Single-stage Depressed Collector Research Articles

Design and experimental results of a 28 GHz, 400 kW gyrotron for electron cyclotron resonance heating

A high-power 28 GHz gyrotron has been successfully developed at the Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron was designed for electron cyclotron resonance heating (ECRH) in the spherical tokamak XL-50. A diode magnetron injection gun was designed to produce the required gyrating electron beam. The gyrotron operates in the TE8,3 mode in a cylindrical open cavity. An internal quasi-optical mode converter was designed to convert the operating mode into a fundamental Gaussian wave beam and separate the spent electron beam from the outgoing microwave power. A tube has been built and successfully tested. The operational frequency of the tube is 28.1 GHz. For beam parameters at an accelerating voltage of 71 kV and beam current of 16 A, the gyrotron has delivered an output power of 400 kW, with a pulse length of 5 s. The output efficiency is about 50% with a single-stage depressed collector. The gyrotron has been installed on the XL-50 and has played an important role in the ECRH experiments.

Gain and Bandwidth Improvement Studies of Millimeter Wave Periodically Dielectric Loaded Gyro-Twystron Amplifier

In this article, the spurious oscillations in the output section of gyro-twystron have been analyzed and the design and modeling of its various subassemblies also been presented. The periodic dielectric loading (PDL) technique has been adapted in the output waveguide section to improve the device gain by suppressing the backward wave oscillations (BWOs). Further, to improve the performance of the amplifier all the cavities were staggered tuned. The particle-in-cell (PIC) simulation of the present PDL gyro-twystron predicted an RF output power of ~120 kW in the fundamental TE01 mode at 94 GHz. The conversion efficiency, 3-dB bandwidth, and saturated gain were calculated as ~34%, ~2.2 GHz, and ~51 dB, respectively. A triode magnetron injection gun (MIG) was designed for 60 kV and 6 A gyrating electron beam and optimized for the velocity spread of ~2.2% with a velocity ratio of 1.6. A single-stage depressed (SSD) collector has also been designed to enhance the efficiency of the amplifier to ~57%. A double-disk window was designed and optimized for the broader bandwidth of ~8 GHz.

Design and Simulation of High-Aspect-Ratio Sheet Beam EIK at 0.22 THz

To improve the output performance of 0.22 THz extended-interaction klystrons (EIKs), we utilized a 20:1-aspect-ratio sheet beam to drive the circuit. The applicable high-frequency structure is discussed to obtain effective characteristic impedance, which is conducive to stronger beam-wave interaction. In addition, the loaded waveguide and the tuning scheme in the input cavity are proposed to provide good electric field uniformity over the whole beamwidth. Moreover, the nonuniform period output circuit based on the phase dynamic synchronization technology is optimized to further enhance power. Meanwhile, considering the feasibility of cathode performance, the beam current density in the tunnel is limited to 330 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The designed four-cavity interaction circuit is comprised of two 9-gap idler cavities and 11-gap input and output cavities. With the operating parameters of 16.2 kV beam voltage and 0.66 A beam current, the particle-in-cell (PIC) simulation predicts a stable output power of 700 W with a 39 mW input excitation. The corresponding gain and 3 dB bandwidth are 42.5 dB and 500 MHz, respectively. An asymmetric single-stage depressed collector is also designed to recover the spent beam energy from the interaction circuit. The collector efficiency is 81.1% with zero electron reflux rate while taking secondary electrons into consideration, and the overall tube efficiency is 26.9%.

Development of a 28-GHz/50-kW/30-s Gyrotron System for Fusion Application

A complete 28-GHz/50-kW gyrotron system was developed by the Institute of Applied Electronics, China Academy of Engineering Physics, Mianyang, China, as a plasma heating source for the EXL50 fusion device, a compact spherical tokamak constructed by the ENN Group in 2019. The gyrotron employs a triode magnetron injection gun, a TE02-mode cylindrical cavity, a built-in quasi-optical mode converter, a single-stage depressed collector, and a single-disk boron nitride window. The gyrotron system features a match optical unit, a superconducting magnet, a calorimetric dummy load, a high-voltage power supply system, and other auxiliary components, was delivered to the EXL50 site. In the acceptance test, five successive shots of each power over 50 kW were generated for 30 s at 28 GHz. An average power of 51 kW, a maximum power of 55 kW, and 46% overall efficiency were obtained. The gyrotron system was assembled in the EXL50 and applied to a plasma discharge experiment. The gyrotron is the first practical domestic gyrotron produced for fusion applications in China. The design and test results of this instrument are presented in this article.

