Competition of modes resonant with arbitrary cyclotron harmonics ina gyrotron with nonfixed axial structure of the high-frequencyfield

A general theory is developed for self-consistent calculations of mode competition in a gyrotron with nonfixed axial structure of the RF field for arbitrary cyclotron harmonics. The theory is applied to the gyrotron operating at the Kernforschungszentrum Karlsruhe with a frequency of 150 GHz. The formalism presented allows a self-consistent calculation of mode competition for the operating and parasitic modes at the cyclotron resonance at arbitrary harmonics. Specific calculations are carried out for the cases n₀=1, n₁ =2 and n₀=2, n₁=1. It is emphasized that the formalism considered applies only to the case in which the azimuthal orthogonality condition is satisfied: n₀m₁≠n₁ m₀. There are circumstances when this condition is not satisfied     

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     

A 1.5-MW, 140-GHz, TE28,16-coaxial cavity gyrotron

The design of a 1.5-MW, 140-GHz, TE-_28,16-coaxial cavity gyrotron is presented and results of experimental operation are given. A cavity with a cylindrical outer wall and a radially tapered inner rod with longitudinal corrugations was used. A maximum output power of 1.17 MW has been measured in the design mode with an efficiency of 27.2%. Single-mode operation has been found over a wide range of operating parameters. The experimental values agree well with the results of multimode calculations. Frequency-step tuning has been performed between 115.6 and 164.2 GHz. In particular, an output power of 0.9 MW has ben measured in the TE_25,14 mode at 123.0 GHz and 1.16 MW in the TE_32,18 mode at 158.9 GHz. At frequencies its with strong window reflections the parameter range for which stable operation is possible is reduced significantly. In order to obtain results relevant for a technical realization of a continuously operated gyrotron, a tube with a radial radio frequency (RF)-beam output through two output windows and a single-stage depressed collector has been designed and is under fabrication. A two-step mode conversion scheme-TE- _28,16 to Te_+76.2 to TEM₀₀-which generates two narrowly directed (60° at the launcher) output wavebeams has been chosen for a quasioptical (q,o) mode converter system. A conversion efficiency of 94% is expected     

Third-harmonic complex cavity gyrotron self-consistent nonlinearanalysis

Starting from general transmission line equations with an electron beam source, a self-consistent nonlinear theoretical model for a complex cavity gyrotron with gradual transitions is presented in this paper. The model accounts for mode coupling in the transition region of the complex cavity. The interaction between the electron beam and H₆₁-H ₆₂ RF field in the complex cavity for a third-harmonic gyrotron is simulated; many calculations are carried out under different cavity dimensions and electron beam parameters     

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     

Design and experimental operation of a 165-GHz, 1.5-MW,coaxial-cavity gyrotron with axial RF output

The development of a coaxial-cavity gyrotron operating in TE_31,17 mode at 165 GHz is presented. The selection of the operating frequency and mode are based on the limitations imposed by the maximum held of the superconducting (sc) magnet at Forschungzentrum Karlsruhe, Institut fur Technische Physik (FZK), the use of the inverse-magnetron injection gun (IMIG) of the 140-GHz, TE_28,16 coaxial gyrotron and the possibility of transforming the cavity mode to a whispering gallery mode (WGM) appropriate for the dual-beam quasioptical (q.o.) output coupler and the two output windows, which are foreseen for the next lateral output version of the tube. The tube with axial output has been tested at FZK to deliver maximum output power of 1.17 MW in the designed TE_31,17 mode with 26.7% efficiency at 164.98 GHz. Maximum efficiency of 28.2% was achieved at 0.9-MW output power. The design operating point with output power 1.36 MW and 36.7% efficiency was net accessible because of beam instabilities at high electron-velocity ratio α, presumably caused due to high electron-velocity spread. Power at higher frequencies was also detected: 1.02 MW at 167.16 GHz in TE_32,17 mode with 26.88 efficiency, 0.63 MW at 169.46 GHz in TE_33,17 mode with 18% efficiency, and 0.35 MW at 171.80 GHz in TE_31,17 mode with 13.3% efficiency     

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。     

Polarization control of microwave emission from high powerrectangular cross-section gyrotron devices

