Gyrotron Interaction Research Articles

The Gyrotron at 50: Historical Overview

Gyrotrons form a specific group of devices in the class of fast-wave vacuum electronic sources of coherent electromagnetic wave radiation known as electron cyclotron masers (ECMs) or cyclotron resonance masers (CRMs). The operation of CRMs is based on the cyclotron maser instability which originates from the relativistic dependence of the electron cyclotron frequency on the electron energy. This relativistic effect can be pronounced even at low voltages when the electron kinetic energy is small in comparison with the rest energy. The free energy for generation of electromagnetic (EM) waves is the energy of electron gyration in an external magnetic field. As in any fast-wave device, the EM field in a gyrotron interaction space is not localized near a circuit wall (like in slow-wave devices), but can occupy large volumes. Due to possibilities of using various methods of mode selection (electrodynamical and electronic ones), gyrotrons can operate in very high order modes. Since the use of large, oversized cavities and waveguides reduces the role of ohmic wall losses and breakdown limitations, gyrotrons are capable of producing very high power radiation at millimeter and submillimeter wavelengths. The present review is restricted primarily by the description of the development and the present state-of-the-art of gyrotrons for controlled thermonuclear fusion plasma applications. The first gyrotron was invented, designed and tested in Gorky, USSR (now Nizhny Novgorod, Russia), in 1964.

EURIDICE: A code-package for gyrotron interaction simulations and cavity design

We report, for the first time comprehensively, on the present status of the code-package EURIDICE for gyrotron interaction simulations and cavity design, developed at the National Technical University of Athens. The area of application, theoretical models, numerical implementation, and features of the codes are discussed.

On the numerical scheme employed in gyrotron interaction simulations

We report on the influence of the numerical scheme employed in gyrotron interaction simulations. Results obtained with the Crank-Nicolson scheme are compared with those obtained with the Backward Time – Centred Space (BTCS) fully implicit scheme. We present realistic cases where, for discretisation parameters in the range usually used in gyrotron simulations, the results can be very different. Hence, the numerical scheme used can be responsible for obscuring the underlying physics if its convergence is not tested carefully.

Numerical design of 120 GHz, 1 MW gyrotron interaction cavity

The design and the numerical simulation of the interaction cavity for 120 GHz, 1 MW gyrotron is presented. The optimizations of the cavity parameters are carried out by using the Particle-in-Cell, 3D-electromagnetic simulation code MAGIC. The co-rotating TE 22,6 mode is selected by using the in-house developed code GCOMS. The cold cavity and the beam–wave interaction analyses are carried out to analyze the eigenmode, eigenfrequency and the output power performance. The output power more than 1 MW is achieved at the magnetic field of 4.82 T. The sensitivity analyses of the output power and the frequency with respect to the various interaction cavity parameters are also performed.

Thermal and Structural Analysis and its Effect on Beam-Wave Interaction for 170-GHz, 1-MW Gyrotron Cavity

In this paper thermal and structural analysis for 170 GHz, 1 MW gyrotron interaction cavity and the effect of structural deformation on beam wave interaction is presented. Finite element analysis codes ANSYS has been used for the thermal and structural analysis. Electromagnetic simulator-MAGIC, a Particle-in-Cell (PIC) code, has been used to carry out the effect of the radial expansion of the interaction cavity on beam wave interaction. The change in output power and resonant frequency for operating mode TE34,10 due to thermal expansion is 10 kW and 0.07 GHz, respectively. These values are under the tolerance limit of power and frequency of the gyrotron. The major variation is found in the power growth stability time.

Thermal and Structural Analysis of Interaction Cavity and Its Effect on the Operating Mode Excitation for a 120 GHz, 1 MW Gyrotron

Thermal and structural analysis for a 120 GHz, 1 MW gyrotron interaction cavity and the effect of structural deformation on the operational mode excitation is presented. Finite element analysis codes ANSYS and COSMOS have been used for the thermal and structural simulation. The values of the water flow rate and the hydraulic diameter of the outer jacket have been optimized. The effect of the maximum radial expansion of the interaction cavity on eigenfrequency and mode excitation has also been analyzed. The change in eigenfrequency for the operating mode due to thermal expansion is 0.606 GHz. This value is very near the theoretically calculated value and under the tolerance limit of the maximum frequency deviation for the gyrotron operating mode.

