Magnetron Injection Gun Research Articles

Design and Development of MIG for 170-GHz Gyrotron

In this paper, the design and development of magnetron injection gun (MIG) for 170 GHz, 1-MW gyrotron have been presented. The triode-type MIG at 80-kV beam voltage and 40-A beam current has been designed and developed. A gun-collector test module for 170-GHz gyrotron has been developed and vacuum processed at 400 °C with continuous 48 h operation. The cathode activation and high-voltage breakdown testing have also been carried out up to desired rating. The simulation result of MIG by EGUN has been validated with 2-D-code TRAK.

An Improved Diagnostic Device for Magnetron Injection Guns of High-Power Gyrotrons

The quality of the electron beam generated by a magnetron injection gun (MIG) is crucial for the performance of high-power high-frequency gyrotrons. The measurement of the electron properties would be very important for interpreting the experimental results and for improving the design of the gyrotron. Considering the increased power of the electron beam, this is a quite challenging procedure. In this paper, we study a diagnostic device based on the retarding field method for the measurement of the beam properties of an MIG. The influence of several factors on the measurement, such as the relativistic properties of the electrons, the magnetic field compression, and the voltage depression are theoretically and numerically investigated. In addition, the possibility to accurately extrapolate the properties of a high-power electron beam based on low-power beam measurements is studied in detail. In this way, an improved analytical methodology is proposed for the accurate estimation of beam quality from the experimental measurements.

Design of a Double Anode Magnetron Injection Gun for Q-band Gyro-TWT Using Boundary Element Method

This paper presents a novel design code for double anode magnetron injection guns (MIGs) in gyro-devices based on boundary element method (BEM). The physical and mathematical models were constructed, and then the code using BEM for MIG’s calculation was developed. Using the code, a double anode MIG for a Q-band gyrotron traveling-wave tube (gyro-TWT) amplifier operating in the circular TE01 mode at the fundamental cyclotron harmonic was designed. In order to verify the reliability of this code, velocity spread and guiding center radius of the MIG simulated by the BEM code were compared with these from the commonly used EGUN code, showing a reasonable agreement. Then, a Q-band gyro-TWT was fabricated and tested. The testing results show that the device has achieved an average power of 5kW and peak power ≥ 150 kW at a 3% duty cycle within bandwidth of 2 GHz, and maximum output peak power of 220 kW, with a corresponding saturated gain of 50.9 dB and efficiency of 39.8%. This paper demonstrates that the BEM code can be used as an effective approach for analysis of electron optics system in gyro-devices.

Prospective THz Gyrotrons for High-Field Magneto-Resonance Spectroscopy

A high-harmonic Large Orbit Gyrotron and a low-voltage gyrotrino placed inside a spectrometer cryomagnet enable greatly simplify terahertz systems for magneto-resonance spectrometers. Large Orbit Gyrotrons provide a powerful third-harmonic generation at frequencies of 1 THz and 0.394 THz in pulsed and CW regimes, respectively, at significantly lower magnetic fields than conventional gyrotrons. According to simulations the gyrotrino with the voltage of 1.5 kV and frequency of 0.264 THz can generate a power of tens of watts; a possibility to operate at such a low voltage is demonstrated in the existing gyrotron with three-electrode magnetron-injection gun.

Electron Gun and Output Coupling System for a 220-/251.5-GHz, 2-MW Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, design studies of a common electron gun and output coupling system for the dual regime operation of a 2-MW coaxial cavity gyrotron are presented. The operating frequencies (220/251.5 GHz) are selected, such that the proposed gyrotron can be used for plasma heating applications in the future experimental fusion reactors. A coaxial magnetron injection gun (MIG) is designed for the generation of hollow electron beam with the desired parameters. Initial geometry of the electron gun is determined through our in-house code Gyrotron Design Suite version 4.1 2016 (GDSv4.1 2016). The MIG is further optimized and simulated through the beam trajectory code ESRAY. A nonlinear taper (NLT) is designed with maximum transmission efficiency at both the operating frequencies. A quasi-optical launcher (QOL) is designed and numerically optimized through the commercial software, launcher optimization tool. The designed launcher efficiently converts both the operating modes into Gaussian-like mode. A single-disk RF window is designed for the extraction of high-power output beam from the gyrotron at their respective operating frequencies. The design of the NLT and the RF window are carried out through GDSv4.1 2016.

