Gyrotron Efficiency Research Articles

Improvement of the helical electron beam quality and the gyrotron efficiency by controlling the electric field distribution near a magnetron injection gun

A technique for controlling the electric field distribution near the cathode of a magnetron injection gun is developed. The feasibility of improving the quality of a helical electron beam by optimizing the electric field distribution in a pulsed 4-mm-wave gyrotron is studied theoretically and experimentally. Field distributions are obtained that minimize the electron velocity spread in the beam, coefficient of electron reflection from a magnetic mirror, and intensity of parasitic low-frequency oscillations. It is demonstrated that the gyrotron efficiency can be increased through a rise in the beam quality at the optimized electric field distribution.

Progress of high-power and long-pulse ECRF system development in JT-60

A new gyrotron operation technique to increase oscillation efficiency was developed using the JT-60 electron cyclotron range of frequency (ECRF) system. In order to increase the efficiency without a significant increase in the anode leakage current by trapped electrons that limits the efficiency of gyrotrons with a collector potential depression technology, we actively optimized the electron pitch factor by controlling only the anode voltage within ∼0.1 s after the start of the gyrotron operation, and a high-efficiency oscillation in the so-called hard-self-excitation region was achieved from the start of the oscillation. As an application of this technique in the JT-60 ECRF system, the gyrotron output power of 1.5 MW for 4 s, which is the longest pulse length in the world at an output power of 1.5 MW, was recorded with a successful reduction in the collector heat load by 20% as compared with the conventional operation. The reduced collector heat load at the 1.5 MW operation was acceptable for a steady-state operation. Further progress was made with respect to the expansion of the long-pulse capability of the ECRF system. A new gyrotron with an improved mode converter was developed in order to demonstrate a reduction in the stray radiation in the gyrotron; such radiation has thus far hindered long-pulse operations by causing an unacceptable heat load. We confirmed that the stray radiation was reduced to 1/3 of that of the original gyrotron; this reduced heat load is acceptable for steady-state operation. A conditioning operation of the improved gyrotron proceeds up to 31 s at 1 MW. This progress significantly contributes to an enhancement in the high-power and long-pulse capability of the ECRF system used in JT-60SA, where a total output power of 9 MW for 100 s is planned.

On the Design of a High-Efficiency Double-Beam Gyrotron

The gyrotron that operates with a double electron beam is designed and presented. The design of a 42-GHz 200-kW gyrotron at TE03 mode has been done and presented elsewhere. To enhance the RF power without losing the interaction efficiency of the 42-GHz gyrotron, double electron beams are launched into the cavity. The beam-wave interaction calculations and power and frequency growth estimation are performed and compared with the single-beam operation results. In the design of the gyrotron, in-house developed codes, GCOMS, MIGSYN, and MIGANS, as well as commercially available codes MAGIC and EGUN are used. The results show that more than 400 kW of power is not possible by single-beam operation while the double-beam operation shows this amount of power with the same mode. To achieve the double electron beams of the desired parameters at the cavity region, the double-beam magnetron injection gun (MIG) is designed. The results of the interaction cavity and MIG show the feasibility of the double-beam gyrotron for high-power and high-efficiency operation.

Effect of the thickness of electron beams on the gyrotron efficiency

In this paper the effect of the spread in the radii of electron guiding centers on the gyrotron efficiency is studied. First, the analytical theory is developed for describing this effect in gyrotrons operating in regimes of soft and hard self-excitations. Then, study of this effect in a 670 GHz gyrotron is carried out both analytically and numerically with the use of the available nonstationary self-consistent code. Comparison of results demonstrates very reasonable agreement between numerical data and analytical predictions. It was found that in the case of beam injection at the inner peak of the function describing the beam coupling to the wave the spread in electron guiding center radii can be significantly larger than is usually allowed. This fact greatly alleviates the design of electron guns for terahertz-range gyrotrons.

Possibilities for reducing the aftercavity interaction effect in gyrotrons

This paper addresses the problem of aftercavity interaction in gyrotrons. The term “aftercavity interaction” is used for the cyclotron resonance interaction between electrons in a spent beam exiting the microwave cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a gyrotron. Aftercavity interaction may reduce the interaction efficiency by several percent and spoil the energy distribution of spent electrons, thus reducing the possibility of efficient utilization of depressed collectors. We have used a simple theory to find conditions resulting in the reduction of this effect. In particular, we have shown that both the axial waveguide wall profile and magnetic field distribution should be modified in order to weaken the effect of aftercavity interaction. We verified results of our simple analysis with advanced simulations and demonstrated that they are in good agreement. Result of these studies can be beneficial for the gyrotron efficiency enhancement.

Excitation of Backward Waves in Beam Tunnels of High-Power Gyrotrons

The starting conditions for excitation of backward waves in the beam tunnel between an electron gun and a cavity of a high-power gyrotron are studied. The excitation of these waves leads to electron energy spread that spoils the beam quality and, hence, degrades the gyrotron efficiency. The suppression of these modes by a resistive coating on the wall of a smooth beam tunnel is examined. The guiding magnetic field and the tunnel wall radius vary along the axis, and thus, the theory is essentially the small-signal theory of a gyro-backward-wave oscillator (gyro-BWO) with tapered parameters. The results obtained are applied to a sample gyrotron.

