Inhomogeneous extended hollow cathode discharge for raising the current density in a forevacuum plasma source of a ribbon electron beam

The plasma parameters and the emissivity of a ribbon electron beam source based on a discharge with an inhomogeneous extended hollow cathode are measured. A constriction in the cathode cavity increases the plasma density near the emitting area boundary, which adds to the electron current density in the beam. The reason for the above effect is the formation of the plasma density distribution nonuniform across the cavity with a maximum in the middle. This maximum is caused primarily by a plasma electron flow from the constriction, which is generated by the electric field and is directed toward a slit emission-extracting aperture.     

On the effect of a magnetic field on fast charged particles passing through a substance

A transport equation in the small-angle approximation is obtained for a curvilinear beam of fast charged particles passing through a substance in a nonuniform magnetic field. Green functions for this equation are found for an annular beam in a weak-focusing field and for a helical beam in the nonuniform magnetic field.     

Particle-in-cell simulation of generation of an HF sequence of short current pulses in the vircator with counter-propagating beams

We performed a particle-in-cell (PIC) simulation of a vircator with counter electron beams in a cylindrical cavity with metal walls. Two pairs of counter-propagating superlimiting electron beams with different energy and current in each pair are injected along the cylinder axis. It is found that the electron charge in the cavity is formed as a sequence of pulses corresponding to slow accumulation and fast discharge. It is shown that the pulses are related to the characteristic dynamics of the squeezed beam state, the distributed virtual cathode. This regime can be used to generate anHF sequence of ultrawideband electromagnetic pulses.     

高电流密度圆柱状电子光学系统设计

设计了高电流密度圆柱状电子枪,并分析了枪体参数对电子注品质的影响;运用径向力平衡理论计算了聚焦磁场.三维模拟软件CSTPARTICLESTUDIO模拟显示,在枪体电极和空间电荷形成的静电场,以及聚焦磁场作用下,电子呈角向先变速后匀速螺旋状运动,其旋转半径始终与出射阴极面时的半径相同,从而在枪区和互作用通道内维持了注包络水平.模拟结果与设计理论相符.注平均电流密度达到24·4A/cm2,层流性良好,填充因子76·7%,流通率100%.     

相对论速调管放大器两腔束流调制的理论与模拟比较分析

采用一维自洽束波互作用的非线性理论,探索了相对论速调管放大器(RKA)的束流调制特性,并与二维PIC结果进行了比较和分析。首先推导了环形电子束的几何因子,然后分别给出了两腔调制时归一化调制电流和电子束动能的积分微分方程。采用调制电子束激励中间腔的非线性理论估算了中间腔间隙电压的幅度和相位,幅度的相对误差为0.07%,相位误差为2.15°。给出了输入腔和中间腔的间隙耦合系数公式,对于束压为715.2 kV、束流为8 kA的电子束,当输入腔和中间腔间隙电压分别为14,315.2 kV,输入腔和中间腔相位差为90.59°时,采用一维非线性理论和二维粒子模拟程序分别计算了基波电流调制系数和距离的关系。对于基波电流调制系数,中间腔之前处于线性区,理论值与模拟值比较一致;在中间腔内部基波电流调制系数出现明显的下降,理论值与模拟值之间的差距较大;中间腔之后处于非线性区,对于基波电流调制系数的最大值和对应的纵向位置,理论值与模拟值基本一致。当束流传输距离为65.8 cm时,计算了归一化电流和归一化电子束动能随时间的变化和电荷守恒参量N(z)与调制电流的n次谐波的电流调制系数随传输距离的变化情况。     

Resonant cavity vircator driven by a thermionic cathode electronbeam gun

A resonant cavity vircator (virtual cathode oscillator) driven by an electron beam emitted from a broad-area thermionic cathode is tested. Narrow-bandwidth (1.0 MHz at the -3 dB level) excitation of the TM₀₂₃ mode of a cylindrical resonant cavity is observed at a frequency of 986 MHz with a pulselength of 1.2 μs. The single-cavity-mode excitation is attributed to the constant-voltage and -current electron beam emitted from the thermionic cathode     

Generation and measurement of relativistic electron bunches characterized by a linearly ramped current profile

We report the first successful attempt to generate ultrashort (1-10 ps) relativistic electron bunches characterized by a ramped longitudinal current profile that rises linearly from head to tail and then falls sharply to zero. Bunches with this type of longitudinal shape may be applied to plasma-based accelerator schemes as an optimized drive beam, and to free-electron lasers as a means of reducing asymmetry in microbunching due to slippage. The scheme used to generate the ramped bunches employs an anisochronous dogleg beam line with nonlinear correction elements to compress a beam having an initial positive time-energy chirp. The beam current profile is measured using a deflecting mode cavity, and a pseudoreconstruction of the beam's longitudinal phase space distribution is obtained by using this diagnostic with a residual horizontal dispersion after the dogleg.     

Generation of electron beams in a multicathode secondary-emission source

A study is made of the generation of electron beams in a system consisting of eight secondary-emission cathodes arranged at regular intervals in the azimuthal direction inside a coaxial cylindrical anode in crossed electric and magnetic fields. In this system with an azimuthally nonuniform electric field, secondary-emission multiplication of electrons is realized and beam generation is achieved. With a cathode voltage of ∼37 kV and a magnetic field of ∼3000 Oe, the total current of all the beams amounts to ∼35 A, the microperveance of each beam being ∼0.7 μA/V^3/2.     

Space charge limiting current of an electron beam transported in a coaxial drift chamber

An expression for the space charge limiting current of an electron beam in a vacuum coaxial drift chamber is obtained in the strong field approximation. The expression is an analogue of the Bogdankevich-Rukhadze interpolation formula for a cylindrical drift chamber. The space charge limiting currents in the coaxial and cylindrical chambers are compared. The space charge limiting current in the vacuum coaxial chamber is numerically calculated and compared to analytical predictions.     

The nondissipative spin Hall current in a nonuniform Rashba effect system

The spin Hall current in a two-dimensional electron system with nonuniform Rashba spin–orbit interaction (SOI) is investigated by means of the lattice Green's function method. Large electric and spin Hall currents are produced by this nonuniform Rashba SOI, while the electric Hall current vanishes in the uniform Rashba SOI system. A nondissipative spin Hall current is also produced, without any longitudinal voltage bias, any external magnetic field and any special class of band insulators.     

Simple Nd:YAG laser generates vector and vortex beam

We report a simple Nd:YAG laser that emits radially polarized beam with helical wavefront. The laser cavity consists of a piece of laser crystal and a plane output coupler, and there is no additional polarization component inside it. The pump light is converted into annular profile through de-focal coupling into a multi-mode fiber. For the continuous-wave(CW) operation, the laser emits radially polarized vortex beam, and it is observed that the helical wavefront of the laser beam is switched from right handedness to left handedness when the output coupler is tilted slightly. For the Q-switched operation under the insertion of a Cr4t:YAG saturable absorber inside the cavity, we obtain radially polarized outputs with left-handedness helical wavefront. By tilting the laser crystal slightly, the laser output switches to azimuthal polarization at pump power larger than 4.5 W and left-handedness helical wavefront of laser beam is preserved.     

Simultaneous generation of helical beams with linear and radial polarization by use of a segmented half-wave plate

Simultaneous generation of helical beams with linear and radial polarization is demonstrated by use of a segmented half-wave plate in a Ti:sapphire laser cavity. A linearly polarized Laguerre-Gaussian beam is converted to a radially polarized beam with a spiral phase shift and vice versa. In addition, these two beams coexist in the cavity, and the cavity emits one of these two beams from an output coupler.     

Electron-optical system for a large-orbit gyrotron

An electron-optical system generating a rectilinear or helical 250 keV/4 A/10 μs electron beam with a high compression factor is developed. For the former beam, a compression factor as high as 4400 and a current density of 25 kA/cm₂ are achieved. In the process of forming the helical beam, the electrons rotating about the system’s axis (paraxial beam) acquire an initial velocity in a transverse magnetic field produced by a kicker. Their pitch factor is increased to a desired (operating) value in an adiabatically growing magnetic field. In tentative experiments with the helical beam in a large-orbit gyrotron, generation was obtained at the second cyclotron harmonic (223 GHz).     

Helical normal waves near a cylindrical cavity in an elastic medium

The properties of helical waves arising near a cylindrical cavity in an elastic medium are described. These waves are a manifestation of aperiodic (in angle) solutions to the dynamic elasticity equations. The dispersion characteristics of the waves are determined, and the spatial structure of the waves is described.     

