Phase-locking of a second-harmonic gyrotron oscillator using aquasi-optical circulator to separate injection and output signals

Phase-locking in a 34.5-GHz special complex cavity gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency was studied. Injection of the locking power was made via a quasi-optical circulator connected to the gyrotron output. Locking bandwidth was measured by comparing the phase of the injection signal and output signal using a balanced mixer. Locking was observed with input power level as low as 40 dB below the gyrotron output power. The locking bandwidth is, however, narrower than in gyrotrons operating at the fundamental cyclotron frequency which may be attributed to the longer resonant cavity in the second harmonic gyrotron and the corresponding larger value of external quality factor. The measurements are roughly in agreement with predictions of Adler's phase-locking equation which is given for our system in terms of powers propagating in the output waveguide toward and away from the gyrotron cavity     

Phase locking and bandwidth in a gyrotron oscillator

For imaging radar and for satelitte and space communication (e.g. NASA's deep space network), it is important that the bandwidth be as large as possible. Here we derive a formalism for computing the phase locking bandwidth that can be achieved in a gyrotron oscillator while varying the beam voltage. As an example, a second harmonic TE_02/03 gyrotron is considered. For this device, the effective bandwidth can be increased by a factor of about 3 compared with the fixed voltage case by allowing the beam voltage to change together with the input locking signal.     

基于预群聚的回旋管注入锁相研究

开展了40 kW预群聚注入锁相回旋管的理论与模拟设计。基于全电磁仿真方法完成了预群聚腔的设计,并采用给定场理论对电子束经过预调制腔后的群聚状态进行了计算。采用自洽理论获得了回旋管的自由振荡工作参数,并计算了振荡频率随各种参数变化的规律,由此提出了锁相带宽的要求。采用PIC粒子模拟进行了锁相状态的模拟,得到7 mm漂移距离下锁定增益可达30.5 dB,相应的锁相带宽为20 MHz。如果进一步增长漂移距离或者进一步增大输入功率,锁相带宽还会增大。理论计算和粒子模拟结果表明40 kW级回旋管注入锁相具有良好的可行性。     

高功率微波振荡器的相位控制

基于相对论返波管振荡器,提出了一种小信号牵引相位控制的方法,通过外加小信号对振荡器起振过程的引导,实现对输出微波的相位控制。相比于传统的方法,该方法可以利用ns量级的脉冲在较宽的带宽和较低的注入功率下实现对高功率微波振荡器的相位控制。在X波段相对论返波管振荡器相位控制实验中,利用百kW级的小信号,实现了对GW级高功率微波的输出相位控制,相位抖动小于±15°。     

Equilibrium and stability of free-running, phase-locked, andmode-locked quasi-optical gyrotrons

Equilibria and stability (both single-mode and sideband) are calculated and contrasted for free-running, phase-locked, and mode-locked oscillator configurations in a quasi-optical gyrotron. The oscillator can be phase locked by direct injection of radiation into the oscillator cavity. The equilibrium and stability properties are not greatly affected at low injection power levels. Alternatively, the oscillator could be phase locked by prebunching the beam. If the beam is prebunched, there are dramatic effects on both equilibrium and stability. The transverse efficiency can be considerably enhanced by prebunching the beam. This prebunching can be done on either a phase-locked (using an external RF source) or mode-locked (using the oscillator output) configuration. The stable locking bandwidth turns out to be about half the ω/Q linewidth of the mode     

W波段连续波30 kW回旋振荡管高频系统和收集极的设计

 根据回旋管的电子回旋脉塞理论,借助于编写的回旋振荡管自洽非线性注-波互作用计算程序,设计出了工作频率94 GHz、工作电压30 kV、工作电流3 A的基次谐波连续波单腔回旋振荡管,工作模式为TE02模。设计的回旋振荡管在电压30.0 kV、电流3.0 A、速度横纵比1.5的条件下,获得了31.8 kW的输出功率,电子效率约35％。利用粒子模拟仿真软件对设计的回旋管收集极辅助线包散焦系统进行了粒子模拟仿真分析,模拟结果表明：借助于辅助线包散焦系统可以有效缩短回旋振荡管的轴向尺寸,并使回旋管收集极上的电子束功率密度低于500 W/cm2; W波段回旋振荡管收集极的热测试验结果表明：利用粒子模拟仿真获得的收集极上的电子束功率密度分布与其试验测量结果比较吻合。     

