Deflection of high-energy negative particles in a bent crystal through axial channeling and multiple volume reflection stimulated by doughnut scattering

Different kinds of deflection in a silicon crystal bent along the 〈111〉 axis was observed for 150 GeV/c   negative particles, mainly π⁻${\pi }^{-}$ mesons, at one of the secondary beams of the CERN SPS. The whole beam was deflected to one side in quasi-bound states of doughnut scattering (DSB) by atomic strings with the efficiency (95.4 ± 0.2)% and with the peak position close to the bend crystal angle, α=185 μrad$\alpha =185\text{μrad}$. It was observed volume capture of π⁻${\pi }^{-}$ mesons into the DSB states with a probability higher than 7%. A beam deflection opposite to the crystal bend was observed for some orientations of the crystal axis due to doughnut scattering and subsequent multiple volume reflections of π⁻${\pi }^{-}$ mesons by different bent planes crossing the axis.     

Observation of parametric X-rays produced by 400 GeV/c protons in bent crystals

Spectral maxima of parametric X-ray radiation (PXR) produced by 400 GeV/c protons in bent silicon crystals aligned with the beam have been observed in an experiment at the H8 external beam of the CERN SPS. The total yield of PXR photons was about 10⁻⁶ per proton. Agreement between calculations and the experimental data shows that the PXR kinematic theory is valid for bent crystals with sufficiently small curvature as used in the experiment. The intensity of PXR emitted from halo protons in a bent crystal used as a primary collimator in a circular accelerator may be considered as a possible tool to control its crystal structure, which is slowly damaged because of irradiation. The intensity distribution of PXR peaks depends on the crystal thickness intersected by the beam, which changes for different orientations of a crystal collimator. This dependence may be used to control crystal collimator alignment by analyzing PXR spectra produced by halo protons.     

重复频率强流电子束的产生和传输实验研究

 电子束真空二极管重复频率运行时,它将表现出与单次运行时不同的特点。在电子束产生过程中,屏蔽半径应尽可能地小,且击穿延时时间较短,故选择石墨作为阴极材料。实验结果表明：在重复频率运行时,当环型阴极环厚较薄时,阴极的发射电流密度较大,因此对阴极的加热效应也加强,等离子体的膨胀速度加快,从而使得二极管阻抗减小,最后几次输出的电子束的电流较大,而电压减小;当重复频率较高时,由于加热效应使得阴极等离子体膨胀速度加快,最后几个脉冲阴极发射能力增强,波形重复性变差;当引导磁场强度增大时,阴极等离子体受到较大的磁场力约束,横向膨胀速度减慢,从而使得电子发射面积减小,总发射电流减小,二极管的阻抗增大。最后取引导磁场为1.5 T,阴极环厚为1 mm,得到重复频率100 Hz、束压827 kV、束流8.22 kA、脉冲波形之间重复性很好的均匀电子束输出。     

扩束准直光学系统中光学元件失调对高斯光束传输变换的影响分析

基于广义惠更斯-菲涅尔衍射积分公式, 以高斯光束为激光束模型,推导了激光光束通过失调扩束准直光学系统的传输公式,分析了光学元件失调对扩束准直光学系统输出光束传输特性的影响,并在此基础上进行了仿真。实验结果表明,高斯光束通过失调扩束准直光学系统时,出射光束变为偏心高斯光束,光学元件失调程度越大,输出光束越偏离光轴,光束质量越差。在同样的失调下,长焦距光学元件对输出光束影响更大,因此在激光扩束准直光学系统中,调整长焦距光学元件更为重要。     

The elliptical micro-lens array in the application of the LDA beam shaping

Based on the analysis of semiconductor laser-diode array (LDA) beam characteristics, a new scheme of beam collimation and focusing is proposed. The elliptical micro-lens shaping the beam of LDA has been analyzed by theoretical and simulated by optical ray tracing. The efficiency for laser beam coupling into the optical fiber is 73.2% of the whole system.     

半导体激光器堆栈填充因子对慢轴光束准直的影响

为实现基于微透镜阵列的高功率半导体激光器堆栈光束整形,对带有快轴准直透镜的高功率半导体激光器堆栈慢轴光束准直技术进行研究。在慢轴光束准直理论分析基础上,着重研究了慢轴填充因子对其光束准直的影响,并对不同填充因子的半导体激光器慢轴光束准直方案进行了分析。针对实际使用的填充因子0.5的高功率半导体激光器堆栈采用以Bar条为单元进行整体准直设计,并采用基于空间扫描法的发散角测试装置对慢轴准直后剩余发散角进行测试,实现准直后剩余发散角半角2.12,实验表明该准直方法的有效性。     

X‐ray collimation by crystals with precise parabolic holes based on diffractive–refractive optics

Two crystals with precise parabolic holes were used to demonstrate sagittal beam collimation by means of a diffractive–refractive double‐crystal monochromator. A new approach is introduced and beam collimation is demonstrated. Two Si(333) crystals with an asymmetry angle of α = 15° were prepared and arranged in a dispersive position (+,?,?,+). Based on theoretical calculations, this double‐crystal set‐up should provide tunable beam collimation within an energy range of 6.3–18.8 keV (Θ_B = 71–18.4°). An experiment study was performed on BM05 at ESRF. Using 8.97 keV energy, the beam profile at various distances was measured. The experimental results are in good agreement with theoretical predictions. Owing to insufficient harmonic suppression, the collimated (333) beam was overlapped by horizontally diverging (444) and (555) beams.     

