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     

Submillimeter-wave harmonic gyrotron experiment

A theoretical and experimental investigation of the operation of a harmonic gyrotron at submillimeter wavelengths is reported. Using a waveguide cavity with an iris at the output end of the straight section, 14 different second-harmonic modes were observed with frequencies of 301-503 GHz, output powers of 1-22 kW, and a 12-MHz emission frequency bandwidth. The highest output power was 22 kW, with a total efficiency of 3.5% at 467 GHz, and an output power of 15 kW with a 6% efficiency was obtained at 417 GHz. Research was conducted using a 65-75 kV up to 10-A electron gun with a 1/1.5-μs pulse length and a 4-Hz repetition rate, which produced a helical electron beam in magnetic fields of up to 14 T. These results represent the first operation of a high-power harmonic gyrotron in the submillimeter region     

Direct EPR detection of transient and continuous wave signals at 2.5 GHz

Direct detection of free induction decays and electron spin echoes, and the recording of echo-detected EPR spectra and electron spin echo envelope modulation patterns at a microwave frequency of 2.5 GHz is demonstrated. This corresponds to the measurement of the transverse magnetization in the laboratory frame, rather than in the rotating frame as usually done by down-converting the signal (homodyne detection). An oscilloscope with a 6-GHz analog bandwidth, a sampling rate of 20 GigaSamples per second, and a trigger frequency of 5 GHz for the edge trigger and 750 MHz for the advanced trigger, is used in these experiments. For signal averaging a 3-GHz microwave clock divider has been developed to synchronize the oscilloscope with the frequency of the EPR signal. Moreover, direct detection of continuous wave EPR signals at 2.5 GHz is described.     

A high power microwave vircator with an enhanced efficiency

A radially extracted vircator (virtual-cathode oscillator) with two identical opposite output ports is presented. The microwave modes propagating inside the WR650 output rectangular waveguide are determined by the calibrated E-probes lined up in the direction perpendicular to the axis of the WR650 waveguide. Velvet-covered cathodes are used to emit electrons. Several diameters of velvet are chosen to see how the emitting area effects microwave generation. Evidence of virtual cathode formation, onset of electron reflexing, and simultaneous microwave emission are discussed. This vircator is found to have the capability of generating a maximum 1.4 GW microwave pulse with the dominant frequency at 8.2 GHz. The radiation pattern is dominated by the TE₁₀ mode and the instantaneous efficiency for producing this microwave pulse is around 6±0.8%. Simple E-probes are applied to pick up the microwave signals inside the WR650 output rectangular waveguide     

High-power, single-beam plasma wave tube

Short-pulse, ultra-broadband sources of RF radiation are needed for a variety of new applications. To meet this demand, we have developed and optimized a single-beam Plasma Wave Tube (PWT), The PWT is a unique microwave/millimeter-wave source which utilizes the interaction between beamexcited electron plasma waves to generate kilowatt-power (~10 kW) radiation at microwave to millimeter-wave frequencies with a beam-to-radiation conversion efficiency of ⩾0.4%. In a single-beam PWT, an electron beam (⩽40 kV, ⩾200 A, 5-to-20-μs pulse width) is injected into a gas-filled (e,g., hydrogen) cylindrical waveguide. The beam first ionizes the gas to generate a plasma, and then nonlinearly interacts with the plasma to generate radiation from 6-to-60 GHz. Slew rates of up to 7 GHz/μs have been measured during a single beam pulse. The radiation has a wide instantaneous bandwidth, typically 10 GHz or wider. Electron-beam transport through the waveguide is accomplished with no externally applied magnetic fields because the beam space charge is cancelled by the background plasma     

Scenario for output pulse shortening in microwave generators drivenby relativistic electron beams

At the present time, microwave generators driven by high current relativistic electron beams are not baked and sealed, so their inner surfaces are densely covered with molecules of gas and oil. This allows the production of microwave pulses of 10^-8 s to 10^-7 s duration, but not longer. A microwave pulse termination scenario is speculated as follows: (1) Electrons oscillating in the strong RF field near the metallic surfaces multiply owing to the secondary emission (the multipactor effect); (2) the multipactor electron bombardment stimulates desorption of gas molecules from the metallic surfaces; (3) the gas undergoes avalanche RF breakdown; and (4) the resultant plasma stops microwave generation and, since electron-ion recombination is slow, does not allow the RF field to revive. At the gigawatt power level, the characteristic time of such a scenario is much shorter than that of the cathode and collector plasma expansion and electron beam instabilities. The energy output parameters of relativistic electron microwave generators can be (and usually are) improved at high pulse repetition rates. A more radical improvement is possible using the technology typical for high vacuum tubes, i.e., baking and sealing     

Variation in the radiation frequency of a plasma relativistic microwave oscillator within its nanosecond pulse

The radiation spectrum of a plasma relativistic microwave oscillator with a pulse power of 50 MW operating in the 10-GHz frequency range is studied experimentally. During a 60-ns-long microwave pulse, the radiation frequency may both remain constant and change by more than 1.5 GHz. The pressure of a gas that ionizes in the microwave field has a significant effect on the radiation frequency and thereby changes the concentration of a pregenerated plasma.     