Theoretical investigation on a multifrequency multimode gyrotron at <italic>Ka</italic>-band

In this paper, a multifrequency, multimode gyrotron has been designed, which can operate at 28, 29, 31, and 32 GHz with corresponding modes TE0.3, TE+5.2, TE−3.3, and TE+6.2, respectively. For all operating status, the mode competitions have been investigated carefully with the help of a new time-dependent, multimode, self-consistent code, which is built on the trajectory approach. For the operating at 28 GHz, it also has been simulated by CST Particle Studio. They have similar results when comparing two results. For the operating at other status, the changes of electron parameters caused by the changed dc-magnetic field have been analyzed in detail. In the analyzes, the guiding center radius and Larmor radius would not be influenced severely, but the velocity ratio would be influenced seriously, such as, the velocity ratio of operating at 32 GHz would be reduced to about 0.9787, which caused a severe reduction in the efficiency. In order to alleviate the influences, a compensation magnetic coil (CMC) has been used in the magnetron injection gun region, in which the changes of electron parameters have been analyzed too. The simulation results show that the electron efficiency of operating at 32 GHz can be increased from 24.6% to 31% by applying CMC. The multimode multifrequency gyrotron operated at ka -band gyrotron has been investigated in theory, which can provide new possibilities in high-power millimeter source development.

Operation of a sub-terahertz CW gyrotron with an extremely low voltage

Decreasing the operating voltage for medium-power sub-terahertz gyrotrons aimed at industrial and scientific applications is highly attractive, since it allows size and cost reduction of the tubes and power supply units. In this paper, we examine such an opportunity both numerically and experimentally for the fundamental cyclotron resonance operation of an existing gyrotron initially designed for operation at the second cyclotron harmonic with a relatively high voltage. Simulations predict that output power higher than 10 W can be produced at the fundamental harmonic at voltages less than 2 kV. To form a low-voltage helical electron beam with a sufficiently large pitch-factor, a positive voltage was applied to the first anode of the gyrotron three-electrode magnetron-injection gun with a negative voltage at the cathode. CW gyrotron operation at voltages down to 1.5 kV has been demonstrated at a frequency about of 256 GHz.

Configuration of the thermal gap in magnetron-injection guns complying with the hydrodynamic flow model

An exact solution of the exterior problem for a planar magnetron is constructed in the case of emission limited by the temperature and the space charge. The configuration of the lateral cathode surface and the electrode with a negative potential, which form a profiled thermal gap, is calculated. In contrast to an arbitrary geometry of the electrode in this region, this configuration does not violate the adopted hydrodynamic flow model. The case of a planar diode with the T-mode emission is also studied.

RF Behavior of a 220/251.5-GHz, 2-MW, Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, RF behavior studies of a dual regime coaxial cavity gyrotron designed for electron cyclotron resonance heating and current drive of magnetically confined plasmas in the future fusion reactors are presented. Considering all the design constraints, the mode pair is chosen as TE48,30 and TE55,34 for operation at 220 and 251.5 GHz, respectively. The interaction circuit is initially designed through the cold cavity design. A triangular corrugated insert offers good mode selection and also reduces the localized heating problem. Single mode computations are carried out to optimize the beam parameters for the maximum efficiency of the chosen mode pair. Time-dependent multimode simulations are carried out to conform the power in the desired mode pair and the possibility of power in the competing modes. Start-up analyses are performed before and after space-charge neutralization with nonuniform magnetic field using nominal electron beam parameters obtained from magnetron injection gun calculations. These studies ensure that the continuous wave operation of a coaxial cavity gyrotron is possible with the output power of ≈2 MW with the chosen mode pair.

Magnetically shielded electron–optical system of a continuous gyrotron with an operating frequency of 24 GHz

It is shown that it is possible to use warm solenoids with copper winding screened with ferromagnetic shields in order to reduce the solenoid supply power by a factor of 1.8–2 while retaining the strength of the magnetic field in the working space for continuous gyrotrons operating in a frequency range of 24–30 GHz. The configuration of the shields as well as the shape and location of the correcting solenoids providing an extended (up to 10 wavelengths long) section of a uniform field in the region of the gyrotron cavity have been determined. It has been shown that introduction of an additional cathode coil into the magnetic system reduces the degree of nonadiabaticity of the magnetic field approximately by an order of magnitude and thereby makes it possible to use the magnetron injection gun for formation of a helical electron beam. Optimization of the configuration of the gun electrodes based on the trajectory analysis makes it possible to obtain an electron beam whose parameters are as good as parameters of the electron beams formed in classical adiabatic electron–optical systems of gyrotrons.