Design and Misalignment Analysis of 140 GHz, 1.5 MW Gyrotron Interaction Cavity for Plasma Heating Applications

In this paper the design of 140 GHz, 1.5 MW gyrotron interaction cavity is described in detail. The interaction cavity is designed and simulated by using Particle-in-Cell code for TE24,8 operating mode. The obtained simulation results show more than 1.5 MW of output power at 139.83 GHz of frequency. A thorough parametric and misalignment study is also presented to support the actual fabrication and assembling of the device.

Numerical Analysis of Interaction Cavity for 1.5 MW/127.5 GHz Gyrotron

This paper presents the design of 127.5 GHz ITER start-up gyrotron interaction cavity. Particle-in-Cell electromagnetic simulation approach has been used for the cold cavity and beam-wave interaction analysis. In-house developed code GCOMS has been used for the mode selection. TE24,8 mode has been chosen as the operating mode. The simulation results show the output power more than 1.5 MW at the operating frequency of 127.8 GHz and the cavity centre magnetic field of 5.1 T. The study of the parametric dependency of the output power and the effi- ciency on the electron beam and the cavity geometry parameters has also been carried out.

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.

Theoretical investigation of a high efficiency and broadband subterahertz gyrotron

This study investigates the electron dynamics of gyrotron interaction in a tapered waveguide and proposes an interaction structure for a 0.2 THz TE02 reflective gyrotron backward-wave oscillator (gyro-BWO). The design shows the advantages of high power, continuous, and broadband frequency tuning, mode converter free, and inherently possesses mode-selective ability to suppress the potentially competing TE22 mode. Calculated results indicate that the reflective TE02 gyro-BWO is capable of generating a peak power of 3.3 kW corresponding to 26.3% interacting efficiency with a broad 3dB tuning bandwidth of 6.9 GHz at beam current Ib=0.5 A and beam voltage Vb=25 kV.

On the Effect of RF-Space Charge on the Beam-Field Interaction in Gyrotrons

The effect of RF-space charge fields on the beam-field interaction in gyrotrons is reviewed. This effect, formerly considered unimportant, is found to cause recognisable differences in simulations of gyrotrons with high current density, such as today’s megawatt level gyrotrons. In this paper, a numerically efficient way of including RF-space charge in the common slow variables model is derived and numerical examples are presented.

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.

MAGY Simulations of Mode Interaction in a Coaxial Gyrotron

Mode interactions in coaxial gyrotrons are studied numerically using the Maryland gyrotron code (MAGY), which has recently been modified to treat coaxial structures. Simulations are focused on the study of the FZK 170 GHz coaxial gyrotron. This device is being considered for electron cyclotron plasma heating in International Thermonuclear Experimental Reactor; however, current performance is below expectations based on previous simulations. The mode competition process during the voltage rise includes at least six modes in two groups (corotating and counterrotating with an electron beam). Simulations show that the sequence of modes achieving a measurable level of output power during start-up depends on several electron beam parameters: voltage, current, beam radius, beam thickness, pitch angle, and spread in transverse velocity. MAGY simulations made with nominal parameters confirm the results of previous simulations. Two beam parameters are not well characterized: pitch angle and spread in transverse velocity. We found that the measured results are consistent with a decrease in the pitch angle and a 3% transverse velocity spread. Also, the effect of modes at the cyclotron harmonic on the fundamental cyclotron harmonic modes is studied and discussed.

Observation of Mode Competition in an 11.4-GHz Magnicon Amplifier

This paper describes the observation of pulse shortening in an 11.4-GHz magnicon amplifier due to competition with a parasitic gyrotron mode. This effect was observed following the installation of a new electron beam collector. The parasitic mode occurred only at high operating powers, when the beam transverse momentum was high, and the growth of the parasitic mode caused the power in the output pulse to drop substantially. We analyze the competition between the gyrotron and magnicon modes in the output cavity using a time-dependent multimode gyrotron simulation code that has been specially modified to model competition between synchronous magnicon and nonsynchronous gyrotron interactions, and compare the results to experimental observations. We also analyze the effect due to the change in the collector.

Experimental observation of the effect of aftercavity interaction in a depressed collector gyrotron oscillator

This paper presents the experimental observation of the effect of an aftercavity interaction (ACI) in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5MW at 110GHz in 3μs pulses with a 96kV and 40A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96kV are compared with simulations from the MAGY code [M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 (1998)]. Excellent agreement is obtained between theory and experiment but only if the ACI is included. In the present experiment, it is estimated that the observed efficiency of 50% would have been about 60% in the absence of the ACI. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.

