High-power oscillator of continuous electromagnetic radiation with a frequency of 300 GHz

We consider a general concept of construction, the possible versions, and specific features of a gyrotron, whose output power in CW oscillation regime can reach a few kilowatts at a frequency of 300 GHz. The gyrotron is designed for work in a high-frequency facility in combination with a “dry” cryomagnet, which ensures a magnetic field of up to 12 T, required for the gyrotron operation. The basic results of numerical simulation and optimization of the electron gun, the resonant cavity, and other subsystems of the gyrotron are presented. The designs used for the gyrotron development are justified. Preliminary experiments showed the efficiency of the pilotproduction gyrotron with an output power of about 2 kW, which is record-breaking in this frequency range. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 6, pp. 461–470, June 2007.     

Possible operation of a 1.5-2-MW, CW conventional cavity gyrotron at 140 GHz

We present a study of the feasibility of continuous wave (CW) operation of a 140-GHz conventional cavity gyrotron at high power levels operating in the TE/sub 31,8/- and TE/sub 25,10/-modes. The question of mode selection is discussed, and a possible design of such a gyrotron with beam energy up to 90 keV and a current of 60-70 A is given. We find that it should be possible to operate a 140-GHz gyrotron at power up to 2 MW if a sufficiently high-order mode is used, although CW power may be somewhat lower.     

System for the formation of an electron beam in a 258 GHz gyrotron designed for experiments on dynamic polarization of nuclei

We consider the features of a system for the formation of an electron beam in a continuouswave high-stability gyrotron at the second gyrofrequency harmonic with an operating frequency of 258 GHz. The problems related to the maintenance of the required beam parameters are analyzed. The procedure and results of numerical simulation of the electron gun and collector of the gyrotron are described. The influence of various physical factors on the characteristics of the electronoptical system of the gyrotron are studied. The design of the electron beam formation system is described along with the results of measuring its parameters.     

Influence of Aftercavity Interaction on Gyrotron Efficiency

We consider theoretically the influence of interaction of a used electron beam with a concurrent wave in the output waveguide transition outside the cavity on the efficiency and energy recovery in a powerful millimeter-wave gyrotron. Without energy recovery, parasitic interaction in the transition reduces the efficiency and output power of a gyrotron by 5–10%. In a gyrotron with energy recovery, losses due to interaction in the transition can become most significant compared with other losses, and the efficiency is reduced by 20–30%. The influence of the transition decreases with decreasing transition length and increasing ratio of the maximum radius to the minimum radius of the transition.     

Multimirror quasi-cylindrical cavity resonators for frequency-tunable gyrotrons

We consider the possibility to create a gyrotron with smooth mechanical frequency tuning on the basis of multimirror quasi-optical resonators. The two-dimensional problem of finding igenmodes at the open quasi-cylindrical resonator of such a type is solved. Parameters of the gyrotron based on such a device, namely, its efficiency and starting current, are estimated.     

The concept of an electron-optical system with field emitter for a spectroscopic gyrotron

We propose a new version of the electron-optical system for a spectroscopic gyrotron, where the primary electron flux appears due to the field emission. Requirements to the choice of the design of a multi-tip cathode are substantiated. The shape of the electrodes and the geometry of the magnetic system, which provide the formation of a helical electron beam with parameters fit for the gyrotron operation are found.     

Specific features of the development of a high-stability continuous-wave gyrotron operated at the second harmonicwith a working frequency OF 258 GHz

We consider specific features of the development of a continuous-wave 258 GHz gyrotron operated at the second gyrofrequency harmonic, which are related to ensuring of high stability of its output parameters. The requirements to accuracy of manufacture of the cavity and adjustment of the electron beam are evaluated, as well as the necessary parameters of the power sources of the gyrotron complex, which should maintain the required level and stability of the generated power and frequency. The influence of the effect of spurious-mode reflection from the output window of the gyrotron is studied.     

Continuous-Wave Operation of a 460-GHz Second Harmonic Gyrotron Oscillator

We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE(0,6,1) mode near 460 GHz. The gyrotron also operates in the second harmonic TE(2,6,1) mode at 456 GHz and in the TE(2,3,1) fundamental mode at 233 GHz. CW operation was demonstrated for a one-hour period in the TE(0,6,1) mode with better than 1% power stability, where the power was regulated using feedback control. Nonlinear simulations of the gyrotron operation agree with the experimentally measured output power and radio-frequency (RF) efficiency when cavity ohmic losses are included in the analysis. The output radiation pattern was measured using a pyroelectric camera and is highly Gaussian, with an ellipticity of 4%. The 460-GHz gyrotron will serve as a millimeter-wave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) experiments at a magnetic field of 16.4 T.     

95 GHz三次谐波回旋管设计

采用线性理论和非线性理论研究了回旋管谐振腔结构、寄生模式抑制及注波互作用等问题。设计了一支工作在95 GHz的三次谐波回旋管,注波互作用结构采用标准开放式谐振单腔,工作模式为TE64, 采用电压45 kV、电流5 A、横纵速度比为1.5的小回旋电子注。在不考虑电子注速度离散及厚度的情况下,非线性理论分析表明,该回旋管可以获得14 kW功率输出,横向互作用效率约为18%,整管效率约11%。     

Development of a Prototype of a 1-MW 105-156-GHz Multifrequency Gyrotron

We present the results of studying the possibility of numerical and experimental optimization of the helical electron beam (HEB) formation system and the processes of mode interaction in the electron beam – cavity system for a prototype of a 1-MW 105–156-GHz step-tunable gyrotron with various operating modes. The system parameters are optimized to achieve the maximum efficiency of the gyrotron with an acceptable ohmic load in the cavity. We also analyze the influence of mode competition and depression of the electron-beam potential as well as the possibility of energy recovery of the collector electron beam. The possibility of optimizing the mode converter system for various operating modes is demonstrated.     