Investigation of electron optical gun and beam collector for 42 GHz, 200 kW second harmonic gyrotron

This paper presents a design investigation of the Electron Optical Gun (EOG) for a 42 GHz, 200 kW second harmonic gyrotron operating in the TE0, 3 mode. The initial design of MIG is done using established design equations and then optimized through numerical simulations. The different software, such as 2D-Numerical codes EGUN, TRAK and 3D-CST Particle Studio, are used to optimize the design and found good agreement in results obtained from these codes. The sensitivity analysis has also been carried out for the sake of complete design. Furthermore in brief, a design has been done for the un-depressed and Single-Stage Depressed Collector (SDC). In addition, the uneven or uniform beam distribution on collector wall is presented with result comparison of both the collectors. The optimized beam voltage and single-stage depressed collector voltage are 65 kV, 30 kV respectively.

Design and Efficiency Enhancement Studies of Periodically Dielectric Loaded W-Band Gyro-TWT Amplifier

In this article, design, simulation, and efficiency improvement of W-band gyrotron traveling wave tube (gyro-TWT) amplifier operating at fundamental TE01 mode have been presented. A triode-type magnetron injection gun (MIG) has been designed and optimized for the desired quality of electron beam with a minimum spread 2% with velocity ratio 1.12 to improve the electronic efficiency of gyro-TWT. Parasitic modes in gyro-TWT have been suppressed by employing a periodic dielectric loading in the RF interaction circuit. The simulated amplifier developed an output power of ~203 kW in fundamental TE01 mode with an electronic efficiency ~29% and power gain ~57 dB for a nonspread gyrating electron beam. To improve the overall efficiency of gyro-TWT, a single-stage depressed collector has been designed as a spent electron beam recovery system. The proposed collector has enhanced the overall efficiency of gyro-TWT to ~48%.

Design and simulation investigations of high-power millimeter-wave gyrotron

ABSTRACTIn this paper, a high power millimeter wave gyrotron cavity was modelled and studied for its beam-wave interaction, beam-misalignment, and thermal issues. The sub-assemblies including the magnetron injection gun (MIG), RF output window, and electron beam collector were investigated. The MIG has produced an annular electron beam with transverse velocity spread less than ∼4%. The Particle–In–Cell simulation predicted an RF output of 470 kW in TE10,4 mode at ∼139.52 GHz for a DC drive of 80 kV, 20 A. The temperature on the inner, outer surfaces of the cavity and the corresponding average radial expansion of the cavity were calculated. Due to the radial expansion of the cavity, the resonant frequency and RF output power has changed to ∼0.514 GHz and ∼50 kW, respectively. Finally, the overall efficiency of the present gyrotron was improved from ∼29% to ∼47% by using a single–stage depressed collector.

CW Operation of a W-Band High-Gain Helical-Waveguide Gyrotron Traveling-Wave Tube

Experimental details are discussed and millimeter wave measurements presented of a W-band gyrotron traveling-wave tube (gyro-TWT) that uses a helically corrugated interaction waveguide operating at the second electron cyclotron harmonic. A method for increasing the gain by using an interaction circuit with two sections separated by a long sub-cutoff drift section has been experimentally verified. A 10-mW driving source was sufficient to enable an amplified signal output power of 3 kWs. The continuous-wave (CW) operation with a single-stage depressed collector and deposited beam power of about 20 kW is believed to be the first demonstration of its kind for a W-band gyro-TWT.

State-of-the-Art of High-Power Gyro-Devices and Free Electron Masers

This paper presents a review of the experimental achievements related to the development of high-power gyrotron oscillators for long-pulse or CW operation and pulsed gyrotrons for many applications. In addition, this work gives a short overview on the present development status of frequency step-tunable and multi-frequency gyrotrons, coaxial-cavity multi-megawatt gyrotrons, gyrotrons for technological and spectroscopy applications, relativistic gyrotrons, large orbit gyrotrons (LOGs), quasi-optical gyrotrons, fast- and slow-wave cyclotron autoresonance masers (CARMs), gyroklystrons, gyro-TWT amplifiers, gyrotwystron amplifiers, gyro-BWOs, gyro-harmonic converters, gyro-peniotrons, magnicons, free electron masers (FEMs), and dielectric vacuum windows for such high-power mm-wave sources. Gyrotron oscillators (gyromonotrons) are mainly used as high-power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control, and diagnostics of magnetically confined plasmas for clean generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 140 GHz, megawatt-class gyrotrons employing synthetic diamond output windows is 30 min (CPI and European KIT-SPC-THALES collaboration). The world record parameters of the European tube are as follows: 0.92 MW output power at 30-min pulse duration, 97.5% Gaussian mode purity, and 44% efficiency, employing a single-stage depressed collector (SDC) for energy recovery. A maximum output power of 1.5 MW in 4.0-s pulses at 45% efficiency was generated with the QST-TOSHIBA (now CANON) 110-GHz gyrotron. The Japan 170-GHz ITER gyrotron achieved 1 MW, 800 s at 55% efficiency and holds the energy world record of 2.88 GJ (0.8 MW, 60 min) and the efficiency record of 57% for tubes with an output power of more than 0.5 MW. The Russian 170-GHz ITER gyrotron obtained 0.99 (1.2) MW with a pulse duration of 1000 (100) s and 53% efficiency. The prototype tube of the European 2-MW, 170-GHz coaxial-cavity gyrotron achieved in short pulses the record power of 2.2 MW at 48% efficiency and 96% Gaussian mode purity. Gyrotrons with pulsed magnet for various short-pulse applications deliver Pout = 210 kW with τ = 20 μs at frequencies up to 670 GHz (η ≅ 20%), Pout = 5.3 kW at 1 THz (η = 6.1%), and Pout = 0.5 kW at 1.3 THz (η = 0.6%). Gyrotron oscillators have also been successfully used in materials processing. Such technological applications require tubes with the following parameters: f > 24 GHz, Pout = 4–50 kW, CW, η > 30%. The CW powers produced by gyroklystrons and FEMs are 10 kW (94 GHz) and 36 W (15 GHz), respectively. The IR FEL at the Thomas Jefferson National Accelerator Facility in the USA obtained a record average power of 14.2 kW at a wavelength of 1.6 μm. The THz FEL (NOVEL) at the Budker Institute of Nuclear Physics in Russia achieved a maximum average power of 0.5 kW at wavelengths 50–240 μm (6.00–1.25 THz).

Design of a Multistage Depressed Collector for ${W}$ -Band Pulsed Traveling-Wave Tubes

This article presents a design method for multistage depressed collectors (MDCs) in W-band traveling -wave tubes (TWTs) using microwave tube simulation studio (MTSS) code. First, the conditions of the spent electron beam, including electron position and velocity distributions, were simulated. Then, to verify the feasibility and effectiveness of the simulation, the output power, electronic efficiency, collector efficiency and overall efficiency of the existing 250-W W-band pulsed TWT with a single-stage collector were also simulated making use of the above-spent beam conditions at the entrance of the collector. The simulated data were compared with the test results, indicating a reasonable agreement between them. Finally, the spent beam conditions at the entrance of the collector were employed to design a four-stage depressed collector, based on the folded waveguide circuit of the TWT. The simulated results showed that the overall efficiency of the TWT with the designed four-stage depressed collector increased by more than 37.26% in the whole frequency band, with the maximum overall efficiency of 44.65%. In comparison with the single-stage depressed collector, the collector efficiency and the overall efficiency of the TWT were increased from 75% to 88.85% and 26.45% to 44.65%, respectively. This article is believed to provide some guidance for the design of MDC of higher frequency band TWTs.

The anode power supply based on the pulse step modulation technology for the ECRH system on J-TEXT tokamak

A 105 GHz/500 kW electron cyclotron resonance heating system on J-TEXT tokamak is under development using a diode type GYCOM gyrotron with a single-stage depressed collector. To modulate the output power of the gyrotron, we designed a 33 kV/1 A anode power supply based on the pulse step modulation technology. The power supply consists of 40 modules with a fixed output voltage of 800 V and 10 modules with a fixed output voltage of 100 V, and these modules are connected in series. In this way, the output voltage of the anode power supply can be adjusted accurately without pulse width modulation. Results of the bench test with dummy load indicates that the output capability and the protection time in fault condition of the designed power supply can meet requirements of the ECRH system on J-TEXT.