Efficient Low-Voltage Operation of a CW Gyrotron Oscillator at 233 GHz

The gyrotron oscillator is a source of high average power millimeter-wave through terahertz radiation. In this paper, we report low beam power and high-efficiency operation of a tunable gyrotron oscillator at 233 GHz. The low-voltage operating mode provides a path to further miniaturization of the gyrotron through reduction in the size of the electron gun, power supply, collector, and cooling system, which will benefit industrial and scientific applications requiring portability. Detailed studies of low-voltage operation in the TE(2) (,) (3) (,) (1) mode reveal that the mode can be excited with less than 7 W of beam power at 3.5 kV. During CW operation with 3.5-kV beam voltage and 50-mA beam current, the gyrotron generates 12 W of RF power at 233.2 GHz. The EGUN electron optics code describes the low-voltage operation of the electron gun. Using gun-operating parameters derived from EGUN simulations, we show that a linear theory adequately predicts the low experimental starting currents.     

Mode interaction through amplitudes and phases in a two-modegyrotron oscillator

An analysis is made of temporal evolution of electromagnetic modes in a two-mode gyrotron oscillator characterized by phase and amplitude interaction through the terms linear in the oscillator power. The problem is solved in the context of amplitudes and phases which vary slowly compared with the period of oscillation. Specific reference is made to competition between TE_11q modes in a closed cavity gyrotron. Qualitative features which are found include phase locking, beat frequencies, periodic pulling, and mode excitation. This work has applicability when the frequency separation between the modes is on the order of the frequency bandwidth of each mode, or the modes are equally spaced in frequency. Gyrotrons of this type include those with low-quality-factor modes or degenerate modes. Phase interaction in the case of equally spaced cavity modes is of importance in analyzing mode-locking phenomena     

Comparison of Electron-Beam-Misalignment Effect on Starting Current and Output Power in Coaxial-Cavity and Cylindrical-Cavity Gyrotron Oscillators

Comparative study is presented to the effect of the electron-beam misalignment on the starting current and output power of the coaxial-cavity and cylindrical-cavity gyrotron oscillators operating in the millimeter wave ranges. The numerical analysis is based on the gyrokinetic formulas for a TE_28,16,1 mode at a frequency of 140 GHz. Results show that the coaxial-cavity gyrotron oscillator has lower starting current and less power loss than the cylindrical-cavity gyrotron oscillator when the electron-beam axis has a misalignment to the cavity axis.     

High-peak power Ka-band gyrotron oscillatorexperiments with slotted and unslotted cavities

A K_a-band gyrotron oscillator powered by a compact pulseline accelerator has been operated using oscillator cavities with and without axial slots. The oscillator was operated at high voltage (~900 keV) and high current (~500 A) in the approximate frequency range of 20-50 GHz. The use of axial slots has been shown to suppress low-starting-current whispering-gallery modes, in particular, modes of the TE_m2 type, allowing stable operation in a linearly polarized TE₁₃ mode. A peak power of 35 MW has been observed at 6% efficiency     

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 μs pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE_27,11 mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed     

Continuous-Wave Operation of a 460-GHz Second Harmonic Gyrotron Oscillator

We report the regulated continuous-wave (CW) operation of a second harmonic gyrotron oscillator at output power levels of over 8 W (12.4 kV and 135 mA beam voltage and current) in the TE(0,6,1) mode near 460 GHz. The gyrotron also operates in the second harmonic TE(2,6,1) mode at 456 GHz and in the TE(2,3,1) fundamental mode at 233 GHz. CW operation was demonstrated for a one-hour period in the TE(0,6,1) mode with better than 1% power stability, where the power was regulated using feedback control. Nonlinear simulations of the gyrotron operation agree with the experimentally measured output power and radio-frequency (RF) efficiency when cavity ohmic losses are included in the analysis. The output radiation pattern was measured using a pyroelectric camera and is highly Gaussian, with an ellipticity of 4%. The 460-GHz gyrotron will serve as a millimeter-wave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) experiments at a magnetic field of 16.4 T.     

