant DC magnetic field and an axis-rotating electron beam. The analytical results of the peniotron using the TE₂₀₁ cavity mode assuming two operating conditions of ω = ω_c, and ω 3ω_c show 51% efficiency for to ω ω_c and 11% efficiency for ω3ω_c.     

An experimental 75 GHz gyrotron

An experimental 75 GHz gyrotron has been designed and manufactured in our research institute. There are two different types of tube operating at the TE₅₂₁ ^○ and TE₀₄₁ ^○ modes, respectively, and at the second harmonic. A laminar flow magnetron injection gun is used so that the velocity spread due to the space-charge effect is greatly reduced.     

Numerical Simulation of a Double-anode Magnetron Injection Gun for 110 GHz, 1 MW Gyrotron

A 40 A double-anode magnetron injection gun for a 1 MW, 110 GHz gyrotron has been designed. The preliminary design has been obtained by using some trade-off equations. The electron beam analysis has been performed by using the commercially available code EGUN and the in-house developed code MIGANS. The operating mode of the gyrotron is TE_22,6 and it is operated in the fundamental harmonic. The electron beam with a low transverse velocity spread (db _^max = 2.26% \delta {\beta_{ \bot \max }} = 2.26\% ) and the transverse-to-axial velocity ratio of the electron beam (α) = 1.37 is obtained. The simulated results of the MIG obtained with the EGUN code have been validated with another trajectory code TRAK. The results on the design output parameters obtained by both the codes are in good agreement. The sensitivity analysis has been carried out by changing the different gun parameters to decide the fabrication tolerance.