Results are summarized of experiments on a gyrotron utilizing a rectangular-cross-section (RCS) cavity region. The major issue under investigation is polarization control of microwave emission as a function of magnetic field. The electron beam driver is the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V=0.8 MV, I_diode=1-10 kA, I_tube=0.1=0.5 kA, and t_e-beam=0.4-1.0 μs. The annular e-beam is spun up into an axis-encircling beam by passing it through a magnetic cusp prior to entering the RCS interaction cavity. Experimental results show a high degree of polarization in either of two orthogonal modes as a function of cavity fields. The RCS gyrotron produced peak powers of 14 MW in one polarization (TE₁₀) and 6 MW in the cross-polarized mode (TE ₀₁). Electronic efficiencies for this device reached as high as 8% with transverse efficiency of 16%. Experimental results on the beam alpha (α=V_⊥/V_∥) diagnostics, where alpha is the ratio of the e-beam's transverse velocity to its parallel velocity, agree well with the single electron trajectory code. MAGIC code results are in qualitative agreement with microwave measurements. Microwave emission shifts from the dominant fundamental mode polarization (TE₁₀□ ), to the next higher order mode polarization (TE₀₁□) as the solenoid magnetic field is raised from 1.4-1.9 kGauss. Frequency measurements using heterodyne mixers support mode identification as well as MAGIC code simulations     

全三维110GHz和220GHz同轴腔双注回旋管数值模拟研究

为了打破传统单注回旋管只能产生较低功率的局限性, 本文基于自主研发的PIC粒子模拟软件CHIPIC 对110 GHz和220 GHz同轴腔双注回旋管进行全三维数值模拟研究. 在理论分析同轴双电子注电子枪设计模型和初始参数的基础上通过CHIPIC对其进行优化设计, 得到了具有横纵速度比为1.0, 最大速度零散约为5.4%的高性能电子束; 并将此优化后的双阳极双注电子枪取代传统回旋管数值模拟时采用的回旋 发射进行110 GHz和220 GHz双注回旋管整管的数值模拟, 并采用MPI四进程并行计算, 最终获得了具有双频分别为110 GHz和220 GHz、模式为TE₀₂模、平均输出功率约在70 kW、 效率达到8.75%的高性能双注回旋振荡管. 关键词： 同轴腔双注回旋管 同轴双电子注电子枪 横纵速度比 速度零散     

Coaxial cavity gyrotron with dual RF beam output

A 140-GHz, 1.5-MW, TE_28,16-coaxial cavity gyrotron with a dual RF beam output has been designed, built, and tested. For the first time, the generated RF power has been split into two parts and coupled out through two RF output windows in order to reduce the power loading in the windows. The quasioptical output system is based on a two-step mode conversion scheme. First, the cavity mode TE_-28,16 is converted into its degenerate whispering gallery mode TE_+76,2 using a rippled-wall mode converter. Then, this mode is transformed into two TEM₀₀ output wave beams. A maximum rf output power of about 950 kW with an output efficiency of 20% has been measured. According to numerical calculations, an rf power above 1.5 MW is expected to be generated in the cavity. Even if all losses are taken into account, a discrepancy between experiment and calculations remains. The power deficit seems to be partly caused by the influence of the stray radiation captured inside the tube. However, the two main reasons are probably an incomplete mode conversion from TE_-28,16 to TE_+76,2 and a large energy spread of the electron beam due to trapped electrons. An increased amount of captured stray radiation resulted in a reduced stability of operation. A single-stage depressed collector was used successfully, increasing the RF output efficiency from 20% to 29%     

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     

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     

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。     

全三维94 GHz回旋管系统数值模拟研究

为了打破传统回旋管数值模拟所采用的回旋发射产生理想电子束的局限性, 本文在理论分析94 GHz双阳极磁控注入式电子枪的结构参数的基础上, 通过共形FDTD算法对网格划分进行优化, 得到了具有横纵速度比为1.42, 最大速度零散为5.92%的高性能电子束, 并将此优化后的电子枪取代传统回旋管数值模拟时采用的回旋发射进行该94 GHz回旋 管系统的数值模拟, 并采用MPI四进程并行计算, 最终获得了具有TE₀₃模、94 GHz、平均输出功率约在40 kW、 效率达到10.5%的高性能回旋振荡管. 关键词： 双阳极磁控注入式电子枪 共形FDTD 横纵速度比 速度零散     

