Large-signal characteristics of a wide-band dielectric-loadedgyro-TWT amplifier

The bandwidth of a gyrotron traveling wave amplifier (gyro-TWT) has been significantly increased by partially filling the interaction waveguide with dielectric to reduce the circuit's dispersion. The proof-of-principle experiment was designed for X-band, and employs the fundamental mode of rectangular waveguide loaded with dielectric slabs along the narrow sidewalls. The amplifier yields a peak output power of 55 kW with 11% efficiency, 27 dB saturated gain, and an unprecedented untapered gyro-TWT constant-drive bandwidth of 11% and saturated bandwidth exceeding 14%. The single-stage amplifier is completely zero-drive stable. The 95-kV 5-A electron beam was produced by a single-anode magnetron injection gun with p_⊥/υ_z=0.6, as determined by the EGUN code, and Δυ_z/υ_z=4%, determined as the best fit to the gyro-TWT large-signal simulation data. Simulation studies predict that by lowering the velocity spread to Δυ _z/υ_z=2%, the amplifier performance will be further enhanced to a constant-drive bandwidth of 20% with 15% efficiency     

Stability of a 95-GHz slotted third-harmonic gyro-TWT amplifier

A low-magnetic-field moderate-voltage gyrotron amplifier has been designed for stable high-performance operation at 95 GHz. A slotted interaction circuit is utilized to achieve strong amplification near the third cyclotron harmonic frequency. The start-oscillation conditions were determined by an analytical theory and confirmed by a multimode particle-in cell simulation code. The dominant threat to the amplifier's stability is from a third-harmonic peniotron backward-wave interaction. A slow-timescale particle-tracing simulation code predicts the three-section slotted third-harmonic gyro-TWT, which utilizes an 11.6-kG magnet and a 50-kV 3-A υ_⊥/υ_z=1.4 axis-encircling electron beam with an axial velocity spread of 6% will yield an output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB, and a constant-drive bandwidth of 2%     

Slotted third-harmonic gyro-TWT amplifier experiment

The first operation of a slotted third-harmonic gyrotron traveling-wave amplifier is reported. The low-magnetic-field moderate-voltage gyrotron amplifier's 62-keV 2.5-A υ_⊥ /υ_∥=1.2 axis-encircling electron beam was supplied by a gyroresonant RF accelerator. The 10-GHz 1.3-kG single-section slotted third-harmonic amplifier is stable and yielded 12.5 dB of small signal gain with a bandwidth of 2.5%. The experiment was performed as a scaled proof-of-principle test of the 95-GHz multisection slotted amplifier under development at CPI (formerly Varian)     

Theory and experiment of ultrahigh-gain gyrotron traveling waveamplifier

Physics and technology issues of importance to the high-gain gyrotron traveling wave amplifier (gyro-TWT) are investigated in theory and experiment. The gyro-TWT is known to be highly susceptible to spurious oscillations, especially in high gain operations. In the current study, oscillations of various origins are classified and characterized with detailed theoretical modeling. They are shown to be intricately connected to the interplay between the absolute/convective instabilities, circuit losses, and reflective feedback. Knowledge of these processes leads to the concept of an ultra high gain scheme which employs distributed wall losses for the suppression of spurious oscillations. A proof-of-principle Ka-band gyro-TWT experiment stable at zero drive has produced 93 kW saturated peak power at 26.5% efficiency and 70 dB gain, with a 3 dB saturated output power bandwidth of 3 GHz. The saturated gain is more than 30 dB beyond that previously achieved     

Multimegawatt relativistic harmonic gyrotron traveling-wave tubeamplifier experiments

The first multimegawatt (4 MW, η=8%) harmonic (ω=sΩ_c, s=2,3) relativistic gyrotron traveling-wave tube (gyro-twt) amplifier experiment has been designed, built, and tested. Results from this experimental setup, including the first ever reported third-harmonic gyro-twt results, are presented. Operation frequency is 17.1 GHz. Detailed phase measurements are also presented. The electron beam source is SNOMAD-II, a solid-state nonlinear magnetic accelerator driver with nominal parameters of 400 kV and 350 A. The flat-top pulsewidth is 30 ns. The electron beam is focused using a Pierce geometry and then imparted with transverse momentum using a bifilar helical wiggler magnet. The imparted beam pitch is a α≡β_⊥/β_∥≈1. Experimental operation involving both a second-harmonic interaction with the TE₂₁ mode and a third-harmonic interaction with the TE ₃₁ mode, both at 17 GHz, has been characterized. The third-harmonic interaction resulted in 4-MW output power and 50-dB single-pass gain, with an efficiency of up to ~8% (for 115-A beam current). The best measured phase stability of the TE₃₁ amplified pulse was ±10° over a 9-ns period. The phase stability was limited because the maximum RF power was attained when operating far from wiggler resonance. The second harmonic, TE₂₁ had a peak amplified power of 2 MW corresponding to 40 dB single-pass gain and 4% efficiency. The second-harmonic interaction showed stronger superradiant emission than the third-harmonic interaction. Characterizations of the second- and third-harmonic gyro-twt experiments presented here include measurement of far-field radiation patterns, gain and phase versus interaction length, phase stability, and output power versus input power     

