Megawatt Gyrotrons Research Articles

Mode Excitation in Gyrotrons With Triode-Type Electron Guns

Gyrotrons operating in high-order modes often suffer from severe mode competition. For the selective excitation of the desired mode, triode-type electron guns are often used. In the case of triode-type guns, where the beam voltage and the mod-anode voltage can be varied independently, the mode excitation during the startup process is studied in this article by using a generalized approach. The term “generalized approach” means that the obtained results can be valid for gyrotrons operating at arbitrary voltages and in any mode. The conditions for excitation of the modes are analyzed for different types of startup scenarios. The goal of the study is to find such relations between the beam and mod-anode voltages during the gyrotron startup that only the desired mode will be excited. It is shown that by using a specific triode-type startup scenario with a proper timing for the rise of the mod-anode voltage with respect to the beam voltage, the initial excitation of the desired mode can be realized in the cases of practically any mode density. The mode interaction in such a case is studied; also, the dependences of the results on the timing relation between the two voltages and on the voltage rise speed are considered. This article also contains the numerical analysis of a specific megawatt (MW) gyrotron with a triode-type gun.

Numerical Simulation and Experimental Study of High-Power Gyrotrons with Energy Recovery

We present the results of numerical simulation and experimental study of high-pow gyrotrons with recovery of the residual energy of the spent electron beam. The results of calculation of megawatt gyrotrons operated at frequencies of 110 and 170 GHz are given. A simplified method for calculating gyrotrons with energy recovery in the case of partial reflection of the electron beam is discussed. The method and results of experimental studies of gyrotrons with energy recovery are described and compared with the calculation data.

Design of master oscillator for frequency locking of a complex of megawatt level microwave sources

AbstractWe present the results of the development of a highly stable gyrotron (with long‐term stability of an operating frequency of 170 GHz being equal to 10−9) for experiments on frequency locking and control of megawatt power sources. The electron‐optical and electrodynamic systems of the gyrotron are calculated and optimized, including the magnetron‐injection gun, interaction space, collector, and quasioptical converter of radiation into a Gaussian wave beam. Main requirements for such systems are described along with their specific features, which allow one to develop a relatively simple, low cost, and reliable master generator for synchronization of a large number of continuous‐wave megawatt gyrotrons. This reveals new possibilities for creation of gyrotron complexes capable of generation coherent radiation of nearly unlimited power levels.

An Experimental Study of the External-Signal Influence on the Oscillation Regime of a Megawatt Gyrotron

The radiation-frequency locking by an external signal is experimentally studied for a megawatt gyrotron. An external signal from a magnetron is applied to the operation space of the gyrotron at a frequency of 35 GHz via a synthesized two-mirror quasioptical converter developed at the Institute of Applied Physics of the Russian Academy of Sciences. The radiation spectra are measured for both the frequency locking regime and the frequency beats. The locking region is constructed on the plane of two parameters, namely, the external-signal power and frequency. The experimental and theoretical results are compared to show a good agreement.

Development of high power gyrotrons for advanced fusion devices

Megawatt (MW) gyrotrons, with a wide frequency range from 14 to 300 GHz, are being developed as part of a collaborative electron cyclotron heating (ECH) study for advanced fusion devices and a demonstration power plant (DEMO). (1) Detailed designs for a 14 GHz 1 MW gyrotron are being developed for fabrication. For a 14 GHz radio frequency (RF) beam with high divergence, a calculated transmission efficiency of 94% to the corrugated waveguide coupling position was obtained initially by introducing the design concept (direct RF beam coupling by built-in waveguide) to minimize the RF transmission path. Installing a double-disk sapphire window will make it possible to develop a 1 MW gyrotron with a continuous wave (CW) at 14 GHz. (2) In experimental tests of a new 28/35 GHz dual-frequency gyrotron, the cooling characteristics of an optimal-structure double-disk sapphire window were evaluated. We confirmed that operating at 0.4 MW with a CW at 28 GHz is feasible, reaching twice the output power reported in previous studies. In a 2 ms short-pulse experimental test, maximum powers of 1.65 MW at 28.04 GHz and 1.21 MW at 34.83 GHz were achieved. (3) A design study of a 77/51 GHz dual-frequency gyrotron was performed. Oscillations above 1.5 MW for 77 GHz and 1.3 MW for 51.88 GHz are expected for a beam voltage Vk = 80 kV and beam current Ik = 60 A. (4) In an experiment with a 300 GHz gyrotron, the influence of the wave reflected from the window was reduced by tilting the output window, and mode competition in the cavity was suppressed. An output power of 0.62 MW with a pulse width of 1 ms, which is the new record for this frequency, was obtained. (5) We also performed a trial design study of a 240 GHz gyrotron for DEMO.

