Experimental study of a gyrotron operated at the second gyrofrequency harmonic with the single-stage energy recovery

We obtain an output efficiency of 60%, which is record-breaking for gyrotrons operated at the second gyrofrequency harmonic, for an output power of 6 kW in the continuous-wave regime. The increase in the efficiency is achieved by using single-stage energy recovery in a gyrotron with an electrically insulated collector. Experimental results confirm the validity of calculation of the electron-beam energy spectra in the gyrotron collector region. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 850–854, October 2008.     

A new approach for a multistage depressed collector for gyrotrons

A feasibility study for a two-stage depressed gyrotron collector has been performed. A new approach for an adiabatic magnetic decompression of the hollow electron beam has been used. It permits control of the radius of the constant magnetic flux surface, which determines the radial extension of the electron beam. Independent of the value of the magnetic field around the beam. For this purpose, either solenoidal coils or a ferromagnetic insert can be placed inside the hollow electron beam. Thus, the radial dimensions of a multistage depressed collector of a high-power high-frequency gyrotron can be kept within limits given by technological constraints. The energy sorting of the electron beam is improved by using electrodes inside the hollow electron beam for controlling the potential distribution. The additional control electrodes make it possible to eliminate almost all of the effect of secondary electrons on the operation of the collector. In order to demonstrate the proposed approach, a compact two-stage depressed collector has been designed for a 1.5-MW coaxial cavity gyrotron operating at 165 GHz in the transverse electric (TE)_31,17 mode, which is under development at FZK, Karlsruhe, Germany. Including the effect due to secondary electrons, a collector efficiency of 73% has been calculated with an average and peak heat dissipation density of about 240 W/cm² and 500 W/cm², respectively. This results in an increase of the output gyrotron efficiency from 36.5% to 62.6% when internal radio frequency (RF)-losses inside the gyrotron tube of 15% are taken into account     

The gyrotron: Constraints on output-power and efficiency increase

With allowance for the results of national and foreign gyrotron developments, we analyze constraints on the gyrotron efficiency and output power stipulated by the properties of the electron beam, the processes in a resonant cavity, the output-window throughput, the scattered-radiation losses in a built-in converter, and the energy release on a collector. A comparative estimate of the influence of the factors limiting the gyrotron power and efficiency is given. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 10, pp. 864–871, October 2006.     

Experimental study of a 110-GHz/1-MW gyrotron with a single-stage depressed collector

The results of the experimental study of a gyrotron with a single-stage depressed collector are reported. Voltage retarding the electron beam was fed to the tube collector, and the dependence of the RF output power on the solenoidal magnetic field was recorded. A 1-MW output power was reached by increasing the gyrotron efficiency from 40 to 65% with a single-stage depressed collector. GIKOM, Ltd. and Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 5, pp. 670–680, May, 1998.     

Experimental investigation of a 110 GHz/1 MW gyrotron with the one-step depressed collector

The results of experimental investigation of high power gyrotron with depressed collector are given. Total efficiency of a gyrotron with one-step energy recovery of 0.65 at the level of output power about 1 MW was achieved experimentally.     

Design of a 1-MW, CW coaxial gyrotron with two gaussian beam outputs

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.     

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.     

A depressed collector system for a quasi-optical gyrotron with precisely controlled magnetic flux lines

Design of a depressed collector system for a quasi-optical gyrotron, which had a severe constraint on the maximum allowable radius of the collector region is outlined. The needs for unwinding of spent beam and for energy sorting could be accommodated by precise control of the magnetic field profile, especially in the collector region. Techniques used for defining and obtaining such profiles; and for dovetailing the profile with the collector geometry are discussed. Results on profiles and electron trajectories are presented, which demonstrate the feasibility of the design. From primary electron trajectories a collector efficiency of up to 68% has been calculated for a three collector design.This work is supported by the U.S. Department of Energy.     

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     

140-GHz gyrotron with multimegawatt output power

The operational features of a 140-GHz, transverse electric TE_22,6 mode gyrotron oscillator with an advanced quasi-optical mode converter, a Brewster window, and a single-stage depressed collector at 140 GHz with an output power of 2.1 MW and an efficiency of 34% without depressed collector and 53% with depressed collector are presented. The high output power level is possible due to an almost reflectionless termination of the radio frequency (RF) beam line outside the tube. The operation of the TE_22,6 mode gyrotron is described in detail, and the significant features for achieving the high-output power are pointed out     

35-GHz 25-kW CW low-voltage third-harmonic gyrotron

A 50-kV third-harmonic gyrotron is shown to be capable of high efficiency. Operation at the third harmonic allows the required magnetic field for 35 GHz generation to be supplied by a 4.5-kG permanent magnet. Two gyrotrons employing sliced circuits for mode control have been evaluated with a large-signal nonself-consistent particle-tracing simulation code and found to be capable of producing 25 kW continuously. The preliminary design of a third-harmonic TE₄₁ gyrotron utilizing a magnetron injection electron gun is predicted to yield a device efficiency of 17%, which can potentially be increased to 46% with an ideal single-stage depressed collector, while an axis-encircling electron beam from a Cusp electron gun is predicted to drive a third-harmonic TE₃₁ gyrotron with a device efficiency of 23%, which can theoretically be increased to 45% through the use of an ideal depressed collector     

High-power twin-beam gyrotrons operating at the second gyrofrequency harmonic

The results of two experiments with a gyrotron operating at the second gyrofrequency harmonic, using two active electron beams, are reported. The internal (additional) beam is coupled weakly to the field by a spurious mode. The currents of the two beams can be controlled independently. The output characteristics of the gyrotons are plotted for various beam currents. The additional beam substantially enhances the operating mode stability and makes an appreciable contribution to the output power. The maximum output power was 0.87 MW at 25% efficiency, beam currents of 57 A and 13 A, and voltage 60 kV. The highest efficiency was 40%. The results suggest that electron-optical systems with two active beams, regardless of some complications of gun design, are promising for short-wave gyrotrons to operate at the fundamental cyclotron resonance. The specific power dissipation in the collector is observed to decrease (in comparison with a single-beam analog).Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 3, pp. 387–392, March, 1994.     