Stable high-power TE01 gyro-TWT amplifiers

The stability of high power gyro-TWT amplifiers operating in the low-loss TE₀₁ mode of cylindrical waveguide has been studied, Linear theory has been used to determine the threshold start-oscillation beam current for absolute instability in the operating mode and the critical section lengths for the dominant gyro-BWO interactions occurring at various cyclotron harmonics in other waveguide modes. The performance of the amplifier was evaluated with a nonlinear, self-consistent slow-timescale simulation code. Utilizing interaction sections whose lengths are less than the threshold start-oscillation length and are separated by attenuating severs for isolation, two stable three-section devices have been designed which are predicted to yield: (1) a peak output power of 230 kW at 35 GHz with an efficiency of 23%, a saturated gain of 46 dB and a constant-drive bandwidth of 6% for a 100 kV, 10 A electron beam with an α=ν_⊥ /ν_z =1.0 and an axial velocity spread Δν_z/ν_z=5% and (2) 105 kW at 94 GHz with 21% efficiency, 45 dB saturated gain and 5% constant-drive bandwidth for a similar 5 A electron beam. In addition, the design of the 0 dB input/output couplers and the MIG electron gun are given. Due to the low loss of the TE₀₁ mode, both of these amplifiers can be operated continuously     

94-GHz 25-kW CW low-voltage harmonic gyrotron

A low-voltage second-harmonic gyrotron intended as a compact lightweight source has been designed and evaluated with a particle-tracing code and the particle-in-cell code MAGIC. The two codes are shown to be in good agreement when applied to a conventional fundamental-frequency gyrotron and also to the novel second-harmonic gyrotron. The 25-kW continuous wave (CW) 94-GHz gyrotron with a predicted conversion efficiency of 32% and device efficiency of 22.5% is driven by a 25-kV 4.5-A (υ_⊥/υ₂=1.5, Δυ_z/υ_z=7%) electron beam from a magnetron injection gun and employs a low-loss TE₀₂₁/TE₀₃₁ complex cavity for mode control. Although the 17-kG CW gyrotron will use a cryogen-free high-T_c superconducting magnet, a 94-GHz prototype will be tested at low duty with a conventional low-T_c superconducting magnet     

W波段回旋行波管放大器的模拟与设计

回旋行波管放大器是高功率毫米波雷达发射系统最重要的候选者.通过对回旋行波管放大器中的绝对不稳定性、回旋返波振荡以及电子注-波互作用的研究，讨论了回旋行波管的稳定性、寄生模式的抑制和工作参数的优化等问题，给出了W波段TE₀₁模回旋行波管放大器的模拟设计结果.PIC粒子模拟结果表明，在电子注电压100kV、电流10A、工作磁场3.52T时，94GHz的基波回旋行波管放大器可获得大于250kW的输出功率、40dB的增益、大于25％的效率和约5％的带宽. 关键词： W波段 回旋行波管放大器 模拟 设计     

Study of gain in C-band deflection cavities for afrequency-doubling magnicon amplifier

An experimental study of the gain between two half-wavelength, 5.7-GHz TM₁₁₀ mode pillbox cavities, separated by a quarter-wavelength drift space, and powered by a 170-A, 500-keV electron beam immersed in an 8.1-kG magnetic field is reported. These cavities constitute the first section of a planned multicavity deflection system, whose purpose is to spin up an electron beam to high transverse momentum (α≡υ⊥/υ_z⩾1) for injection into the output cavity of a frequency-doubling magnicon amplifier. A gain of ~15 dB was observed in the preferred circular polarization, at a frequency shift of approximately -0.18%, in the opposite circular polarization, at a frequency shift of approximately +0.06%. These results are in good agreement with theory     