回旋振荡管收集极横向磁扫描系统的设计与仿真

从降低收集极表面峰值功率和平均功率入手,设计了一种由12个横向椭圆交流线圈和2个轴向直流线圈组成的磁场扫描系统。针对140 GHz、TE_28,8模、1 MW功率输出回旋振荡管收集极的设计参数,结合电磁场理论和带电粒子电磁软件仿真,获得收集极表面上电子注的峰值和平均功率密度分别不超过404.91 W/cm2和244.01 W/cm2,以及扫描长度达到443.33 mm的扫描效果,有效地缓解了收集极功率耗散和冷却的压力。     

Ytterbium doped all-fiber-path all-normal dispersion mode-locked laser based on semiconductor saturable mirror

In this paper, we describe a compact all-fiber-path picosecond pulse based on Ytterbium doped fiber oscillator. A home-made novel SESAM mounted on fiber is reported, by which stable mode locking is obtained. The SESAM possesses the low saturation flux 20 μJ/cm² (versus prior low saturation flux 32 μJ/cm²), which effectively reduces the pump power threshold of mode locking. The fiber laser generates 15 ps pulse trains without a dispersive delay line or anomalous dispersion in the cavity. Mode locking pulse with 30 MHz basic repetition-rate was produced, with 10–30 mW scale average output power at 1064 nm. Through 60 h of uninterrupted laser operating, mode locking is steady as ever.     

Response of a gyrotron to small-amplitude low-frequency-modulated microwaves reflected from a plasma

Experiments in the L-2M stellarator revealed the intense noise modulation of the gyrotron power and the change in its mean value under the action of the noise modulation of radiation reflected from the plasma column. The effect observed is explained in terms of the resonant locking of the gyrotron self-oscillations due to wave reflection from the fluctuating plasma load.     

Long-pulse operation of a 0.5 MW TE10.4 gyrotron at 140GHz

The operation features of a TE_10.4-mode gyrotron oscillator with a quasi-optical mode converter and a single-stage depressed collector at 140 GHz with an output power of 500 kW in long pulses of 0.2 s are presented. Measurements on long-pulse operation of the tube are described in detail, and the significant differences between short- and long-pulse operation concerning efficiency and output power are pointed out. The variation of frequency during a pulse and an irreversible frequency shift during long-pulse operation were measured and are discussed with respect to gyrotron design     

140-GHz gyrotron with multimegawatt output power

The operational features of a 140-GHz, transverse electric TE_22,6 mode gyrotron oscillator with an advanced quasi-optical mode converter, a Brewster window, and a single-stage depressed collector at 140 GHz with an output power of 2.1 MW and an efficiency of 34% without depressed collector and 53% with depressed collector are presented. The high output power level is possible due to an almost reflectionless termination of the radio frequency (RF) beam line outside the tube. The operation of the TE_22,6 mode gyrotron is described in detail, and the significant features for achieving the high-output power are pointed out     

Network-theoretical approach to orotron analysis

For the use of oscillators in microwave networks, the knowledge of the behavior of the isolated oscillator is not sufficient. Of major interest is the interaction of the device with the external circuit, where questions about stability, modulation, injection locking, noise, etc. arise. If it is possible to describe the behavior of the active element in the frequency domain, the answers to the above mentioned questions can be found utilizing the standard methods of network theory. Recently the gyrotron oscillator has been described successfully in the frequency domain [1].     

Gyrotron Backward Wave Oscillator Experiments with a Relativistic Electron Beam Using an X-Band Rectangular Waveguide

A mildly relativistic electron beam (500keV, 200A, 10ns) injected into an X-band rectangular waveguide immersed in a uniform axial magnetic field (4-10kG) produced magnetically tunable microwave radiation in the 9-13 GHz frequency range with an estimated output power of 1MW. The frequency range and tunability of the radiated microwave agreed with a theoretical model for a gyrotron backward wave oscillator taking into account the low energy component of the beam electron.     

Mode competition, suppression, and efficiency enhancement in overmoded gyrotron oscillators

Gyrotron oscillators are of great interest as sources of high power mm wave radiation for electron cyclotron resonance heating and current drive in magnetic fusion research devices. Gyrotrons capable of efficiently generating cw power 1 megawatt will be required in future magnetic fusion studies. However, as gyrotron power approaches the megawatt level, a very large, overmoded cavity must be employed in order to keep ohmic power losses in the cavity at an acceptable level, and the problem of mode overpopulation becomes severe. Also, it becomes increasingly important to optimize gyrotron efficiency for a number of important reasons including minimizing the problem of collecting the electron beam energy. In the present paper, a detailed experimental and theoretical study of mode competition and mode locking in an overmoded gyrotron is presented. Efficiency enhancement (to 60%) and high peak power (430 kW) were achieved in the TE_2,4,1 mode using magnetostatic profiling in the cavity. With selective mode suppression, peak power of 475 kW was generated in the TE_0,4,1 mode.This work was performed at the Naval Research Laboratory. Some of the authors have affiliations with other institutions, as indicated: