Modeling the effect of spin polarization of magnetic moments of electrons on crossover formation

We present the results from investigations of the effect of spin polarization on crossover formation in electron microscopes. Spatial distributions of magnetic states of electrons for the electron beams in the crossover of the electron gun are analyzed by the example of a self-consistent spin-polarization field. The factors able to affect the spatial resolving ability and sensitivity of scanning microscopy are evaluated.     

集装箱检测系统用加速器电子枪的特性研究

 研究了固定式海关集装箱检测系统用9MeV电子行波直线加速器电子枪的结构特点和束流特性。该电子枪采用钪酸盐阴极,在灯丝电流为2.4A,阳极引出电压为-40kV的工作条件下,由电子枪发射到加速器入口处的脉冲电流强度为300mA,在加速器出口处,束斑截面小于φ2mm;打靶后,产生的X射线剂量率大于30cGy/(min× m)。电子枪的良好特性及束流强度保证了集装箱检测图像所需的高分辨率的要求。为保证满足更换电子枪时的安装要求,利用EGUN程序模拟计算了电子枪的结构及束流特性,为实际装配和使用提供了参考尺寸和调试参数。     

Constructions and Preliminary HV Conditioning of a Photocathode Direct-Current Electron Gun at IHEP

As one of the most important key technologies for future advanced light source based on the energy recovery linac, a photocathode dc electron gun is supported by Institute of High Energy Physics(IHEP) to address the technical challenges of producing very low emittance beams at high average current. Construction of the dc gun is completed and a preliminary high voltage conditioning is carried out up to 440 kV. The design,construction and preliminary HV conditioning results for the dc gun are described.     

Analysis of the triode gun and the beam dynamics of a standing waveelectron linac

The charge-density integral method is used to analyse a triode gun which will inject electrons into a standing wave electron linac. The effect of space charge on the gun output beam parameters is estimated. The beam dynamics of the entire linac system is studied. The energy spectrum, beam size and divergence at the end of the linac are evaluated neglecting space charge     

Electron optics for high throughput low energy electron microscopy

Novel electron-optical components and concepts aiming at improving the throughput and extending the applications of a low energy electron microscope (LEEM) have been developed. An immersion magnetic objective lens can substantially reduce e-e interactions and the associated blur, as electrons do not form a sharp crossover in the back-focal plane. The resulting limited field of view of the immersion objective lens in mirror mode can be eliminated by immersing the cathode of the electron gun in a magnetic field. A dual illumination beam approach is used to mitigate the charging effects when the LEEM is used to image insulating surfaces. The negative charging effect, created by a partially absorbed mirror beam, is compensated by the positive charging effect of the secondary beam with an electron yield exceeding 1. On substrates illuminated with a tilted beam near glancing incidence, large shadows are formed on even the smallest topographic features, easing their detection. On magnetic substrates, the magnetic flux leaking above the surface can be detected with tilted illumination and used to image domain walls with high contrast.     

Size-driven dimensional crossover in the quasi-two-dimensional Ising model

We have studied the finite size effect in the quasi-two-dimensional Ising model by using a Monte Carlo simulation. A marked finite size effect was found with decreasing interlayer interaction. Aside from the well-known three- to two-dimensional crossover, a three- to one-dimensional crossover at a crossover size L_c∼(λ/2)^−ν/φ was revealed as an origin of the marked finite size effect, where λ is the interlayer to intralayer interaction ratio, and ν and φ are the critical exponent for the correlation length and the crossover exponent, respectively. While the former crossover is driven by temperature, the latter is driven by size at a fixed λ.     

Efficient Low-Voltage Operation of a CW Gyrotron Oscillator at 233 GHz

The gyrotron oscillator is a source of high average power millimeter-wave through terahertz radiation. In this paper, we report low beam power and high-efficiency operation of a tunable gyrotron oscillator at 233 GHz. The low-voltage operating mode provides a path to further miniaturization of the gyrotron through reduction in the size of the electron gun, power supply, collector, and cooling system, which will benefit industrial and scientific applications requiring portability. Detailed studies of low-voltage operation in the TE(2) (,) (3) (,) (1) mode reveal that the mode can be excited with less than 7 W of beam power at 3.5 kV. During CW operation with 3.5-kV beam voltage and 50-mA beam current, the gyrotron generates 12 W of RF power at 233.2 GHz. The EGUN electron optics code describes the low-voltage operation of the electron gun. Using gun-operating parameters derived from EGUN simulations, we show that a linear theory adequately predicts the low experimental starting currents.     