High efficiency, phase-locked operation of the harmonic-multiplyinginverted gyrotwystron oscillator

The inverted gyrotwystron (phigtron) is a millimeter wave frequency-doubling amplifier that has been demonstrated to produce over 300 kW peak power at twice the input frequency (centered at f_in =16.85 GHz and f_out=33.7 GHz) over a 0.5% bandwidth with a saturated gain of 30 dB and efficiency greater than 35%. The device has also been studied both theoretically and experimentally in a different operating regime where frequency-doubled, phase-locked oscillation is possible. A signal, injected via a fundamental gyro-traveling wave tube input section, modulated a 55 kV, 10 A electron beam. After transit through a drift section, the prebunched electron beam produced phase-locked, second harmonic oscillations in a TE₀₃ mode output cavity. RF output centered at either of two frequencies, 34.42 and 34.62 GHz, with a maximum output power of 180 kW, an efficiency of 32% and a locked signal gain of 35 dB was measured. A theoretical prediction of locking bandwidth, a design overview, and the experimental results are presented followed by a summary and discussion of the results     

Study of high-power Ka-band second-harmonic gyroklystron amplifier

The self-consistent nonlinear theory of two-cavity high-harmonic gyroklystron amplifier has been developed. The efficiency and gain of a second-harmonic gyroklystron were calculated numerically. The results obtained were used to choose the optimal parameters of the experimental second-harmonic tube. The experimental study was carried out to test high-harmonic amplifier concept. Two-cavity 35 GHz second harmonic gyroklystron with the TE₀₂₁ cavity mode has been designed and tested in pulse operation. Output power of about 260 kW with efficiency 18% and 17 dB gain have been produced at 72 kV beam voltage and 20 A beam current. Bandwidth of about 0.1% has been observed. The restriction of the output power, efficiency, and gain was caused by spurious oscillations excited in the second cavity in the TE₀₁₁ mode at the fundamental cyclotron frequency     

LARGE-SIGNAL SIMULATIONS OF A GYROTRON TRAVELING-WAVE AMPLIFIER WITH A MODE-SELECTIVE INTERACTION CIRCUIT

A self-consistent nonlinear theory is used to analyze the saturated performances of a Ka-band gyrotron traveling wave amplifier (gyro-TWA) operating at the fundamental with a mode-selective interaction circuit involving a tapered vane-slot mode converter. The amplifier is predicted to generate 140 kW saturated output power with 33.3% efficiency, a saturated gain of 33dB, and a 3dB bandwidth of 2.7 GHz (8%) for a 70 kV, 6A electron beam with a velocity ratio of 1.0 and an axial velocity spread of 5%.     

连续调谐太赫兹回旋管非稳定振荡状态研究

太赫兹回旋管可实现高功率输出,并具有一定的频率调谐范围,是核磁共振波谱系统理想的高功率太赫兹辐射源。设计了263 GHz,TE_5,2基波连续调谐回旋管,通过磁场调节实现频率调谐范围为1.39 GHz,利用时域多模多频自洽非线性理论对设计的连续调谐回旋管非稳定振荡状态进行了研究。结果表明,在低次纵向谐波模式工作磁场范围内,当工作电流大于起振电流时,连续调谐回旋管先进入稳定状态,高次纵向谐波模式被抑制,工作模式TE_5,2的输出功率随时间不变;当电流增大,纵向谐波模式间的竞争引起回旋管由稳定状态进入到非稳定振荡状态,工作模式TE_5,2的输出功率随时间呈振荡变化且互作用效率大大降低;随着电流的进一步增大,回旋管又回到与低电流不同的稳定状态,互作用效率进一步降低。同时发现非稳定振荡状态的起始电流随着磁场增加而增大。本研究对需工作于稳定状态的面向DNP-NMR应用的连续调谐太赫兹回旋管的研制具有一定指导意义。     

W-Band Second-Harmonic Gyrotron Traveling Wave Amplifier with Distributed-Loss and Severed Structures

The second harmonic TE₀₂ gyrotron traveling wave amplifier (gyro-TWT) is a high-power, broadband, millimeter-wave amplifier with a low applied magnetic field. Mode-selective interaction circuits were applied to suppressing spurious oscillations. However, the mode-selective interaction circuit may perturb the operating mode in the gyro-TWT. A multi-stage gyro-TWT design with distributed-loss and severed structures is proposed to stabilize the amplification. This study presents a nonlinear analysis of typical oscillations, including absolute instability, gyrotron backward oscillation (gyro-BWO) and reflective oscillation. The lossy and severed sections of the multi-stage gyro-TWT seem to increase effectively the start-oscillation currents of the absolute instability, gyro-BWO, and reflection oscillation. The multi-stage gyro-TWT is predicted to yield a peak output power of 215 kW at 89.9 GHz with an efficiency of 14.3 %, a saturated gain of 60 dB and a bandwidth of 1.7 GHz for a 100 kV, 15 A electron beam with an axial velocity spread _z/ _z = 5%.     

