Design of a high-efficiency C-band backward wave oscillator

For practicability of the high power microwave source,a C-band backward wave oscillator(BWO)which has high conversion efficiency is designed.When the axial guiding magnetic field is 0.83 T,the electron energy and the beam current of the diode are respectively 80 keV and 2.1 kA,a microwave output power of100 MW at 7.4 GHz microwave frequency with 65% conversion efficiency is achieved in simulation.     

过模返波管的正反馈机制

作为一个典型的高功率微波振荡器,过模返波管(backward wave oscillator,BWO)的束波互作用过程复杂,束流负载效应影响明显,但是作为振荡器本身,其本质就是一个正反馈电路,电子从阴极发射后,穿过谐振反射腔和慢波结构(slow-wave structure,SWS),在SWS区电子动能转化为微波能,其中的一部分微波反馈到谐振反射腔,实现对电子束的调制,其他微波通过后面输出端口向外辐射.本文根据这种正反馈机制,建立器件工作模式等效电路和束波互作用的自洽过程,从理论上给出正反馈机制对器件模式控制、起振电流等参数的影响,并模拟研究了这种反馈机制对模式控制的影响,由此设计了一个能够在(1 MV,20 kA)电子束条件下克服模式竞争的过模BWO,其微波输出功率为7.9 GW,频率为8.68 GHz,相应的效率为39.5%.     

捷变频相对论返波管振荡器设计

基于低磁场返波管振荡器的工作原理,设计了一个捷变频相对论返波管振荡器,该器件由两段对电子束参数要求基本一致的慢波结构串接而成,通过调节引导磁场强度实现器件频率的调节,使其分别工作于C波段和X波段。在电子能量和束流分别为670keV和8kA的条件下,当引导磁场强度为0.5T时,采用2.5维PIC程序模拟得到频率为6.28GHz、功率为1.0GW的微波输出;而当引导磁场强度为0.8T时,得到频率为9.25GHz、功率为0.75GW的微波输出。     

Experimental investigation of backward wave oscillator with low magnetic field

A backward wave oscillator (BWO) is introduced in the paper. On the accelerator of Simus-700,it is experimentally investigated. Under the condition that the electron energy is 740 keV, the beam current is 7 kA and the guiding magnetic field is at 0.68 T, the performance of 1.15 GW microwave output power at 9.1 GHz microwave frequency with 22 ns pulse width and 22% conversion efficiency are reached.     

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm₀₁ mode prevailed when the background plasma density was below 6×10¹¹ cm^-3 . At a critical background plasma density of ≃8×10¹¹ cm^-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse     

Scaling studies and time-resolved microwave measurements on arelativistic backward-wave oscillator

A relativistic backward-wave oscillator (BWO) operating at a frequency near 8 GHz has been built. The parameters of the 60-ns electron beam driving this microwave source are varied over the ranges 0.8-1.5 MV and 2-10 kA. Several different annular cathodes for launching the electron beam are tried, varying the outer radius and shape. The axial magnetic field guiding the beam through the BWO is varied between 0.6 and 3 T. The power transfer downstream to an output waveguide is investigated as a function of the shape of the transition from the BWO to the waveguide. The scaling of the output power and frequency with these variations is discussed. Time-resolved measurements of 2-ns-long segments of the microwave output are shown. In observations of the microwave signal, it is found that the frequency shifts as the output power envelope passes through a sharp dip. It is proposed that this shift corresponds to a change in the longitudinal operating mode of the BWO     

Efficiency enhancement of high power vacuum BWO's using nonuniformslow wave structures

The Sinus-6, a high-power relativistic repetitively-pulsed electron beam accelerator, is used to drive various slow wave structures in a BWO configuration in vacuum. Peak output power of about 550 MW at 9.45 GHz was radiated in an 8-ns pulse. We describe experiments which study the relative efficiencies of microwave generation from a two-stage nonuniform amplitude slow wave structure and its variations without an initial stage. Experimental results are compared with 2.5 D particle-in-cell computer simulations. Our results suggest that prebunching the electron beam in the initial section of the nonuniform BWO results in increased microwave generation efficiency, Furthermore, simulations reveal that, in addition to the backward propagating surface harmonic of the TM₀₁ mode, backward and forward propagating volume harmonics with phase velocity twice that of the surface harmonic play an important role in high-power microwave generation and radiation     