Simulation of a depressed collector for millimeter wave gyrotron

The trajectories of spent electron beam were simulated from the mode converter to the single-stage depressed collector for a W-band gyrotron. The spent beam trajectories and power distribution data obtained from large signal analysis were fed to the simulation model. The collector geometry and the magnetic field were profiled to ensure the landing of the electrons to the wider cylindrical section of the collector electrode for better thermal management, which was ensured with ±5% variation in the magnetic field and no electron interception. The power dissipation and the maximum thermal loading on the collector were 80.55 kW and 0.377 kW/cm2, respectively. The collector efficiency was ∼48% for 120 kW RF generation. The thermal analysis of the collector in pulsed condition with 10 and 40 L per minute flow rates predicted the maximum temperature at the collector electrode as 151.8°C and 133.3°C, respectively.

Gain- and Phase-Matched Mini-TWTs

Traveling-wave tubes (TWTs) in broadband power-combining applications should have closely matched gain and phase to achieve high system-level efficiency. A gain- and phase-matched TWT operating over a frequency band of 7.5–18 GHz with a minimum output power of 60 W is currently under production at Teledyne e2v. The tube design is based on an existing mini-TWT that incorporates focus electrode switching gun, phase velocity tapered helix slow wave structure, single-stage depressed collector, subminiature A connector at the input, and threaded Neill–Concelman connector at the output. During development and production stages, a number of devices were manufactured and their performances were matched. In this paper, we have described gain- and phase-matching techniques of such devices. Experimental results of 132 matched devices have been collated that show a gain match within ±3 dB and phase match within ±15°.

Numerical Investigation on Spent Beam Deceleration Schemes for Depressed Collector of a High-Power Gyrotron

Single-stage depressed collector systems are widely used in high-power gyrotrons in order to increase the overall efficiency. The position where the deceleration of the beam takes place influences the collector efficiency. In this paper, several deceleration schemes are numerically investigated for the short-pulse prototype of the 170-GHz, 1-MW European gyrotron for International Thermonuclear Experimental Reactor. The results of this paper are in a qualitative agreement with the results of dedicated experiments. A simple idea is also proposed for the significant increase of the overall gyrotron efficiency without an important technological modification.

Design of 170 GHz, 1.5-MW Conventional Cavity Gyrotron for Plasma Heating

In this paper, an overall conceptual design of a 170 GHz, 1.5 MW, continuous wave (CW) conventional cavity gyrotron is presented for plasma heating applications in thermonuclear fusion reactors. The operating mode is carefully selected with due consideration of design constraints/goals and mode competition. The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">36</sub> ,10 mode is selected as operating mode for the present study. A weakly tapered conventional cavity resonator is considered for the study of the RF-behavior. Single mode and multimode time dependence self-consistence calculations are carried out for power and efficiencies. In addition, the design studies of a triode type magnetic injection gun, magnetic guidance system, output system that consists of an optimized nonlinear taper, a highly efficient dimpled-wall quasi-optical launcher and a single disk Chemical Vapor Deposition diamond window are also reported. Results obtained support an output power of 1.5-MW CW power at 170 GHz with a conventional cavity gyrotron with 35% efficiency without single stage depressed collector. This device is intended to serve as heating source for international thermonuclear experimental reactorlike machines.

A Study on 94 GHz Low-Voltage, Low-Current Gyrotron

A theoretical and experimental study on a 94 GHz low-voltage, low-current gyrotron is presented in this paper. In order to obtain stable radiation and make great use of the equipment in the lab, the TE6,2 mode is selected as the operating mode of the desired gyrotron. The efficiency of the electron beam interaction with RF fields under different conditions has been calculated by a code, which is based on the self-consistent nonlinear theory for gyrotrons. The gyrotron is equipped with a single-stage depressed collector and a quasi-optical mode converter. Then, a gyrotron with optimized parameters has been designed, constructed, and tested. An output power of 23 kW is obtained at an accelerating beam voltage of 31.5 kV, a beam current of 1.8 A, and a collector depression voltage of 7.4 kV, corresponding to an overall efficiency of 53%.