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。     

Millimeter-wave gyroklystron amplifier experiment using arelativistic electron beam

A fundamental-mode TE₁₁₁° two-cavity intense-beam gyroklystron amplifier experiment, operating at an accelerating voltage of 1 MV, is reported. The two cavities that were tested are designed to serve as bunching cavities for a high-power output cavity. The two-cavity amplifier has demonstrated a linear gain of 15 dB and an unsaturated output power of ~40 kW, with the intracavity gain and power ~4 dB higher. The frequency of the second cavity has been found to track the frequency of the driven cavity over a range of 300 MHz around a center frequency of 35 GHz. Stable amplifier operation was achieved with beam currents as large as 150 A and a velocity pitch ratio of 0.36. The stable operating range was limited by spurious oscillation in the TE₁₁₂° mode. Theoretical calculations indicate that higher gains might be attainable if this mode could be suppressed     

X-band dielectric Cerenkov maser amplifier experiment

An X-band dielectric Cerenkov maser amplifier experiment is reported. The amplifier system consisted of a solid, thermionically generated electron beam propagating through a cylindrical waveguide partially filled with an annular, dielectric liner. The input signal was provided by a tunable (9-10.3 GHz) magnetron with power up to 10 kW. Electron beam voltages and currents of up to 250 kV and 100 A could be generated for 1 μs pulse durations. The system was configured to operate in the TM₀₁ mode of the dielectric-lined waveguide. In this experiment the gain of the system with respect to the length of the dielectric liner was studied at a fixed input frequency of 10.3 GHz. At electron beam parameters of 160 kV and 60 A, a power gain of 24 dB over 56 cm of interaction length was measured for an input power of 4.5 kW, corresponding to a maximum RF amplified power of 1.15 MW and 12% efficiency     

Preliminary Design of a Harmonic-doubling Gyrotron Traveling Wave Amplifier

This study proposes a Ka-band harmonic-doubling gyrotron traveling-wave amplifier (gyro- TWT), using distributed wall losses in the input stage and mode-selective interaction circuit in the output stage, to improve the stability of the amplification. Based on a large signal simulation code, a saturated peak power of 163 kW with an efficiency of 15.5%, a gain of 31.1 dB, and a 3 dB bandwidth of 0.9 GHz is predicted for the gyro-TWT driven by 70 kV, 15 A electron beam with a velocity ratio of 1.2 and velocity spread 5% at 33.2 GHz.     

High-gain wide-band gyrotron traveling wave amplifier with a helically corrugated waveguide

First bandwidth measurements of a novel gyrotron amplifier are presented. The coupling between the second harmonic cyclotron mode of a gyrating electron beam and the radiation field occurred in the region of near infinite phase velocity over a broad bandwidth by using a cylindrical waveguide with a helical corrugation on its internal surface. With a beam energy of 185 keV, the amplifier achieved a maximum output power of 1.1 MW, saturated gain of 37 dB, linear gain of 47 dB, saturated bandwidth of 8.4 to 10.4 GHz ( 21% relative bandwidth), and an efficiency of 29%, in good agreement with theory.     

Design of a high-efficiency low-voltage axially modulatedcusp-injected second-harmonic X-band gyrotron amplifier

We present the theoretical design of a second-harmonic small-orbit gyrotron amplifier which utilizes the interactions between a 35-kV 4-A beam and a TE₀₁₁ cavity to produce over 70 kW of amplified power at 9.9 GHz in a 1.83-kG magnetic field. One of the novel features of this device is that the electron gun produces an axially streaming annular beam which is velocity modulated by a short TM_0n0 input cavity. Perpendicular energy is imparted to the beam via a nonadiabatic magnetic transition at the end of a 13-cm drift region. An electronic efficiency of 53% is predicted with a large signal gain near 20 dB by a single particle code which takes into account nonideal effects associated with finite beam thickness and finite magnetic field transition widths