Laser-driven semiconductor switch for generating nanosecond pulses from a megawatt gyrotron

A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 μs, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532 nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures.

Frequency Conversion of High-Power Gyrotron Radiation under Conditions of Raman Backscattering on an Auxiliary Electron Beam

The regime of Raman backscattering of a pumping wave that is cocurrent with an electron beam into the counterpropagating wave can be used for the radiation frequency conversion in high-power gyrotrons. The absolute instability developing in this system ensures generation of a scattered signal in the absence of external resonators, which allows the frequency of scattered radiation to be smoothly controlled within 20–40% by varying the electron energy in the case of high-power (megawatt) gyrotrons used as sources of pumping radiation.

Преобразование частоты излучения мощных гиротронов в условиях обратного рамановского рассеяния на дополнительном электронном пучке

AbstractThe regime of Raman backscattering of a pumping wave that is cocurrent with an electron beam into the counterpropagating wave can be used for the radiation frequency conversion in high-power gyrotrons. The absolute instability developing in this system ensures generation of a scattered signal in the absence of external resonators, which allows the frequency of scattered radiation to be smoothly controlled within 20–40% by varying the electron energy in the case of high-power (megawatt) gyrotrons used as sources of pumping radiation.

Development of 28 GHz Gyrotron for Cooperative ECH Study

At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron–driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.

The Collector of a Megawatt Gyrotron with a Static Nonadiabatic Magnetic Field

We have developed the concept and design of a collector with a static nonadiabatic magnetic field, which allow one to exclude the influence of pulsed thermal loads and accumulation of nonelastic deformations in the collector body, as well as achieve the nearly uniform distribution of the heat release power density over its surface for an energy load of about 500 W/cm2. Such a collector is characterized by moderate criticality with respect to variations in the electron energy distribution and the effect of an external scattering magnetic field. The decrease in criticality is achieved by choosing the cylindrical shape of the collector and using two pairs of counter-wound low-power solenoids in the collector system.

Development of 154 GHz 1 MW Gyrotron for ECRH of LHD

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. Development of a new 154 GHz 1 MW gyrotron for ECRH system of Large Helical Device (LHD) has been done. The design studies of electron gun (MIG), cavity oscillator, mode converter, transmission mirrors, collector and superconducting magnet (SCM) have indicated more than 1 MW output with efficiency ~33 % for cavity oscillation mode of TE28,8. In the short pulse test of the first test tube, the maximum output power of 1.06 MW and the maximum total efficiency of 35.2 % with CPD were obtained.

Electrical and thermo-mechanical analysis of beam recovery system for megawatt power gyrotron

The paper presents the electrical and thermo-mechanical design of single stage beam recovery system for 120GHz, 1MW gyrotron. The electrical study shows that the cylindrical shape single stage beam recovery system enhances the efficiency by 66.26%. The maximum power deposited to collector in depressed collector operation is 0.48MW for electronic efficiency, 30% and 1.44MW for DC electron beam. The thermo-mechanical analysis has been performed to evaluate the water cooling system. The cooling system has capability of accommodating a peak wall loading, 0.9kW/cm2 at flow rate of 1500l/min for safe operating time, 60ms. Further, a high voltage analysis is also carried out to appraise the electric field distribution in the collector.

Suppression of spurious mode oscillation in mega-watt 77-GHz gyrotron as a high quality probe beam source for the collective Thomson scattering in LHD

Collective Thomson scattering (CTS) diagnostic requires a strong probing beam to diagnose a bulk and fast ion distribution function in fusion plasmas. A mega-watt gyrotron for electron cyclotron resonance heating is used as a probing beam in the large helical device. Spurious mode oscillations are often observed during the turning on/off phase of the modulation. The frequency spectra of the 77-GHz gyrotron output power have been measured, and then one of the spurious modes, which interferes with the CTS receiver system, is identified as the TE(17,6) mode at the frequency of 74.7 GHz. The mode competition calculation indicates that the increase of the magnetic field strength at the gyrotron resonator can avoid such a spurious mode and excite only the main TE(18,6) mode. The spurious radiation at the 74.7 GHz is experimentally demonstrated to be suppressed in the stronger magnetic field than that optimized for the high-power operation.