General scaling of pulse shortening in explosive-emission-drivenmicrowave sources

Microwave generation in devices that depend on synchronization between an electron beam and a resonant cavity or slow wave structure can be disrupted by changes in either. Explosive-emission-driven microwave sources use plasma as the electron source in the diode. This plasma is conductive enough to act as the boundary for both the applied diode voltage and the microwave electric field. The motion of this plasma can effectively change the dimensions of either the electron beam diode or the cavity and will thereby cause resonance destruction. This shortens the microwave pulse length τ_μ. A general model of the process predicts that, for a Child-Langmuir diode, microwave power falls as P∝τ_μ^-5/3 and that pulse energy falls as E∝τ_μ^-2/3. Therefore, energy efficiency declines as the pulse length is extended. We compare with data from magnetrons, MILO's and BWO's, and find that over some regions of operation the pulse length and energy from these explosive-emission-driven microwave sources agree with the plasma motion model scaling. At these applied drive voltages and output powers the microwave pulse length can be increased by finding cathode materials that generate slower plasmas     

微波输出窗内表面闪络击穿3维全电磁等离子体流体模拟北大核心CSCD

通过建立电磁场等离子体流体耦合物理模型,基于自主研发的3维全电磁粒子模拟大规模并行程序NEPTUNE3D,编制了3维电磁场与等离子流体耦合程序模块,对1.3GHz高功率微波窗内表面闪络击穿物理过程进行了数值模拟。研究结果表明:微波窗内侧表面形成的等离子体构型与初始种子电子分布形式密切相关。中心点源分布下,等离子体发展为"蘑菇"形状,输出微波脉冲缩短并不严重,等离子体吸收微波功率大于反射微波功率;面源分布下,等离子体发展为"帽子"形状,输出微波脉冲缩短严重,输出微波完全截断,开始阶段等离子体吸收微波功率占优,待等离子体密度增加到一定程度后,反射微波功率占优。通过降低窗体表面场强、表面释气率及初始种子电子密度等方法,可不同程度地延长输出微波脉冲宽度。窗体表面不同气体层厚度对闪络击穿下的输出微波脉冲宽度影响不大。     

射频击穿等离子体对高功率微波传输特性的影响

利用极化正交的高功率微波合路器,开展了等离子体对于微波传输特性的实验研究.通过改变前级源的功率和脉冲宽度,使得在合路器耦合缝处发生射频击穿,产生等离子体.等离子体扩散进入微波传输主通道,对于高功率微波的传输产生明显的影响,导致微波能量吸收和极化的偏转.初步实验结果表明,等离子体扩散到主通道中心的时间约为3μs,扩散速度约为1μs/cm,等离子体的恢复时间约为5μs.实验测得等离子体导致的微波极化方向最大偏转角度约为4.1?,此时通道内电子个数约为3.7×1015,极化偏转角度与电子数密度以及微波频率相关.     

Experimental observation of further frequency upshift from dc to ac radiation converter with perpendicular dc magnetic field

A frequency upshift of a short microwave pulse is generated by the interaction between a relativistic underdense ionization front and a periodic electrostatic field with a perpendicular dc magnetic field. When the dc magnetic field is applied, further frequency upshift of 3 GHz is observed with respect to an unmagnetized case which has typically a GHz range. The radiation frequency depends on both the plasma density and the strength of the dc magnetic field, i.e., the plasma frequency and the cyclotron frequency. The frequency of the emitted radiation is in reasonable agreement with the theoretical values.     

Operational features and microwave characteristics of the VircatorII experiment

The Vircator II oscillating virtual-cathode microwave source operates with diode voltages between 600 and 800 kV and diode current between 50 and 120 kA. Maximal microwave output power between 200 and 500 MW is achieved when the diode aspect ratio, cathode surface, charge voltage, and extraction coupling are arranged to simultaneously (1) maximize diode voltage, (2) satisfy magnetic insulation criteria, (3) avoid nonuniform or unstable electron emission, and (4) optimize microwave transmission from the virtual cathode to the launching antenna. Broadband radiation between 0.4 and 5.5 GHz is generated. The central frequency follows the beam plasma frequency. It is tuned by varying the current density with anode-cathode gap adjustments     

大面积等离子体片密度分布分析

利用空心阴极放电产生了尺寸为60 cm× 60 cm× 2 cm的大面积等离子体面. 在实验室条件下对大面积等离子体片的密度分布进行了测量. 由于高压放电脉冲脉宽较短, 实验中改变了测量方法, 同时, 在中等磁场影响下, 为了得到真实的等离子体密度, 进行了必要的数值修正.在放电电流为1---6 A时, 测量了二维的电子密度分布. 另外, 测量并讨论了其他环境参数对等离子体密度的影响. 电子密度的分布情况对与微波波束切换相当重要. 由空心阴极增强型放电产生的大面积等离子体面具有反射X波段(8---12 GHz) 微波需要的足够稠密的电子密度和足够均匀的密度分布, 这是等离子体面在雷达系统中取代金属面板的有利条件.     