毫米波速调管电子光学系统的研究

电子光学系统是毫米波速调管长寿命和整管性能实现的关键,毫米波速调管零件尺寸较小,为了在Ka波段和W波段实现千瓦量级的输出功率,要求具有高的电子注通过率及低的阴极负荷。对Ka波段和W波段电子光学系统特性进行了分析,确定了Ka波段10 kW分布作用速调管和W波段1 kW分布作用速调管电子光学系统的设计方案,利用软件对电子枪和聚焦系统的结构进行计算,并采用CST仿真软件对设计的电子枪发射的电子注在聚焦磁场中的状态进行优化。设计出的Ka波段速调管电子光学系统,电子枪工作电压26 kV,发射电流2 A,互作用区长度30 mm,磁场强度大于0.6 T,流通达到100%。设计的W波段速调管电子光学系统,电子枪工作电压17 kV,电流0.65 A,互作用区长度20 mm,磁场大于0.9 T,流通达到100%。已制成Ka波段速调管和W波段速调管,设计的电子光学系统能够满足速调管工程化需求。     

NEPCⅡ新电子枪热结构分析与测试

北京正负电子对撞机重大改造工程(BEPCⅡ)要求其直线注入器提供更高的能量和流强, 为此必须设计大电流和小发射度的新电子枪, 以提高正电子的产额. 利用ANSYS对BEPCⅡ新电子枪的温度场分布及结构的热变形进行了模拟分析. 对于电子枪复杂的内部结构形态以及能量转换方式, 分析了传导传热方式对温度场的影响. 在此基础上进行了温度场与结构变形的耦合分析, 利用EGUN对电子枪形变前后束流光学特性进行模拟分析, 并对模拟结果与试验 测试结果进行了分析比较.     

放电参数对爆燃模式下同轴枪强流脉冲放电等离子体的影响

同轴枪强流脉冲放电常见有爆燃模式和预填充模式两种放电模式,爆燃模式放电可以得到杂质少、准直性高、输运速度更快的等离子体射流.本实验主要对不同电压及进气量下同轴枪强流脉冲爆燃模式放电的等离子体特性进行了研究.结果表明,在相同放电电压下,进气量少时会有多团等离子体从枪口喷出.随着进气量的增加,同轴枪放电产生的等离子体密度增加,输运速度减小,最终等离子体只有一团从枪口喷出;而在相同进气量下,随着电压的增加,等离子体密度增加,输运速度增大,开始出现有多团等离子体从枪口喷出的现象.产生该现象的原因主要是在放电过程中,当气体持续进入枪底部时,同轴枪底部会产生新的电流通道向前运动,使得在同轴枪出口处观察到了多团等离子体喷出的现象;随着放电电压的增加,在放电过程中回路电流也增加.当电流增加到一定程度时,同轴枪底部就会产生新的电流通道,从而有多个等离子体团从枪口喷出.通过改变充电电容以及对磁探针信号的分析,进一步分析并验证了同轴枪底端多次放电的现象.     

Ultrashort pulse electron gun with a MHz repetition rate

We report the construction of an electron gun emitting ultrashort pulses with a repetition rate of 2.7 MHz. The gun works at an acceleration voltage of 20 kV and is operated with a laser oscillator having an ultralong cavity. A low number of electrons per pulse eliminates space charge broadening. Electron yield and beam profiles are measured for operation with laser wavelengths of 800, 400, and 266 nm. The initial energy spread of the electrons is determined for these three wavelengths, and pulse durations of 600, 390, and 270 fs are inferred from the data.     

同轴对称型主耦合器的研制

 为更进一步提高注入器的性能,注入器使用的耦合器采用同轴对称耦合。给出了同轴对称耦合器的理论计算、具体设计以及实验情况。经过对模型耦合器多次微波传输测量调试以及对各个部位反射的细致分析,最后获得了较好的结果,证明了该耦合器的实用性和可行性。     

Experimental observation of structural crossover in binary mixtures of colloidal hard spheres

Using confocal microscopy, we investigate the structure of binary mixtures of colloidal hard spheres with size ratio q=0.61. As a function of the packing fraction of the two particle species, we observe a marked change of the dominant wavelength in the pair-correlation function. This behavior is in excellent agreement with a recently predicted structural crossover in such mixtures. In addition, the repercussions of structural crossover on the real-space structure of a binary fluid are analyzed. We suggest a relation between crossover and the lateral extension of networks containing only equally-sized particles that are connected by nearest-neighbor bonds. This is supported by Monte Carlo simulations which are performed at different packing fractions and size ratios.     

