Studies of continuous-wave submillimeter-wave gyrotrons for spectroscopy and diagnostics of various media

We present the results of experimental studies of submillimeter-wave gyrotrons, which were obtained in the Research Center for Development of Far-Infrared Region of Fukui University (Japan). Distinctive features of the systems forming helical electron beams in submillimeter-wave gyrotrons are described. The results of the theoretical studies, which were aimed at excitation of higher harmonics in submillimeter-wave gyrotrons, reducing the operating voltage, and creating devices with smooth wideband tuning of the generation frequency, are presented. It is shown that modern gyrotrons are capable of ensuring continuous-wave generation at the frequencies from 200 to 400 GHz at a power level of up to hundreds of watts, and up to tens of kilowatts in the pulse-generation regime.     

Low-frequency gyrotrons for fusion studies

A number of experimental plasma devices require high-power sources of electromagnetic radiation in a range of 5 to 30 GHz. In this paper, we consider gyrotrons as such sources. The difficulties of creating long-wavelength gyrotrons are discussed and the possibility to overcome them is shown for already existing and currently developed oscillators. We present the results of studies aimed at development of high-power, long-pulse, low-frequency gyrotrons in Russia, as well as the designs and performance parameters of industrial gyrotrons operated at frequencies 5 and 28 GHz with an output power of 0.5 MW. Several design versions of the gyrotrons with operating frequencies 17.5 and 28 GHz, an output power of up to 1 MW, and a pulse duration of up to 5 s are discussed and the design features which make it possible to overcome the difficulties of their manufacturing are considered. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 207–218, March 2006.     

Cyclotron resonance masers: State of the art

Cyclotron resonance masers, gyrotrons and gyroklystrons, are dominant high-energy sources at millimeter wavelengths and still have considerable margins of power and efficiency. The present-day challenge of creating CW megawatt gyrotrons is associated with many problems, among which the output window problem is a key one. New results of the investigation of gyrotrons and gyroklystrons in Russia are reported. A state-of-the-art analysis of gyrotrons for plasma heating in fusion installations and technological applications as well as gyroklystrons for millimeter-wavelength radars is performed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 6, pp. 635–670, June, 1996.     

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HL-2A装置电子回旋共振加热系统的主要指标是2MW/1s/68GHz,系统由4个单元组成,每个单元包括一只回旋管,微波传输系统,控制保护测量和冷却等子系统。通过对ECRH系统和回旋管的调试,每只管子微波输出功率500kW,脉冲宽度1s,四管并联运行时总输出功率达到1.63MW,系统使用效率高于80%。     

Some perspectives on the use of powerful gyrotrons for the electron-cyclotron plasma heating in large tokamaks

Conclusion The main problems for the further development of high efficiency powerful gyrotrons are obviously the elaboration of electron guns with a large current density and the ensuring of a stable single-mode operation in resonators with a large cross-section. For the increase in microwave pulse energy (with a pulse length of the order of the time of plasma confinement in large tokamaks) the investigation of the possibilities to diminish heating loads in the resonator and output window of a gyrotron is necessary.     

Influence of the axial misalignment of the electron beam and the cavity on the gyrotron parameters

We consider some effects connected with the axial misalignment of the electron beam and the cavity in gyrotrons. These effects are studied by the example of three gyrotrons with different parameters of the electron beam and output characteristics. The experimental data and the results of computer calculations are compared on the basis of a multi-mode nonstationary model of the electron-wave interaction in the gyrotron cavity.     

Improved gyrotron cavity with high quality factor

For gyrotrons operating at the second harmonic of the electron cyclotron frequency cavities with high quality factors are needed. Up to now steep output tapers or abrupt output irises are used in order to provide a high reflection coefficient. Such circuits suffer from the problem of strong conversion to unwanted output modes. A promising possibility to achieve a high reflection at the cavity output (and thus a high quality factor) combined with high mode purity is the use of a rounded iris and an optimized nonlinear taper with very low unwanted mode conversion at the output of the resonator.     

A rigorous time domain analysis of gyrotrons

A self-consistent time domain analysis of gyrotrons is presented which allows studying multi-mode, multi-frequency operation. The electromagnetic field in the gyrotron cavity is expanded with respect to complete sets of eigenfunctions so that space charge effects are included in the analysis. Two improvements of the modal expansion inside gyrotron cavities are suggested which significantly increase the accuracy and the numerical efficiency of this method, namely, the removal of the non-uniform convergence of some field series at the coupling apertures and the estimation of the asymptotic values of some slowly converging series related to the modal analysis by a moderate number of cavity eigenfunctions. Discrete Fourier transform is used to obtain the time dependence of the electromagnetic field. The electron beam is described by a set of relativistic single particles. It is demonstrated that the strong numerical requirements of the suggested method can be overcome by using a vector computer. Two gyrotrons are investigated, namely, a low Q 35 GHz TE₀₁- and a 150 GHz TE₀₃-gyrotron. Both oscillation build-up and steady state operation are investigated including mode competition and window reflections. The simulations show that the assumption of a monofrequent steady state operation of gyrotrons, which is made by the commonly used frequency domain methods, is not always justified.     

Numerical Analysis of Weakly Relativistic Large Orbit Gyrotron with Permanent Magnet System

High-harmonic gyro-devices with axis-encircling electron beams known as large orbit gyrotrons (LOG) represent an appealing alternative to the conventional gyrotrons. In this paper we investigate the feasibility of such device operating with low current and low energy electron beams formed by a novel electron gun with a permanent magnet system. The results from the numerical experiments indicate the possibility to excite TE₄₁ mode at fourth harmonic of the cyclotron frequency. Simulations predict generation of microwave radiation with frequency 104 GHz and output power near 1 kW.     

Rigorous electrodynamic analysis of resonator systems of coaxial gyrotrons

An efficient and rigorous numerical method is developed for numerical analysis of resonators of coaxial gyrotrons used for plasma heating, drag current generation, and for other purposes in controlled thermonuclear fusion facilities with magnetic confinement. Results obtained may be used to simulate operation of coaxial gyrotrons, to optimize their resonators, and to develop efficient software intended for studying the physics of coaxial gyrotrons and for their design.     

Cold-cavity and self-consistent approaches in the theory of modecompetition in gyrotrons

On the basis of a generalized theory, the cold-cavity and the self-consistent methods of calculation of mode competition in gyrotrons are compared. This comparison enables the conditions when the cold cavity approximation is justified to be determined. Specific examples are presented and numerical problems are discussed. It is concluded that mode competition can be studied by neglecting the interaction of the electrons and the RF field in the output taper in those cases when the parameter of irregularity of the output taper is >15. Here, the results obtained in the cold-cavity approximation overestimate the starting current of the parasitic mode by ~25% in comparison with the self-consistent approach. For values of the parameter of irregularity <15, the mode competition in gyrotrons must be studied only self-consistently, taking into account the interaction of the electrons and the RF field in the output taper as well as the factors which influence the interaction along the axis     

Reduction of Ohmic Losses in the Cavities of Low-Power Terahertz Gyrotrons

Radiophysics and Quantum Electronics - In the gyrotrons designed for the terahertz frequency range, the issue of primary interest is to ensure stable single-mode generation during operation at...     

Detailed Consideration of Experimental Results of Gyrotron FU CW II Developed as a Radiation Source for DNP-NMR Spectroscopy

A CW gyrotron for the sensitivity enhancement of NMR spectroscopy through dynamic nuclear polarization has been designed. The gyrotron operates at the second harmonic and frequency of 394.6 GHz with the main operating mode TE_0,6. Operating conditions of other neighboring cavity modes such as TE_2,6 at frequency of 392.6 GHz and TE_2,3 at frequency of 200.7 GHz were also considered. The experimental conditions of the gyrotron at low and high voltages are simulated. The output power of 56 watts corresponds to the efficiency of 2 percent at low voltage operation and frequency of 394.6 GHz is expected.     

Nonlinear dynamics of planar gyrotrons with transverse diffraction coupling of radiation

We propose that the planar geometry of the interaction space and transverse (relative to the direction of translatory motion of electrons) power extraction be used for increasing the integrated radiation power of short-wavelength gyrotrons. The advantage of such a scheme over the traditional cylindrical geometry of gyrotrons is the possibility to ensure coherence of radiation for a large superdimensionality factor due to diffraction mechanism of mode selection in the open transverse coordinate and locking of radiation from various fractions of the ribbon-shaped polyhelical electron beam by transverse energy fluxes. Simulation of the non-linear dynamics of a planar gyrotron demonstrates the possibility of attaining the output power of hundreds of kilowatts at frequencies of up to 1 THz. A further increase in the output power to the megawatt level can be attained by using resonators with a nonuniform profile, for which the gap between the plates increases from the center to the periphery. Such a configuration is analogous to unstable optical resonators used in some powerful lasers.     

High-power terahertz-range planar gyrotrons with transverse energy extraction

To increase the output power of terahertz gyrotrons to several hundred kilowatts, we suggest using a planar geometry of interaction space with a sheet electron beam and transverse energy extraction. An advantage of this scheme in comparison with conventional cylindrical geometry is the possibility to ensure effective mode selection over the open transverse coordinate in combination with radiation outcoupling that leads to a substantial reduction of Ohmic losses. Similar to unstable resonators in optics for further growth of the radiation power it is beneficial to introduce waveguide tapering.     

W波段交错双栅返波振荡器高频系统

将矩形交错双栅结构作为慢波电路并提出与之配套的过渡结构和输出耦合器,设计了利用带状电子注工作在W波段的返波振荡器。提出的过渡结构和耦合器解决了该类直波导型器件的信号传输衰减大、反射强等难题。相对于传统圆形电子注器件,该器件得到了较大的功率提升。利用三维粒子模拟计算的方法,在电流12 mA时通过调节工作电压,在92~98 GHz频带内得到了数W的稳定平均功率输出,信号中心频率非常接近设计频率,且单色性好,谐波分量小。     

High power laser operation with crystal fibers

We present the very last results on the development of high-power lasers with crystal fibers in Nd:YAG and in Yb:YAG grown by the Micro-Pulling Down technique. An overview of the main optical properties of the grown crystal fibers is given as well as the principles of the diode-pumped systems are developed. The average output power obtained with those materials reaches now several tens of watts in the CW regime and in high repetition rate Q-switched operation. Pulses with an energy of several millijoules have been obtained with pulse durations from 10 to 20 ns and peak powers from 100 kW to 350 kW. In each case, the measured M ² quality factors remained in the range 2.5 to 5. In addition, the first demonstration of high-power laser emission with an Yb:YAG crystal fiber of 0.4 mm in diameter is reported. In this case, we obtained an output power of 27 W at 1030 nm under 100 W of pump power at 940 nm in CW regime. To our knowledge, those results represent the highest powers ever generated with crystal fibers obtained directly from the growth. We finally conclude this work by exposing the potential of crystal fiber lasers for a new generation of high-power laser systems.     

W波段交错双栅返波振荡器高频系统

将矩形交错双栅结构作为慢波电路并提出与之配套的过渡结构和输出耦合器,设计了利用带状电子注工作在W波段的返波振荡器。提出的过渡结构和耦合器解决了该类直波导型器件的信号传输衰减大、反射强等难题。相对于传统圆形电子注器件,该器件得到了较大的功率提升。利用三维粒子模拟计算的方法,在电流12 mA时通过调节工作电压,在92~98 GHz频带内得到了数W的稳定平均功率输出,信号中心频率非常接近设计频率,且单色性好,谐波分量小。     

Forward-wave enhanced radiation in the terahertz electron cyclotron maser

Based on the principle of electron cyclotron maser (ECM), gyrotrons are among the most promising devices to generate powerful coherent terahertz (THz) radiation and play a vital role in numerous advanced THz applications. Unfortunately, THz ECM systems using a conventional high-Q cavity were theoretically and experimentally demonstrated to suffer from strong ohmic losses, and, accordingly, the wave output efficiency was significantly reduced. A scheme to alleviate such a challenging problem is systematically investigated in this paper. The traveling-wave operation concept is employed in a 1-THz third harmonic gyrotron oscillator, which strengthens electron-wave interaction efficiency and reduces the ohmic dissipation, simultaneously. A lossy belt is added in the interaction circuit to stably constitute the traveling-wave interaction, and a down-tapered magnetic field is employed to further amplify the forward-wave (FW) component. The results demonstrate that the proportion of ohmic losses is nearly halved, and output efficiency is nearly doubled, which is promising for further advancement of high-power continuous-wave operation of the ECM-based devices.     

A Beam Waveguide Launcher for High Power ECRH Heating in Large Tandem Mirror Machines

We explore the possibility of applying a quasi-optical beam waveguide of the Cassegrain type as a launching system for RF heating of a fusion plasma. The plane wave required for proper operation of the Cassegrain antenna can be obtained from the azimuthally polarized TE0n gyroton output by means of, e. g., a horn reflector with a stepped-twist reflector mounted in its throat. Our launcher has the potential to keep the loss level extremely low, continue the outputs of a number of gyrotrons, and direct the radiation on the plasma in specific directions and polarization states.