Tolerance Studies on an Inverse Magnetron Injection Gun for a 2-MW 170-GHz Coaxial-Cavity Gyrotron

The magnetron injection gun (MIG) is the most critical part of any gyrotron. Small tolerances in the manufacturing process and alignment of the subcomponents directly affect the electron beam quality and therefore the beam wave interaction. At the Karlsruhe Institute of Technology (KIT), an innovative new Inverse MIG (IMIG) is proposed for the European 2-MW 170-GHz coaxial-cavity gyrotronwhich is under the developmentand test at KIT. The design of the IMIG has been done under strict consideration of the gun design criteria published earlier. In order to find the maximum allowed tolerances of that IMIG which allows operation within the allowed gun design criteria, systematic theoretical studies have been done. Commonly used “conventional” MIGs have been shown that a small emitter, anode, and cathodemisalignment have a significant influence to the electron beam quality.

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 TE46,17 mode.

A gyrating electron beam source for frequency tunable, 200 GHz gyrotron

Magnetron injection gun (MIG) is an important and critical part of any gyrotron. The overall performance of gyrotron deeply depends on the quality of gyrating electron beam. In this article, the design of triode type magnetron injection gun for 200 GHz frequency tunable gyrotron is discussed. It is planned to design and develop a 200 GHz gyrotron for 300 MHz DNP-NMR spectroscopy. The triode MIG is designed in such a way that it can be used in 200 GHz gyrotron of higher RF power for other applications also such as electron spin resonance, imaging. First, geometrical and electrical parameters of MIG are calculated using trade-off and scaling equations. Based on the calculated parameters, further, electron beam trajectory simulations are performed in EGUN. The geometry of electrodes are optimized through numerous simulation iterations to achieve the required beam parameters, such as, minimum spread in velocity and guiding center radius, an optimum value of velocity ratio (1.3–1.5), guiding center radius as per the maximum beam-wave coupling strength. The magnet system is designed to achieve the smooth Gaussian type of magnetic field profile with peak value of 7.35 T at cavity center. Further detail parametric analyses are performed which would be helpful in the frequency tuning and long-time stable operation of gyrotron device.

A 220/247.5/275-GHz, 1.0-MW, Triple Frequency Regime Gyrotron

In this paper, a complete design methodology of a triple frequency gyrotron is presented, which can also be further applied to multifrequency operations. Frequencies for operation are selected as 220, 247.5, and 275 GHz for the localized and intense heating of magnetically confined plasmas (i.e., electron cyclotron heating and current drive) for future fusion reactors. A cautious selection procedure of the mode triplet is portrayed in accordance with the all possible physical and technical constraints. Typical cold cavity (beam absent) and single-mode (beam present) calculations are performed and presented with extended interaction structure (including an optimized nonlinear taper section). A triode type configuration is adopted for magnetron injection gun to produce the electron beam with desired characteristics as required for RF behavior. Time-dependent multimode calculations are presented with nonuniform magnetic field and beam parameters optimized by gun simulations. These rigorous calculations affirm proper working of the design with $\approx 1$ -MW continuous wave power for chosen mode triplet and efficiency $\approx 35$ %.

Theory and Experiment Investigate of a 400-kW Ku-Band Gyro-TWT With Mode Selective Loss Loading Structure

A high-power Ku-band TE01 mode gyro-traveling wave tube (gyro-TWT) with a loss loading structure is designed, fabricated, and tested. A high beam quality magnetron injection gun generating 68-kV, 20–30-A electron beam is optimized to provide a high power radiation. A novel lossy ceramic loaded structure with mode selective attenuation ability is proposed to suppress the potential backward wave oscillation. The interaction circuit is analyzed by linear, nonlinear self-consistent theory, and experiment. An improved wideband input structure design is proposed accordingly. In the hot test, the proposed gyro-TWT produces a 420-kW maximum peak power, about 35-dB saturated gain, 23% efficiency, and more than 1.6 GHz (10%)−1-dB bandwidth in a stable operating condition.