Numerical Design of Megawatt Gyrotron with 120 GHz Frequency and 50% Efficiency for Plasma Fusion Application

The design of 120 GHz, 1 MW gyrotron for plasma fusion application is presented in this paper. The mode selection is carried out considering the aim of minimum mode competition, minimum cavity wall heating, etc. On the basis of the selected operating mode, the interaction cavity design and beam-wave interaction computation are carried out by using the PIC code. The design of triode type Magnetron Injection Gun (MIG) is also presented. Trajectory code EGUN, synthesis code MIGSYN and data analysis code MIGANS are used in the MIG designing. Further, the design of MIG is also validated by using the another trajectory code TRAK. The design results of beam dumping system (collector) and RF window are also presented. Depressed collector is designed to enhance the overall tube efficiency. The design study confirms >1 MW output power with tube efficiency around 50% (with collector efficiency).

Development of Mega-Watt Gyrotrons for Fusion Research

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. We are developing a new 28 GHz 1 MW and a 77 GHz 1.5 MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. In the short pulse test of 77 GHz gyrotron, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec. with 1 MW and 4500 sec. with 0.2 MW. The design study of 154 GHz 1MW gyrotron for LHD has been started. For each cavity oscillation mode, TE28,8, TE28,12 and TE31,8, cavity, electron gun (MIG), mode converter, collector and SCM design are being examined.

Experimental Investigations and Analysis of Parasitic RF Oscillations in High-Power Gyrotrons

Megawatt gyrotrons are found to suffer from various parasitic oscillations, in particular, RF oscillations in the beam tunnel prior to the desired interaction zone (the cavity). This paper describes the experimental results from a gyrotron experiment which was dedicated to investigate parasitic oscillations in the beam tunnel and to verify improved beam-tunnel structures. A system for improved spectral measurements and a new analysis method are presented. The results verify theoretical predictions on the parasitic oscillations, and in effect validate the corresponding improved beam-tunnel structure. In addition, other types of parasitic oscillations were observed and explained.

Scattering by structural inhomogeneities of CVD diamonds in the 100–1000 GHz range

We study the scattering of electromagnetic radiation by structural inhomogeneities of CVD diamond windows used to output the power produced by megawatt gyrotrons in the 100–1000 GHz range. It is shown that the scattering can make a significant contribution to the attenuation of the radiation starting from 200 GHz, and becomes a determinant factor near 1 THz. Even in the framework of the single-scattering approximation, about one-third of the scattered power is released beyond the window.

Numerical simulation and experimental study of an electron-optical system of a megawatt gyrotron with step frequency tuning in the range 100–170 GHz

We present the results of numerical simulation and experimental studies of the systems of forming helical electron beams with different topologies for a 1-MW gyrotron with step frequency tuning in the range 100–170 GHz. We analyze variations in the beam parameters including the distribution of electrons over the oscillatory velocities, as functions of the beam current for various accelerating voltages and magnetic fields. The results of experimental studies of a prototype of the multifrequency gyrotron in the oscillation regime such that the designed optimized electron-optical system forms an intense helical electron beam with specified parameters in a wide interval of magnetic fields are shown.

Megawatt Gyrotrons for ECR Heating and Current-Drive Systems in Controlled-Fusion Facilities

We discuss the results and main trends in developing megawatt gyrotrons used as microwave sources for electron-cyclotron wave systems in controlled-fusion facilities. Such systems require gyrotrons with power no less than 1 MW operable during a time of about 1000 s in the frequency range 110–170 GHz. We also highlight the main design principles of such gyrotrons and the most striking aspects and prospects of their development in the last years.

The absorption investigation in CVD-diamond plates and windows at 50–200 GHz

Absorption investigations into a number of CVD-diamond plates, as well as absorption investigations into the output windows of megawatt gyrotrons in the range of frequencies 50–200 GHz have been carried out. Significant additional surface absorption was found and its influence was defined. The theory for measurement of this additional absorption was developed. The possible mechanism of microwave absorption in CVD-diamond is briefly discussed.