Experimental results from a tandem BWO-TWT system used to generatehigh-power microwaves

A relativistic backward wave oscillator (BWO) in tandem with a traveling wave tube (TWT) amplifier has been used to generate relatively long pulses of high-power X-band microwaves. In these experiments, a BWO is used to modulate the annular relativistic electron beam, which subsequently drives a TWT producing high-power microwave radiation. A special RF sever located between the two structures cuts off microwaves generated in the BWO from the TWT. Peak powers in excess of 100 MW are observed with overall beam-to-microwave efficiencies as high as 35%. By operating the BWO below saturation levels, pulse-shortening effects are minimized so that microwave pulses of duration comparable to that of the beam (100 ns) are possible. The operating frequency of the tandem system is tuned from 11 to 12 GHz by varying the effective energy of the beam     

Powerful electromagnetic millimeter-wave oscillations produced by stimulated scattering of microwave radiation by relativistic electron beams

The stimulated scattering of electromagnetic waves in microwave generators, in which a high-current electron beam excites either a backward wave (BWO) or a quasi-critical frequency wave (orotron) is investigated experimentally. The scattering occurs at the same electron beam and the high-frequency feedback is provided by the inhomogeneities of the electrodynamic system of the microwave generator itself. A power of several tens of megawatts has been achieved in the 3-mm range. The mode selection permitted to obtain single-mode scattering.     

On the amplification mechanism of the ion-channel laser

The amplification mechanism of the ion-channel laser (ICL) in the low-gain regime is studied. In this concept, a relativistic electron beam is injected into a plasma whose density is comparable to or lower than the beam's density. The head of the electron beam pushes out the plasma electrons, leaving an ion channel. The ion-focusing force causes the electrons to oscillate (betatron oscillations) about the axis and plays a role similar to the magnetic field in a cyclotron autoresonance maser (CARM). Radiation can be produced with wave frequencies from microwaves to X-rays depending on the beam energy and plasma density: ω~2γ^3/2ω_pe, where γ is the Lorentz factor of the beam and ω_pe is the plasma frequency. Transverse (relativistic) bunching and axial (conventional) bunching are the amplification mechanisms in ICLs; only the latter effect operates in free-electron lasers. The competition of these two bunching mechanisms depends on beam velocity ν_0z; their dependences on ν_0z cancel for the cyclotron autoresonance masers. A linear theory is developed to study the physical mechanisms, and a PIC (particle-in-cell) simulation code is used to verify the theory. The mechanism is examined as a possible explanation for experimentally observed millimeter radiation from relativistic electron beams interacting with plasmas     

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。     

紧凑型L波段同轴相对论返波振荡器的粒子模拟

 设计了紧凑型L波段同轴相对论返波振荡器，通过粒子模拟研究了L波段同轴相对论返波振荡器相互作用的物理过程，并对器件的电磁结构进行了优化和改进。分析表明，采用同轴慢波结构可以在较低的外加磁场下实现L波段返波振荡器的微波输出，同时可以大大减小微波器件的径向尺寸。这是因为同轴慢波结构的TM01模式有类似于TEM模的性质，没有截止频率，但纵向电场不为零，电子束能够与它发生强相互作用过程。粒子模拟优化结果表明，在器件半径仅为4.0 cm，电子束能量240 keV，电子束流1.8 kA，导引磁场仅为0.75 T时，返波振荡器可以在频率1.60 GHz处获得较大功率的微波输出， 平均峰值功率达140 MW，平均峰值功率效率约为32%。     