Quasilinear theory of mode interaction in gyrotrons with azimuthally inhomogeneous electron emission

A theory is developed which describes the effect of the azimuthal inhomogeneity of the electron current on the gyrotron operation. The theory is simplified by using a quasilinear approach, which allows one to study analytically the stability of oscillations in free-running and phase-locked gyrotron oscillators with inhomogeneous current emission. Also the numerical analysis of nonstationary processes in a three-mode gyrotron oscillator is carried out. The results show that, at low currents, the inhomogeneity may cause excitation of additional modes at the frequency of the operating one. At higher currents, one can observe the onset of phase-locked triplets with equidistant frequency spectrum. As the inhomogeneity increases, this enhances the beating effects in such triplets, e.g., the mode amplitudes exhibit some oscillations whose period is inversely proportional to their frequency separation and whose modulation increases with the inhomogeneity in the beam current.

Efficient Ka-band second-harmonic slotted peniotron

A harmonic peniotron has been designed for high efficiency and relative immunity to mode competition from gyrotron interactions. The device employs a 70-kV, 3.5-A, axis-encircling electron beam. To ensure stability from competing gyrotron interactions, the peniotron will operates in the lowest-order mode of a slotted four-vane circuit, the /spl pi//2-mode. The cavity is strongly diffraction coupled for a high circuit efficiency. The 33-GHz, slotted second-harmonic peniotron is predicted to yield 125 kW with an electron efficiency of 58% and a device efficiency of 52%. The device is being constructed at the University of California at Davis and is driven by a recently developed Northrop Grumman (Rolling Meadows, IL) cusp electron gun.

Electromagnetic Field and Cutoff Frequencies of the Azimuthally Rippled Wall Waveguide

The cylindrical waveguide with azimuthally periodic corrugations described by a sinusoidal function is analyzed. Relevant to magnetron and gyrotron interactions, TE-mode oscillations are treated by deriving a dispersion relation—linking the cut-off frequency and corrugation parameters—upon expansion of the field quantities in terms of space harmonics. From numerical examples, the present paper examines how the geometrical properties (area and perimeter) of the corrugated cross section relates with the cutoff frequencies of 2π and π modes having one radial variation. The waveguiding system studied here with continuously corrugated azimuthal profile looks attractive to relativistic, slow-wave electron devices for it is more resistant to RF breakdown in comparison with traditional magnetron cavities with vane-type side resonators.

A novel hybrid slow-wave/fast-wave traveling-wave amplifier

We introduce a new broad-band amplifier configuration which contains elements of both linear and cyclotron resonance maser microwave tubes. A helix slow-wave structure is used to bunch a linearly streaming annular electron beam. A nonadiabatic magnetic transition, which is placed after a drift region at the point where the ac beam current is nearly maximized, converts much of the beam's axial momentum to rotational momentum. The beam then travels through a tapered right-circular waveguide, where microwave energy is extracted via the gyrotron interaction. The design of a proof-of-principle X-band amplifier is given, and the codes that are used in the simulation are described. The nominal beam voltage and current are 45 kV and 8 A, respectively. The tube is predicted to be stable, to possess a peak efficiency above 43%, and to have an instantaneous bandwidth of over 9%.

Gyrotron Experiments Employing a Field Emission Array Cathode.

The design and operation of a field emission array (FEA) cathode for a millimetre wave gyrotron are presented. The FEA chip employed achieved 10 mA reproducibly while operating in the environment of a gyrotron, namely in a vacuum of 1/spl times/10/sup -8/ mbar at voltages of up to 40 kV. New methods are presented of mounting such FEA chips involving the shielding of the micron devices from voltages of up to 40 kV, as experienced in a microwave device. The design parameters of the FEA gun were similar to those of a magnetron injection gun with cathode radius 12.5 mm and slope angle 4/spl deg/. An electron beam current of up to 100 mA was achievable from the FEA gyrotron cathode. An electrically isolated, fibre-optic, 10-channel control unit was developed to allow individual control of each chip. Electron beam powers of /spl sim/1 kW were measured from the FEA cathode. To obtain oscillation, cavity inserts were employed in the electron beam-microwave interaction region to increase the cavity quality factor. Results are presented which verify the gyrotron interaction from the novel FEA cathode.