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.     

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.     

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

Shortening of the radiation pulse from a plasma relativistic microwave generator in numerical calculations with plasma simulation by the particle-in-cell method

Spontaneous shortening of radiation pulses of Cherenkov microwave generators based on the interaction of a high-current relativistic electron beam with preliminarily generated plasma was studied in a numerical model. Microwave pulse shortening is caused by the appearance of a gradually expanding region near the collector, from which plasma is expelled by an electrostatic field of relativistic electrons. The absence of plasma results in a severalfold decrease in the plasma wave reflectance from the collector and violation of generator self-excitation conditions. The microwave emission duration increases with the plasma ion mass.     

充氦气对多波切伦柯夫振荡器工作特性的影响

采用PIC模拟方法,研究了具有谐振腔的多波切伦柯夫振荡器在充入不同密度的氦气下工作的物理过程,分析了等离子体产生物理机制及其对微波输出影响。结果表明,等离子体的产生是由于电子束对氦原子的碰撞电离及其雪崩效应引起的。由于电离产生的正离子有利于束的传输和群聚,当在一定范围内增加氦气密度时,可减小微波起振时间,提高束波能量转换效率,但并不改变微波频率;进一步增大气体密度,微波起振时间增大、效率下降,甚至出现脉冲缩短现象。     

Plasma parameters of a nonself-sustained microwave discharge created by a programmed pulse

The kinetics of the electrons in the plasma of a nonself-sustained discharge formed at the focus of a microwave beam when the gas is exposed to electromagnetic radiation with an energy flux density that varies with time in a programmed-pulse mode are investigated. It is shown that the temperature of the electrons in the plasma of the localized microwave discharge is of the order of 1 eV and varies weakly during the pump pulse and as the air pressure is varied, while the electron density is an order of magnitude or more lower than the critical density and depends on the level of the pump generator power. It is shown that the degree of ionization of the plasma can be regulated by altering the programmed-pulse mode. Zh. Tekh. Fiz. 67, 19–23 (July 1997)     

Numerical investigation of subcritical microwave discharges in a high-pressure gas

Subcritical microwave streamer discharges are investigated using a two-dimensional model that describes gas-dynamic processes in the ideal gas approximation and a self-consistent electromagnetic field in the wave approximation and takes into account the minimum required number of kinetic processes (such as ionization, attachment, recombination, diffusion, and electric conduction). The initial conditions imitate the initiation of a discharge from a small cavity with a reduced gas density and an arbitrarily small degree of gas ionization. The possibility of describing streamer discharges without reference to ionizing hard radiation is confirmed.     

Microsecond Microwave Radiation Bursts from a Relativistic Electron Beam Rotating in a Plasma

Coherent curvature radiation from an intense relativistic electron beam rotating in a plasma yielded in the past [1] only very short bursts of microwave radiation. Here we report on extension of the microwave-burst duration to the full width of the beam pulse for two different electron beams: a) 1 MV, 50 kA, 110 ns b) 0.6 MW, 15 kA, 1 , ?s. We show that the short bursts in the past resulted from sudden changes in the plasma density caused by ionization of neutrals, present in the drift tube, by the beam itself. This density change caused a shift away from the resonance condition necessary for coherent radiation. The extension of the pulsewidth was achieved by introducing a plasma gun which did not emit many neutrals. The radiation is mostly in the Ka band (? ? 1 cm) and the power level was ~1 MW.     

Dependence of Output Power on Cavity Length in a Gyrotron Backward Wave Oscillator

A relativistic electron beam (500 keV, 200 A, 10 ns) generated magnetically tunable microwave radiation in a frequency range of 9-13 GHz when it is injected into an X-band rectangular waveguide immersed in a uniform axial magnetic field (4-10 kG). The mechanism of the microwave radiation was identified as the gyrotron backward wave interaction. The output power of the radiated microwave increased exponentially with the increase of the cavity length.     

低环径比托卡马克堆中的α粒子输运

基于当前等离子体物理，本文初步讨论了低环径比托卡马克堆中等离子体的特征。在自洽的低环径比堆芯参数下，计算了α粒子约束和损失，以及不同环径比对它们的影响。     

Wideband antenna for detecting high-power microwave radiation pulses

The plasma relativistic generator (PRG) at the Sinus 550-80 accelerator has a number of unique properties, such as the single- and multiple-pulse (up to 50 pulses per second) generation modes, generation of microwave pulses with a duration of up to 80 ns and frequencies of 2 to 25GHz, and generation of monochromatic and wide-spectrum radiation. The carrier frequency and the spectrum width can be controlled by varying the experimental parameters, e.g., the plasma density. To investigate the PRG operation modes, appropriate devices for detection of the PRG microwave radiation are needed. In this work, numerical models of antennas with different geometrical parameters are developed using the Ansoft HFSS 13.0 CAD and an antenna with the optimum parameters is proposed for measuring microwave radiation spectra in the desired frequency range at a power level of 50 to 100MW.     