重复频率X波段类周期加载微波腔的实验研究

在CHP-01加速器上对我所提出的X波段类周期加载微波腔进行了实验研究. 在实验中,首先对加速器进行了调试,使其能够稳定运行;然后对磁场、电压等参数与微波输出的关系进行了研究; 最后在二极管电压约为790kV、电流约为6.7kA时得到了微波输出功率为1.4GW、微波频率为9.4GHz、微波脉宽为30ns、束波转换效率为26%的实验结果.     

高功率微波窗内外表面闪络击穿流体模拟研究

建立理论模型,将电磁场时域有限差分方法与等离子体流体模型结合,编制一维电磁场与等离子流体耦合程序,数值研究了3 GHz高功率微波窗内外表面闪络击穿的不同物理过程.研究结果表明:外表面闪络击穿中,输出微波脉宽缩短(未完全截止),窗体前均方根场强呈驻波分布,波节与波腹位置不变,窗体外表面形成有一层高密(约10~(21)·m~(-3)量级)极薄(约mm量级)等离子体(扩散缓慢),入射波可部分透过该薄层等离子体,脉宽缩短主要源于等离子体吸收效应;降低初始等离子体密度、厚度、入射波场强及缩短入射波脉宽等方式,可不同程度地改善输出脉宽缩短效应.内表面闪络击穿中,窗体前均方根场强亦出现驻波分布f但波节与波腹位置随时间变化),等离子体向波源方向运动;强释气下,输出脉宽缩短(未完全截止),形成多丝状高密(约10~(21)·m~(-3)量级)极薄(约mm量级)等离子体区域(扩散缓慢),间距1/4微波波长,脉宽缩短主要源于等离子体吸收效应;弱释气、低场强下,脉宽缩短有所改善(但最终截止),形成多带状致密(约10~(18)·m~(-3)量级)略厚(mm-cm量级)等离子体区域(扩散较快),间距1/4波长,脉宽缩短主要源于等离子体吸收效应;弱释气、高场强下,脉宽缩短严重(很快截止),形成块状高密(约10~(21)·m~(-3)量级)较厚(约cm量级)等离子体区域(扩散迅速),脉宽缩短主要源于等离子体反射效应.     

强流相对论环形电子束自调制振荡研究

 主要研究了强流相对论环形电子束在等位谐振腔内的非线性自调制振荡，用小信号理论分析了电子起振的条件，得出了电子起振的扰动频率。基于该理论分析，又提出了一种不外加磁场的新型高功率微波器件，该器件主要由一个圆柱谐振腔和一个同轴波导输出腔构成。用2.5维MAGIC粒子模拟软件对该非线性过程进行数值模拟，分析了输入电压、电流对输出微波功率的影响。模拟结果表明这种结构中确实存在非线性不稳定性，自调制的扰动频率则由电子束的初始能量、电荷密度和电子束的半径以及谐振腔的空间结构给定。利用谐振腔长为4.7 cm、二极管电压为2.8 MV、电流为20 kA的电子束，可以得到频率为4.29 GHz、功率为6 GW的微波输出，束-波转换效率约为11%。     

S波段长脉冲相对论返波振荡器实验研究

从物理机制上定性地分析了导致脉冲缩短的主要原因,给出了长脉冲重复频率运行下的相对论返波振荡器(RBWO)设计原则。结合传统谐振式返波振荡器的基本设计理论,设计和模拟优化了工作在S波段的长脉冲RBWO,并利用本实验室现有长脉冲脉冲功率驱动源开展了S波段长脉冲RBWO的实验研究。实验结果表明:在单次运行条件下,微波输出功率达到约2 GW、脉宽约90 ns;在10 Hz重复频率运行条件下,输出微波功率达到约1 GW、脉宽约100 ns。器件产生的微波频率为3.6 GHz,输出模式为TM01模,效率约20%。对实验结果分析表明,器件截止颈和第一个慢波结构结合处的爆炸发射是导致脉冲缩短的主要原因之一。     

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     

同轴电缆头和转接头HPM击穿现象初步分析

介绍了同轴电缆头和转接头的HPM击穿实验研究方法,给出了几种电缆头和转接头微波击穿功率随微波频率、脉冲宽度、重复频率和脉冲持续时间变化规律的实验研究结果。结果表明:微波击穿发生在同轴电缆头连接处,是电缆接头沿面滑闪,且击穿功率随同轴电缆及转接头尺寸的减小而降低;击穿功率也随微波脉冲宽度（30 ns～1 μs）的增大而减小,并且在100 ns附近有一拐点;在低重复频率（1～1000 Hz）下,重频对击穿功率的影响不大;微波频率在2.856～9.37 GHz变化时,微波频率对击穿功率的影响不明显;微波脉冲宽度较窄时（几十ns以下）,击穿功率随持续时间变化不大,脉冲宽度较宽时（百ns以上）,击穿阈值随持续时间的增大而下降。