Analysis of aftercavity interaction in gyrotrons

This paper is devoted to the analysis of a so-called “aftercavity interaction” in gyrotrons. The term “aftercavity interaction” is used for defining the cyclotron resonance interaction between electrons in a spent beam exiting the cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a tube. A simple theory allowing one to analyze the role of aftercavity interaction in gyrotron operation has been developed. With the use of only two parameters characterizing the profile of the waveguide wall and the profile of the magnetic field, one can find conditions resulting in weakening this parasitic effect. Also, the simulations for the gyrotron efficiency in the presence of aftercavity interaction are performed for an existing device. Results of these simulations agree with analytical predictions. They also show how the axial distribution of the magnetic field in an existing cryomagnet should be modified in order, first, to use the aftercavity interaction for improving the interaction efficiency and, second, for weakening the role of this interaction in degrading the wall-plug efficiency in gyrotrons with depressed collectors. Results of such analysis can be beneficial for the gyrotron efficiency enhancement.

Increasing Power and Efficiency of Gyrotrons

Until recently, the development of new gyrotrons was directed mainly at the increase of their operating frequency, power, and efficiency. The output power of modern continuous-wave (cw) gyrotrons has reached 1 MW, and there is a clear tendency to increase this power further to at least up to 1.5 to 2 MW. The efficiency of the best gyrotron tubes reaches 40% without recovering the residual energy of the spent electron beam [collector potential depression (CPD)] in the continuous regimes and 50% in the pulsed one and achieves 50% with one-step CPD in the cw regimes and near 70% in the pulsed regimes. We analyze limitations of the gyrotron output power and efficiency imposed by systems forming helical electron beams, the cavity interaction processes, the transmitting capability of the output window, and the losses of stray radiation in the built-in converter and power dissipation on the collector (including CPD). Some specific examples in applying the different limits to real cases of gyrotrons are discussed. Ways to enhance the power and efficiency of gyrotrons based on the results of this analysis are shown.

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.

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.

On potentials of gyrotron efficiency enhancement: measurements and simulations on a 4-mm gyrotron

In a 74.2-GHz 100-kW pulse gyrotron, the characteristics of parasitic low-frequency oscillations and their influence on electron energy spread have been determined. A method for suppressing parasitic oscillations by varying the magnetic field distribution in the beam compression region is studied. The results of measurements demonstrate the influence of azimuthal inhomogeneity of electron emission on the excitation of parasitic oscillations and on the gyrotron output parameters. Obtained data allows one to determine the ways of gyrotron efficiency enhancement by improved quality of the helical electron beam

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.

Self-oscillations in a gyro-backward-wave tube with coupled electrodynamic structures

Self-oscillations in a gyro-backward-wave tube with coupled guiding structures are studied. It is shown analytically and numerically that the critical current for the onset of self-modulation in such a structure can be significantly increased and a high power can be achieved in the single-frequency oscillation mode. Numerical simulations predict a significant increase in the gyrotron efficiency at the optimal coupling between the guiding structures.

Influence of Voltage Fluctuations on Gyrotron Efficiency and Stability

Influence of power supply voltage instability on gyrotron operation regime is investigated. Dependence of efficiency versus variations of external parameters is given and conditions of oscillations cutoff are determined. It is shown that positive voltage fluctuations result in sharp falling efficiency and failure of self-oscillations and the steadiest gyrotrons are those with short cavities and the large ratio of transverse electron speed to longitudinal one around 1.5 - 2. Comparison of theoretical and experimental results is given.

Investigations of Advanced Coaxial Gyrotrons at IAP RAS

Main results of experimental testing of the 140 GHz/1.5 MW coaxial gyrotron are summarized. High selective properties of the coaxial cavities and a possibility to increase considerably the efficiency of a coaxial gyrotron only by applying independently an appropriate voltage on its electrodes (without any design modification) have been confirmed. Successful uses of the two-potential connection scheme for the frequency step tuning and output power modulation in a coaxial gyrotron has been also demonstrated.

Generalized gyrotron theory with inclusion of adiabatic electron trapping in the presence of a depressed collector

The effect of electrostatic adiabatic electron trapping in the presence of a depressed collector has been included in gyrotron efficiency computations. The results are presented as general contour plots in the control parameter plane. A condition for trapping is derived in terms of the retarding voltage. The overall electron efficiency as a function of the retarding voltage has been calculated for a representative set of control parameters including the effect of electron velocity spread. It is found that the onset of trapping seriously decreases the efficiency.

Efficiency Enhancement of the Relativistic Gyrotron

The possibilities to increase efficiency of relativistic gyrotrons by optimizing gyrotron parameters have been considered and different calculated models of gyrotrons, compared. Dependencies of the efficiency and optimal parameters on the accelerating voltage have been obtained. It is shown that efficiency of 35-45% can be achieved in a strongly relativistic gyrotron without energy recovery, and 60-70%, in the device with single stage energy recovery of the electron beam.

The energy spectrum of an electron beam after interaction with an RF field in a gyrotron

The electron distribution over residual energy after electron interaction with an rf field was determined using the method of decelerating electric field. It is demonstrated that, under the maximum efficiency conditions, electrons with an energy lower than 30% of their initial energy are absent in the spectrum. Hence, the efficiency of energy-recuperation gyrotrons can substantially be increased.

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)/sub 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/cm/sup 2/ and 500 W/cm/sup 2/, 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.

Experimental test of the natural scheme of electron beam energy recovery in a coaxial gyrotron

A possibility to increase considerably the efficiency of a coaxial gyrotron by only applying independently an appropriate voltage on its electrodes (without any design modification) has been shown. The used scheme is acceptable also for frequency step tuning and the deep output power modulation.