带有反射腔的相对论返波管的数值模拟

 阐述了带有反射腔的相对论返波管的数值模拟研究。利用线性理论[1]设计了返波管的慢波结构，应用SUPERFISH软件设计了谐振反射器。用KARAT软件对谐振反射腔返波管进行了宏观粒子模拟，得到了优化的返波管结构参数，并研究了外加磁场对输出效率的影响。模拟结果表明：谐振反射腔不仅起到截止颈的作用，还有预调制的作用；在低外加磁场条件下，该返波管也能输出较高功率的微波。显示了其在重复频率工作方面的重要意义。     

X波段永磁包装相对论返波管研制

设计了一个X波段相对论返波管,通过对器件结构的特殊设计,提高了器件的Q值,同时增大了电子束与慢波结构之间的耦合阻抗,从而实现器件的低磁场运行以便对其进行永磁包装;当引导磁场强度0.46 T、电子束束压750 kV、束流约5.5 kA时,得到频率9.1 GHz、功率1.24 GW的微波输出。根据模拟结果设计加工了一个磁场强度为0.46 T的小型化永磁磁体,该磁体长48 cm,最大外半径15 cm,总重量约116 kg。开展了永磁包装返波管的实验研究,得到以20 Hz的频率运行1 s时功率900 MW、单次运行时功率940 MW的X波段微波输出。     

Study of superradiance based on a low-voltagebackward wave oscillator

According to the small size requirement for wide-band high-power microwave radiation, a superradiance backward wave oscillator (BWO) is proposed to generate such high-power microwave radiation with a low voltage (～20 kV) pulse power supply and low guiding magnet field (～0.1 T). In order to get a high-efficiency C-band superradiance BWO with a low beam voltage and a low guiding magnet field, the mechanism of superradiance in a BWO is explored in particle-in-cell simulation. With the oversized structure, the simulation shows that a microwave power of 405 kW with a frequency of 5.6 GHz and a spectrum width of 500 MHz can be obtained with a voltage of 23 kV and magnetic field of 0.1 T.     

Plasma density measurement in a gas-filled X-band backward wave oscillator with a double conversion heterodyne microwave interferometer

Plasma density was measured with a heterodyne microwave interferometer in both a gas-filled X-band backward wave oscillator (BWO) and in a smooth tube. Plasma is generated by impact ionization of a 650 kV, 2 kA electron beam. For fixed gas pressure we found that the plasma density rise in the operating BWO was much faster than in a smooth tube, indicating that Trivelpiece-Gould modes, or high power microwaves, increase plasma generation. Additional plasma enhanced BWO microwave output power. Measured plasma density at optimum power levels was n_cr ≈ 6 × 10¹²cm⁻³ at onset of emitted microwaves.     

An Unmagnetized Plasma-Loaded Relativistic Backward-Wave Oscillator: Experiment and Simulation

Reported in this paper are the results of an experiment to produce high-power microwave radiation from a gas-filled backward wave oscillator (BWO) driven by a relativistic electron beam without external guiding magnetic field. A peak power for background gas pressure at 5.25 mTorr has been observed as argon pressure from 0.75 mTorr to 15 mTorr. Operating frequency of the oscillator has been measured at 9.6 ± 0.4 GHz. The results of PIC simulation are in good agreement with the experimental results. A reasonable explanation is given for experimental results by PIC simulation.     

Development of the relativistic backward wave oscillator with a permanent magnet

Firstly, an X-band relativistic backward wave oscillator with a low guiding magnetic field is simulated, whose output microwave power is 520 MW. Then, an experiment is carried out on an accelerator to investigate a relativistic backward wave oscillator with a permanent magnetic field whose strength is 0.46 T. When the energy of the electron is 630 keV and the current of the electron beam is 6.7 kA, a 15 ns width pulsed microwave with 510 MW output power at 8.0 GHz microwave frequency is achieved.     

充中性气体相对论返波振荡器的粒子模拟研究

 用PIC粒子模拟方法研究了充中性气体相对论返波管的物理机制，成功模拟了电子束碰撞充入返波管中的中性气体电离产生等离子体的过程，在电子束传输的路径上形成离子通道，有效中和电子束径向空间电荷力，有利于电子束的传输及束波相互作用产生微波。增加中性气体密度，返波管的输出频率明显上移，其辐射的功率和效率比相同的真空器件也有明显的提高。