I/O System for A 77/154-GHz, 0.5-MW Dual Regime Gyrotron

This paper contributes to the design of a common input/output (I/O) system to support a dual regime gyrotron operation. I/O system comprises of different subassemblies such as magnetron injection gun (MIG), quasi-optical launcher, and RF window. The operating frequencies selected are 77/154 GHz with the desired mode-pair TE15,4/TE28,8, respectively. The necessary selection criteria of such unified design for these subassemblies has been briefly discussed. The initial design parameters have been selected using our in-house code Gyrotron Design Suite version 3.0 2014 (GDSv3.0 2014). Final optimization of MIG to generate electron beam with proper parameters to support the maximum power transfer has been carried out using a particle trajectory code ESRAY. A dimpled wall, helical cut launcher has been designed to convert the complex higher order modes to a simple Gaussian-like beam and has been optimized with a commercial software launcher optimization tool. The design and optimization of RF window has been done using GDSv3.0, which will allow the chosen frequency beam to be extracted out from the gyrotron.

Magnetron Injection Gun Design for Multifrequency Band Operations

In this paper, a novel triode-type magnetron injection gun (MIG) with three emitters designed for gyrotron traveling wave amplifiers has been presented. This MIG can be operated in Ku/Ka/Q single band or Ku-Ka/Ka-Q dual band. The optimization of the MIG is accomplished by a multiobjective genetic algorithm method. It is anticipated that this MIG possesses an axial velocity spread of <5% in all the operating frequency bands. The results of sensitivity study show that this MIG can work stably within reasonable parametric windows. Velocity spreads considering cathode surface roughness and initial thermal temperature are analyzed. Cathode configuration with heat insulation chambers is introduced to cut off the thermal flow among the emitters. Thermal analysis has been performed.

The Electron-Optical System of a Gyrotron with an Operating Frequency of 263 GHz for Spectroscopic Research

We describe specific features of modeling numerically the operation of magnetron-injection guns, which form high-quality helical electron beams in gyrotrons operated in the short-wave part of the millimeter-wave band (at a wavelength of 1 mm). As an example, we consider the gun of a gyrotron having an operating frequency of 263 GHz designed for spectroscopic research. It is shown that there are good reasons to perform calculations and optimization of the magnetroninjection un in two steps. At the first step, a simplest two-dimensional model can be used, which allows only for the influence of the field of the electrodes and the intrinsic space charge of the beam on the beam parameters. At the second, final stage one should allow for such factors as roughness of the emitting surface and thermal velocities of electrons. The electron distribution function in oscillatory velocities and the coefficient of electron reflection from the magnetic mirror should be calculated. It is demonstrated that the magnetron-injection gun, which is optimized by the method presented, is sufficiently universal and can be operated both at the first and second cyclotron-frequency harmonics. This opens up the possibility of developing gyrotrons for spectroscopy applications at frequencies of 263 and 526 GHz, respectively, which are required for biological and medical research.

Extended RF Behavior of a 77/154 GHz, 0.5 MW Continuous Wave Gyrotron

An extended RF-behavior study of a dual regime gyrotron for large helical devices (LHDs) is presented in this paper. The frequencies of taken operation are 77 and 154 GHz as used in the LHD experimental campaign. The mode-pair selected, after careful consideration of practical tradeoff parameters, for successful realization of the design is the TE15,4 and TE28,8 modes as the mode-pair for operation at 77 and 154 GHz, respectively. Rigorous mode competition and cold-cavity analysis have been done to ensure the feasibility of this mode-pair. For RF behavior, an extended cavity, which is comprised of a down-taper and mid-section conventional cavity followed by a nonlinear uptaper section, is considered. Self-consistent single-mode and time-dependent multimode computations are also carried out for both frequencies. These calculations were performed with a nonuniform magnetic field over the extended cavity section before and after space-charge neutralization using realistic beam parameters obtained from the magnetron injection gun calculations. From these rigorous calculations 0.5 MW, continuous wave power is obtained for chosen mode-pair with efficiency more than 38%.

An Inverse Magnetron Injection Gun for the KIT 2-MW Coaxial-Cavity Gyrotron

An inverse magnetron injection gun (MIG) has been designed for the 2-MW, 170-GHz, coaxial-cavity gyrotron built at the Karlsruhe Institute of Technology. The inverse gun design could offer the possibility for the implementation of a larger emitter ring without the need for a bigger bore hole in the magnet compared with the conventional type of MIGs. Considering the fundamental beam parameters, an excellent beam quality has been achieved in numerical simulation. Electron-trapping suppression criteria were considered during the design phase of the MIG.