140-GHz gyrotron experiments based on a confocal cavity

We have designed and experimentally demonstrated the operation of a novel quasioptical gyrotron oscillator based on an overmoded confocal waveguide cavity. This cavity effectively suppresses undesired modes, and therefore has extremely low mode density. Stable single-mode, single-frequency operation was achieved in the TE₀₆ mode at 136 GHz. A peak RF output power of 66 kW, corresponding to an efficiency of 18%, was measured. By varying the cavity magnetic field, high-power generation was observed at 136 GHz in the TE₀₆ mode and at 114 GHz in the TE₀₅ mode. These frequencies correspond to the high Q modes of the confocal resonator. The low Q modes were either weak or not observed. In this paper, we will review the design procedure for this cavity and present experimental data verifying its effectiveness in reducing the number of modes that can be excited. The confocal waveguide could also be used in high-power, gyro-TWT amplifiers to provide greater operating stability and bandwidth, especially in an overmoded waveguide structure     

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     

Effect of a high-frequency field on the reflection of ion acousticwaves from the sheath at a grid

Experiments are described which show that the reflection coefficient for ion acoustic waves (IAW) from the sheath at a grid is affected by an HF electric field with a frequency f_HF≲5f_pi(f_pi =ion plasma frequency). For peak-to-peak amplitudes of the HF voltage drop across the sheath Φ₀≲k_B T_e/e and f_HF>f _pi, the energy distribution of the ions passing through the grid develops a hot tail and the reflected wave suffers enhanced Landau damping. If Φ₀≳k_BT_e/e and f_HF<f_pi, a large-amplitude IAW is excited at the grid; a well-defined ion beam is formed; and local growth of the reflected wave is observed. Test waves launched from the grid show the same propagation characteristics as the reflected waves     

Multimode simulations without particles in the quasi-opticalgyrotron

A set of coupled nonlinear differential equations, involving the slow amplitude and phase variation for each mode, is used to simulate the multimode dynamics in the quasi-optical gyrotron. The interaction among various modes is mediated by coupling coefficients of known analytic dependence on the normalized current I, the interaction length μ, and the frequency detunings Δ_i corresponding to the competing frequencies ω_i. The equations include all the possible resonant combinations of up to four different frequencies, ω_i-ω_j+ω _k-ω_l≃0, among a set of N participating modes, keeping terms up to fifth order in the wave amplitudes. The formalism is quite general and can be used to study mode competition, the existence of a final steady state and its stability, and its accessibility from given initial conditions. It is shown that when μ/β_⊥≫1, μ can be eliminated as an independent parameter. The control space is then reduced to a new normalized current I and the desynchronism parameters ν_i=Δ_iμ for the interacting frequencies. Numerical simulations for cold beams of various cross sections demonstrate that ν_i is the most important parameter for the system behavior. Overmoding is not determined by the frequency separation δω among the cavity modes per se, but by the separation among the corresponding desynchronism parameters     

Analytical solution for capacitive RF sheath

A self-consistent solution for the dynamics of a high voltage, capacitive radio frequency (RF) sheath driven by a sinusoidal current source is obtained under the assumptions of time-independent, collisionless ion motion and inertialess electrons. Expressions are obtained for the time-average ion and electron densities, electric field and potential within the sheath. The nonlinear oscillation motion of the electron sheath boundary and the nonlinear oscillating sheath voltage are also obtained. The effective sheath capacitance and conductance are also determined. It was found that: (1) the ion-sheath thickness S _m is √50/27 larger than a Child's law sheath for the DC voltage and ion current density; (2) the sheath capacitance per unit area for the fundamental voltage harmonic is 2.452 ϵ₀ /S_m, where ϵ₀ is the free space permittivity; (3) the ratio of the DC to peak value of the oscillating voltage is 54/125; (4) the second and third voltage harmonics are, respectively, 12.3 and 4.2% of the fundamental; and (5) the conductance per unit area for stochastic heating by the oscillating sheath is 2.98 (λ_D/S_m)^2/3 (e ²n₀/mu_e), where n ₀ is the ion density, λ_D is the Debye length at the plasma-sheath edge, and u_e is the mean electron speed     

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