强流电子束碰撞电离背景气体研究

利用数值计算与粒子模拟两种方法,结合实际的实验数据,对高功率微波二极管中相对论电子束与背景气体相互作用碰撞产生的等离子体密度进行了研究.研究结果表明：碰撞产生的等离子体密度数值计算结果与粒子模拟结果基本一致,背景气压在0.01 Pa—0.05 Pa时,碰撞产生的等离子体密度在4—12×10⁹cm^-3,即便在考虑电子离子复合的情况下,数值计算结果与粒子模拟结果依然符合得很好.另外,粒子模拟结果表明：随着气压的增加,等离子体密度呈现先增大再减小然后又逐渐增大的过程, 关键词： 相对论电子束 等离子体 数值计算 粒子模拟     

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     

Kinetic theory of the plasma-dynamical modes and the transport coefficients of a relativistic plasma

The kinetic equation of an inhomogeneous relativistic plasma, consisting of an electron gas and a radiation field, is studied with particular regard to its eigenvalues in the hydrodynamical limit. The treatment is classical for the particles and quantum-mechanical for the field oscillators.After a suitable regularization, the eigenvalues are obtained by a perturbation theory through second order in the strength of the gradients. It is shown that these eigenvalues are in exact correspondence with the macroscopic relativistic plasma-dynamical modes. The important role played by the Vlassov operator in building up the peculiar structure of these modes is underlined. From a comparison of the macroscopic and microscopic eigenvalues we obtain general expressions for the thermal conductivity, the shear viscosity and the bulk viscosity of a relativistic plasma. The contribution of the radiation field to these quantities is a noteworthy feature of these expressions.     

填充等离子体的介质同轴波导中的切伦科夫辐射

 利用自洽线性场理论，导出了薄环形相对论电子注通过填充等离子体的介质同轴波导中的注波互作用色散方程，得到了注波互作用产生切伦科夫辐射的同步条件和波增长率。分析了填充等离子体后的波与电子注之间的能量交换及等离子体密度对色散特性、波增长率和注波能量交换的影响。分析结果表明：切伦科夫辐射是由沿介质同轴波导传播的慢波与沿薄环形相对论电子注传播的负能空间电荷波耦合所致，且其耦合强度与电子注的密度成正比；输出频率和波增长率随着填充等离子体密度的增大而提高；保持一定的输出频率，增大电子注的束流可得到高的微波输出功率。     

相对论全腔轴向提取磁控管阳极释气数值模拟研究

相对论电子轰击阳极产生二次电子和释气以及释气电离产生的击穿现象是限制全腔轴向提取透明阴极磁控管（TCMAC）工作性能的一个重要因素。本研究对TCMAC中轰击产生的阳极二次电子以及释气电离现象进行了物理建模以及三维数值模拟研究,考察了其对TCMAC运行性能的影响。初步计算结果表明,二次电子发射与阳极释气对TCMAC工作都有一定影响,释气后电离产生的正离子数大于系统中电子数时,会导致TCMAC击穿。     

Microwave and optical breakdowns of gases and field harmonic generation

The gas breakdown in strong electromagnetic fields of microwave and optical frequency ranges is studied. Strong fields are understood as the fields in which the electron oscillation energy exceeds the ionization potential of gas atoms, but which are still weak for field ionization of atoms. In such fields, the inelastic collision frequency of electrons is higher than the elastic collision frequency. As a result, ionization of atoms is accompanied by efficient generation of high field harmonics.     

大气压微波等离子体射流装置放电特性研究

基于同轴传输线结构设计了两种不同喷嘴结构的大气压微波等离子体射流(MW-APPJ)装置,其工作频率2.45 GHz,工作气体为氩气,分别研究了两种不同喷嘴结构对等离子体放电特性产生的影响。仿真结果表明,MW-APPJ在气体喷嘴处会产生高强度的电场,经过优化结构,实现在频率2.45 GHz下,喷嘴处的场强满足氩气电离的击穿场强阈值要求。同时,利用多物理场耦合仿真软件对装置的气流分布进行了稳态模拟,并通过实验对比分析了两种喷嘴结构下大气压氩等离子体射流的基本特性。实验结果表明,不同的喷嘴结构会影响等离子体装置的反射系数随输入功率的变化规律,但并不影响等离子体射流长度随输入功率的变化规律和反射功率随进气流量的变化规律;同时,在大气压下,稳态微波等离子体射流呈现出类金属性,等离子体中的电子只能在很薄的区域中吸收微波能量,因而造成微波的反射功率较大。     

On the development of plasma relativistic microwave oscillator without strong magnetic field

A method for generating microwave pulses with a power of ～1 GW and a frequency controlled in a band from 2 to 15 GHz during the interaction of relativistic electrons with plasma without strong magnetic field is proposed in the numerical model. The combination of a broadband plasma relativistic microwave oscillator and a monochromatic magnetically insulated transmission line oscillator (MILO) allows the development of a device combining advantages of both prototypes.