Towards a 1 MW, 170 GHz gyrotron design for fusion application

The electrical design of different components of 1MW, 170GHz gyrotron such as, magnetron injection gun, cylindrical interaction cavity and collector and RF window is presented in this article. Recently, a new project related to the development of 170GHz, 1MW gyrotron has been started for the Indian Tokamak. TE34,10 mode is selected as the operating mode after studied the problem of mode competition. The triode type geometry is selected for the design of magnetron injection gun (MIG) to achieve the required beam parameters. The maximum transverse velocity spread of 3.28% at the velocity ratio of 1.34 is obtained in simulations for a 40A, 80kV electron beam. The RF output power of more than 1MW with 36.5% interaction efficiency without depressed collector is predicted by simulation in single-mode operation at 170GHz frequency. The simulated single-stage depressed collector of the gyrotron predicted the overall device efficiencies>55%. Due to the very good thermal conductivity and very weak dependency of the dielectric parameters on temperature, PACVD diamond is selected for window design for the transmission of RF power. The in-house developed code MIGSYN and GCOMS are used for initial geometry design of MIG and mode selection respectively. Commercially available simulation tools MAGIC and ANSYS are used for beam–wave interaction and mechanical analysis respectively.

Design of single-stage depressed collector for 42 GHz, 200 kW gyrotron

A single stage depressed collector has been designed to dissipate the spent electron energy of 45 keV. This results in an increase of overall efficiency of 42 GHz, 200 kW gyrotron from 33% to 57%. The design of depressed collector has taken into account the performance on several sectors such as maximum beam spread, minimum power dissipation including the effect of high voltage and thermal efficiency. Different computer codes have been used in order to design this particular collector. The simulated efficiency of the single stage depressed collector is 63%.

Progress on Gyrotrons for ITER and Future Thermonuclear Fusion Reactors

The prototype of the Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW, 3600 s and 1 MW, 800 s) and the efficiency record of 57%, whereas the Russian 170-GHz ITER prototype tube achieved 0.8 MW with a pulse duration of 800 s at 55% efficiency and 1 MW at 280 s and 53%. The record parameters of the European megawatt-class 140-GHz gyrotron for the stellarator Wendelstein W7-X are as follows: 0.92-MW output power at 1800-s pulse duration, nearly 45% efficiency, and 97.5% Gaussian mode purity. These gyrotrons employ a cylindrical cavity, a quasi-optical output coupler, a synthetic diamond window, and a single-stage depressed collector (SDC) for energy recovery. In order to reduce the costs of the ITER 24-MW 170-GHz ECH&CD system, 2-MW millimeter-wave power per gyrotron tube is desirable. Cylindrical gyrotron cavities are not suitable for the 2-MW power regime because of high ohmic wall losses and mode competition problems. However, in coaxial cavities, the existence of the longitudinally corrugated inner conductor reduces the problem of mode competition, thus allowing one to use even higher order modes with lower ohmic attenuation than in cylindrical cavities. Synthetic diamond windows with a transmission capability of 2-MW CW are feasible. A 2-MW CW 170-GHz coaxial-cavity gyrotron for ECH&CD in ITER is under development in cooperation with European research institutions (EGYC, collaboration among the CRPP, Switzerland, the KIT, Germany, the HELLAS, Greece, the CNR, Italy, and the ENEA, Italy). At the Karlsruhe Institute of Technology (KIT), the short-pulse (1-ms) preprototype tube delivered 2.2 MW at 30% efficiency (without SDC) with 96% Gaussian output mode purity. Design studies for a 4-MW 170-GHz coaxial-cavity gyrotron with two synthetic diamond output windows and two 2-MW millimeter-wave output beams for future fusion reactors are currently being performed at KIT. The availability of sources with fast frequency tunability would permit the use of simple fixed nonsteerable mirror antennas for local current drive experiments and plasma stabilization. IAP Nizhny Novgorod develops in collaboration with IPP Garching and KIT an industrial multifrequency 1-MW gyrotron with approximately 50% efficiency (SDC). A four-frequency tube (105, 117, 127, and 140 GHz) delivered 0.8 MW at 105 GHz and 0.95 MW at 140 GHz in 10-s pulses. After the installation of a broadband diamond window, this gyrotron will be operated also at the two intermediate frequencies.

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.