Design of a 35 GHz gyrotron

The conceptual design of a 35 GHz gyrotron has been developed consistently with the complex formulation of the electric field longitudinal distribution in the resonant cavity. Some models of magnetron injection guns able to produce laminar beams have been investigated leading to the design of an electron gun capable of generating a current of 5 A with a perpendicular velocity dispersion of 0.5%. The device includes three magnetic systems producting flat axial magnetic induction profiles of 1.05 kG, 13.2 kG and 0.65 kG in the cathode, cavity and collector regions, respectively. The gyrotron has been designed for pulsed operation in the TE₀₂₁ mode. Under the soft self-excitation condition, the maximum attainable efficiency is 40% with an output power of 100 kW. An analysis of the collector thermal behaviour has been carried out as well as a study of the thermophysical properties of the alumina window to be used.     

Development of 170 GHz/500 kW gyrotron

A development of 170GHz/500kW level gyrotron was carried out as R&D work of ITER. The oscillation mode is TE_31,8. In a short pulse experiment, the maximum power of 750kW was achieved at 85kV/40A. The efficiency was 22%. In the depressed collector operation, 500kW/36%/50ms was obtained. The maximum efficiency of 40% was obtained at P_RF=470kW whereas the power decrease by the electron trapping was observed. Pulse extension was done up to 10s at P_RF=170kW with the depressed collector operation. The power was limited by the temperature increase of the output window.     

Submillimeter-wave harmonic gyrotron experiment

A theoretical and experimental investigation of the operation of a harmonic gyrotron at submillimeter wavelengths is reported. Using a waveguide cavity with an iris at the output end of the straight section, 14 different second-harmonic modes were observed with frequencies of 301-503 GHz, output powers of 1-22 kW, and a 12-MHz emission frequency bandwidth. The highest output power was 22 kW, with a total efficiency of 3.5% at 467 GHz, and an output power of 15 kW with a 6% efficiency was obtained at 417 GHz. Research was conducted using a 65-75 kV up to 10-A electron gun with a 1/1.5-μs pulse length and a 4-Hz repetition rate, which produced a helical electron beam in magnetic fields of up to 14 T. These results represent the first operation of a high-power harmonic gyrotron in the submillimeter region     

Low-Frequency Parasitic Space-Charge Oscillations in the Helical Electron Beam of a Gyrotron

We developed a medium-power pulsed gyrotron equipped with diagnostic systems for studying low-frequency parasitic space-charge oscillations in the helical electron beam and the energy spectrum of electrons in the collector region. Methods for suppressing parasitic oscillations are proposed and tested. An explanation of the mechanism of suppression of these oscillations is given.     

165 GHz, 1.5 MW-coaxial cavity gyrotron with depressed collector

A further step in the development of a coaxial-cavity gyrotron operated in the transverse electric TE_-31,17 mode at 165 GHz is presented. The gyrotron has been equipped with a quasi-optical output system consisting of a Vlasov launcher with a single cut and two mirrors, one with a quasi-elliptic and the other with a nonquadratic phase correcting surface. The radio frequency (RF) power is transmitted through a single output window. A maximum output power of 1.7 MW has been achieved. At the nominal operational parameters an RF power of 1.3 MW with an efficiency of 27.3% has been measured. The efficiency increases to 41% in operation with a single-stage depressed collector     

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.     

Recent results of the 1-MW, 140-GHz, TE22,6-modegyrotron

The TE_22,6-mode gyrotron operated at Forschungszentrum Karlsruhe at a frequency of 140 GHz has been investigated with respect to the behavior of different emitter materials, step tunability and reflections of the output beam. Two different materials of an emitter ring, LaB₆ and a coated dispenser cathode, were used to test the features of the gyrotron. The output power was found to be independent from the cathode material, as long as a new emitter ring was used. Aging of the emitter led to a slightly decreased output power. The gyrotron also was operated with a Brewster window. The broad-band characteristics of this window made it possible to measure the neighboring frequencies in a frequency range extending from 114 to 166 GHz. Only a slight dependence of the output power has been found over the whole frequency range. The Brewster window also allows us to investigate the influence of reflections on the output power. A strong decrease of the output power was found even for very small reflections. Tilting the power calorimeter (the reflections were measured to be less than 1%) increased the output power by about 20% to 1.6 MW at an efficiency of 36.2%. With a collector depression voltage of 35 kV for energy recovery, efficiencies of 60% at the above-mentioned output power were obtained     

Numerical simulation of processes in a 170 GHZ/1 MW CW gyrotron cavity with operating mode TE 25.10 for iter

We consider a gyrotron for the ECR plasma heating complex of ITER. Gyrotron cavity parameters are optimized with allowance for ohmic losses, reradiation of waves due to profile inhomogeneities, potential depression, and velocity spread of the electron beam. We study the effect of ion compensation for the space charge, the onset of oscillations after switching on the gyrotron, and the competition between operating and spurious modes. The possibility of obtaining 35–39% efficiency without electron-beam energy recovery and 55–60% efficiency with a one-stage depressed-potential collector is shown for cavity RF ohmic loss power less than 2 kW/cm². Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 3, pp. 225–234, March 1999.