Technological gyrotron with low accelerating voltage

We consider the design of a gyrotron with an operating frequency of 24 GHz and an accelerating voltage of about 5 kV and describe a nonadiabatic electron gun forming an electron beam with a pitch factor of 1.8 and a velocity spread of about 10%. Longitudinal distribution of high-frequency field in a cavity with a step is optimized by choosing the step height and length. The design value of the gyrotron output efficiency is 35% at the second gyrofrequency harmonic.     

Optical spectroscopy of plasma in high power microwave pulseshortening experiments driven by a μs e-beam

Microwave pulse shortening experiments have been performed on a rectangular-cross-section (RCS) gyrotron driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters V=-800 kV, I_tube=0.3 kA and pulselengths of 0.5-1 μs. Pulse shortening typically limits the highest (10 MW level) microwave power pulselength to 100-200 ns. Potential explanations of pulse shortening are being investigated, particularly plasma production inside the cavity and at the e-beam-collector. We report the first optical spectroscopy diagnostic measurements inside an operating gyrotron as a means of exploring plasma effects on pulse shortening. Plasma hydrogen H-α line radiation has been characterized in both time-integrated and temporally-resolved measurements and correlated with microwave power/cutoff. Hydrogen is believed to originate from water absorbed on internal tube surfaces in the gyrotron     

Determination of gyrotron wave beam parameters

The output radiation of a gyrotron has the form close to the Gaussian beam. Knowing parameters of the wave beam along its way from a gyrotron to an object permits one to optimize the transmission losses. By this, the structure of the wave beam can be recorded with an infrared camera in several cross-sections of the beam. Real gyrotron wave beam is approximated with high precision by the Gaussian beam.     

Observations of Phase Locking in a Single-Cavity Gyrotron Oscillator

Observations of phase locking in a single-cavity gyrotron oscillator operating at 35 GHz are reported. Injection of the locking power was made via a circulator into the gyrotron output guide. Locking was observed with input power levels as low as 35 dB below the gyrotron power. For low input powers, good agreement is found with theory based on lumped-element circuit modeling and a new distributed oscillator model.     

The influence of reflections on the stability of gyrotron autooscillations

We study the influence of some types of reflections on the oscillatory processes in a gyrotron. The oscillation stability conditions in the presence of a reflected signal are estimated, the processes in a gyrotron with a fixed structure of the HF field are simulated numerically, and the enrichment of the signal spectrum in the presence of reflections is studied. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novogorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 10, pp. 1348–1357, october 1998.     

Phase-locking of a second-harmonic gyrotron oscillator using aquasi-optical circulator to separate injection and output signals

Phase-locking in a 34.5-GHz special complex cavity gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency was studied. Injection of the locking power was made via a quasi-optical circulator connected to the gyrotron output. Locking bandwidth was measured by comparing the phase of the injection signal and output signal using a balanced mixer. Locking was observed with input power level as low as 40 dB below the gyrotron output power. The locking bandwidth is, however, narrower than in gyrotrons operating at the fundamental cyclotron frequency which may be attributed to the longer resonant cavity in the second harmonic gyrotron and the corresponding larger value of external quality factor. The measurements are roughly in agreement with predictions of Adler's phase-locking equation which is given for our system in terms of powers propagating in the output waveguide toward and away from the gyrotron cavity     

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 μs pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE_27,11 mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed     

Experimental study of a continuous-wave high-stability second-harmonic gyrotron for spectroscopy of dynamically polarized nuclei

We present the results of the experiments on a continuous-wave gyrotron with an output frequency of about 260 GHz, which has been developed for dynamic polarization of nuclei and is operated at the second cyclotron-frequency harmonic. For an accelerating voltage of 16 kV and a beam current of 0.3 A, a maximum continuous-wave radiation power of about 200 W has been obtained. Measurements of the thermal load on the resonator cavity allow evaluating its actual ohmic Q-factor. Gyrotron radiation is also observed at other frequencies and modes. Their generation zones and the structure of the output radiation have been studied. Long-term stability of gyrotron parameters has been determined.     

Numerical simulation and experimental study of an electron-optical system of a megawatt gyrotron with step frequency tuning in the range 100–170 GHz

We present the results of numerical simulation and experimental studies of the systems of forming helical electron beams with different topologies for a 1-MW gyrotron with step frequency tuning in the range 100–170 GHz. We analyze variations in the beam parameters including the distribution of electrons over the oscillatory velocities, as functions of the beam current for various accelerating voltages and magnetic fields. The results of experimental studies of a prototype of the multifrequency gyrotron in the oscillation regime such that the designed optimized electron-optical system forms an intense helical electron beam with specified parameters in a wide interval of magnetic fields are shown. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 9, pp. 773–784, September 2007.