Development of a Prototype of a 1-MW 105-156-GHz Multifrequency Gyrotron

We present the results of studying the possibility of numerical and experimental optimization of the helical electron beam (HEB) formation system and the processes of mode interaction in the electron beam – cavity system for a prototype of a 1-MW 105–156-GHz step-tunable gyrotron with various operating modes. The system parameters are optimized to achieve the maximum efficiency of the gyrotron with an acceptable ohmic load in the cavity. We also analyze the influence of mode competition and depression of the electron-beam potential as well as the possibility of energy recovery of the collector electron beam. The possibility of optimizing the mode converter system for various operating modes is demonstrated.     

体布拉格光栅外腔半导体激光器光谱特性研究

用体布拉格光栅(VBG)作为反馈元件与瓦级半导体激光器(LD)以及快轴准直柱透镜构成一个可以将半导体激光器的工作波长稳定在体布拉格光栅布拉格波长处的外腔激光器。测量了体布拉格光栅外腔激光器的波长稳定性与其工作电流、热汇温度、激光束准直装置等因素的关系。分析了波长稳定效果与半导体激光器增益谱特性、外腔结构参量等因素的关系。研究表明,在相同的工作电流、热汇温度下,当准直柱透镜直径为0.4 mm时的波长稳定效果较好;在此情况下,当热汇温度控制在30℃,工作电流从0.5 A增加到1.5 A的测量范围内,以及当工作电流固定在1.5 A,热汇温度从20℃增加到35℃时,测得的光谱特性表明,半导体激光器的工作波长可以很好地稳定在体布拉格光栅的布拉格波长处。与该激光器在同样条件下自由运转的光谱比较,可以看到,自由运转激射波长与体布拉格光栅的布拉格波长差值小于2.6 nm情况下,可以获得很好的波长稳定效果。实验也表明,当该值大于4.8 nm时波长稳定效果变差。     

Interference properties of a light beam having a helical wave surface

The spatial wave front characteristics of a laser were examined by two beam interference after amplitude division. Complex interference patterns were observed and are interpreted as due to a helical form of the phase front. This unusual form is implicit in previous analyses of cavity eigenfunctions, but the properties of such a helical co-phasal wavefront have not apparently been previously studied.     

基于外腔反馈二极管线阵列的smile效应测量方法

采用外建激光谐振腔,在低于原芯片阈值的电流激励下对LDA的每个发光点进行单独测量,从而分析整个半导体激光阵列(LDA)的smile效应。实验中利用镀膜反射率大于半导体前腔面的外腔镜形成外腔半导体激光器。在外腔中插入曲面平行于p-n结的柱面镜,使只在光轴上的发光点与外腔镜形成外腔激光器,降低该发光点的激光阈值,从而使其在正常的阈值以下的电流激励下输出激光,在平行于p-n结的方向移动柱面镜,可以逐个对半导体激光器中的发光点进行选择测量,从而获得LDA smile效应的测量值。测量中的低电流激励产生的热量对芯片寿命没有影响,对LDA的发光点的单个测量也避免了其他发光点对CCD的影响。     

Limiting current of helical electron beams in gyrotrons

Potential depression (PD) and limiting current (LC) of helical electron beams (HEB) are investigated. Nonneutral electron beams and two velocity distributions: uniform and Rayleigh distribution are taken into account. For Rayleigh distribution electron reflection from magnetic mirror is taken into account. It is shown that velocity spread may lead to substantial reduction of LC for real values of the mean pitch-factors of helical trajectories. PD and LC are an unmonotonous functions of the velocity spread for Rayleigh distribution. A connection of PD with the change of gyrofrequency and with the beam broadening in gyrotron operating space is retraced.     

同轴波导环形相对论电子束空间极限电流

得到了1维条件下电子均匀分布时,环形电子束在同轴波导内轴向传输的空间极限电流解析表达式。通过讨论解析式在薄电子束和波导内导体半径为0等极限条件下的近似情况,分析了其合理性。在电子束非均匀分布的条件下,对空间极限电流进行了数值求解,并与解析计算结果进行了比较。理论研究表明:环形电子束在同轴波导内具有较圆柱波导内更高的空间极限电流,而且当电子束靠近波导壁时,空间极限电流会显著提高。