双注磁控注入电子枪

为了增加回旋管的功率,采用双注磁控注入电子枪产生相对论电子注。与双阳极磁控注入电子枪相比,双注磁控注入电子枪产生双束电子注,在不影响电子注质量的基础上,增加电子枪的电流;电子枪产生相同电流时,双注磁控注入电子枪电子注电流小,电子注电子之间的空间电荷效应小,能够降低电子注的速度零散,提高电子注的质量。采用MAGIC软件数值模拟双注磁控注入电子枪,设计出一支大束流、低速度零散的双注磁控注入电子枪。     

Unheated magnetron gun as an electron source for a resonant linear accelerator

Investigations of the operation of a cold-cathode magnetron gun as part of a traveling-wave linear accelerator are described. Two operating regimes of the gun are observed: with and without the microwave field of the prebuncher near the gun. In the regime with no microwave field, short (around 2 ns) pulses of accelerated electrons of up to 0.5A were obtained with a gun current up to 20 A. The presence of a microwave field near the gun makes it possible to obtain a beam of longer duration (up to 1.0 μs), but with a current of up to 20mA at the accelerator exit and 1A from the gun. The operating mechanism of the gun is attributed to a secondary-electron current rise and the establishment of self-sustained secondary emission. A comparison between the conditions of beam production from a thermionic gun and that studied for the same injection energies indicates that the characteristics of this gun are acceptable for injection into an accelerator. Zh. Tekh. Fiz. 69, 130–134 (July 1999)     

BEPCⅡ Positron Source

BEPCⅡ— an upgrade project of the Beijing Electron Positron Collider (BEPC) is a factory type of e^＋e^－ collider. The fundamental requirements for its injector linac are the beam energy of 1.89GeV for on-energy injection and a 40mA positron beam current at the linac end with a low beam emittance of 1.6μm and a low energy spread of ±0.5% so as to guarantee a higher injection rate (≥50mA/min) to the storage ring. Since the positron flux is proportional to the primary electron beam power on the target, we will increase the electron gun current from 4A to 10A by using a new electron gun system and increase the primary electron energy from 120MeV to 240MeV. The positron source itself is an extremely important system for producing more positrons, including a positron converter target chamber, a 12kA flux modulator, the 7m focusing module with DC power supplies and the support. The new positron production linac from the electron gun to the positron source has been installed into the tunnel. In what follows, we will emphasize the positron source design, manufacture and tests.     

Critical thermodynamics of the two-dimensional +/-J Ising spin glass

We compute the exact partition function of 2d Ising spin glasses with binary couplings. In these systems, the ground state is highly degenerate and is separated from the first excited state by a gap of size 4J. Nevertheless, we find that the low temperature specific heat density scales as exp(-2J/T), corresponding to an "effective" gap of size 2J; in addition, an associated crossover length scale grows as exp(J/T). We justify these scalings via the degeneracy of the low lying excitations and by the way low energy domain walls proliferate in this model.     

紧凑型高平均流强光阴极注入器

根据北京大学1+1/2单元超导腔直流--超导(DC-SC)光阴极注入器的实验研究结果, 证明了这种新型光阴极注入器的可行性, 在此基础上对这种新型注入器进行了升级设计, 使之成为实用的紧凑型高平均流强的光阴极注入器, 为北京大学自由电子激光装置提供品质良好的电子束流. 给出了升级的3+1/2单元超导腔注入器的详细设计, 包括注入器核心结构的电磁场设计、束流动力学模拟、机械设计. 并对整个注入器的恒温器进行了初步设计.     

Effect of gun current on the microstructure and crystallinity of plasma sprayed hydroxyapatite coatings

Hydroxyapatite (HA) is a bioactive material because its chemical structure is close to the natural bone. Its bioactive properties make it attractive material in biomedical applications. Gas tunnel type plasma spraying (GTPS) technique was employed in the present study to deposit HA coatings on SUS 304 stainless steel substrate. GTPS is composed of two plasma sources: gun which produces internal low power plasma (1.3-8 kW) and vortex which produces the main plasma with high power level (10-40 kW). Controlling the spraying parameters is the key role for spraying high crystalline HA coatings on the metallic implants. In this study, the arc gun current was changed while the vortex arc current was kept constant at 450 A during the spraying process of HA coatings. The objective of this study is to investigate the influence of gun current on the microstructure, phase crystallinity and hardness properties of HA coatings. The surface morphology and microstructure of as-sprayed coatings were examined by scanning electron microscope. The phase structure of HA coatings was investigated by X-ray diffraction analysis. HA coatings sprayed at high gun current (100 A) are dense, and have high hardness. The crystallinity of HA coatings was decreased with the increasing in the gun current. On the other hand, the hardness was slightly decreased and the coatings suffer from some porosity at gun currents 0, 30 and 50 A.     

Design of a 3-MW 140-GHz gyrotron with a coaxial cavity

A design for a 3-MW 140-GHz gyrotron based on the use of a coaxial cavity is given. The cavity mode is TE_21,13, chosen so that the ohmic heating on both the inner and outer conductors would be low enough for CW operation. The mode selection process, nonlinear, multimode and time-dependent modeling of the beam wave interaction, and gun design are discussed in detail. An inverted magnetron injection gun (MIG) is used to accommodate the inner conductor. The radiation is coupled out via a quasi-optical mode converter, consisting of an irregular cylindrical waveguide section followed by a step-cut launching aperture and a single near-parabolic mirror. The design of these components is also described