Dynamical chaos and stochastic mechanism of high-energy negatively charged particle deflection by bent crystals

Deflection of high-energy negatively charged particles in straight and bent crystals through multiple scattering by crystal atomic strings was considered for the case in which the initial angle between the particle momentum and one of the main crystallographic axes was approximately four critical angles of axial channeling. It was shown that in a bent crystal with a small crystal thickness, when the crystal bend was less than the beam incidence angle, the beam deflected in the direction opposite to the direction of the crystal bend. At larger crystal thicknesses, the large part of the beam starts to deflect in the direction of the crystal bend. In addition, there is a group of particles that follow the crystal axis bend in the angular region of approximately the critical angle of axial channeling with respect to the current direction of the crystal axis. It was shown that in all of these deflection processes, the periodicity of the location of atomic strings in the crystal does not influence the angular distributions of scattered particles. This fact is connected with the effect of dynamical chaos in particle motion in the periodical field of bent crystal atomic strings. It was also shown that observed in a recent CERN experiment effect of beam deflection, when the angle between the initial particle momentum and the crystal axis was approximately four critical angles of axial channeling, is due to peculiarities of the stochastic multiple scattering of particles by bent crystal atomic strings. These peculiarities are connected with the effect of dynamical chaos in particle motion in crystals.     

Study on collimation and shielding of the back-streaming neutrons at the CSNS target

The back-streaming neutrons from the spallation target at CSNS are very intense, and can pose serious damage problems for the devices in the accelerator-target interface region. To tackle the problems, a possible scheme for this region was studied, namely a specially designed optics for the proton beam line produces two beam waists, and two collimators are placed at the two waist positions to maximize the collimation effect of the back-streaming neutrons. Detailed Monte Carlo simulations with the beams in the two different CSNS phases show the effectiveness of the collimation system, and the radiation dose rate decreases largely in the interface section. This can ensure the use of epoxy coils for the last magnets and other devices in the beam transport line with reasonable lifetimes, e.g., thirty years. The design philosophy for such an accelerator-target interface region can also be applicable to other high-power proton beam applications.     

Fission of Pb nuclei induced by 0.5, 1.0, 1.5, 3.7 and 7.4 GeV protons in the volume of massive U/Pb and Pb targets

Experiments with relativistic protons accelerated at the Synchrophasotron LHE, Dubna, with energies of 0.5, 1.0, 1.5, 3.7 and 7.5 GeV hitting massive targets of (nat. U)/Pb and Pb were carried out using SSNTD during the years 1996–1999. The beam profiles and intensities of both primary particles and fast secondary neutrons were measured inside the massive cylinder blocks of Pb and U by counting fission fragment tracks due to the induced fission of Pb nuclei. The beam diameter typically increases by 20–30% at the depth 10 and 20 cm. With increasing the energy of protons the number of secondary neutrons increases with the depth of the target.

Further studies on beam profile measurements inside the massive heavy metal targets are discussed.     

Analysis of the calibration results obtained with Liulin-4J spectrometer-dosimeter on protons and heavy ions

We are developing a portable dosimeter (Liulin-4J) based on a silicon semiconductor detector for use in measuring the absorbed dose from primary or secondary cosmic rays to astronauts and airplane crews. The dosimeter can measure not only the flux and dose rate, but also the deposited energy spectrum for silicon in per unit time. In order to calibrate the dosimeter, we have carried out exposures at the NIRS cyclotron and HIMAC heavy ion synchrotron facilities. We obtained a detector response function for using in measuring energy deposition and LET.     

Ranges and profiles of distribution of low-energy ions channeling in metal and semiconductor single crystals

In the present work peculiarities of trajectories and energy losses, ranges and profiles of distribution of low-energy different-mass ions channeling in thin single crystals of metals and semiconductors have been thoroughly studied by computer simulation in binary collision approximation. The character of oscillations of channeled-ion trajectories depending on their energies, aiming points from the axis of a channel, kind of interaction potential, crystal lattice type and temperature has been determined. It has been found that, in the case of light ions even at low energy, the main contribution to energy loss is made by inelastic energy losses, whereas for heavy ions, already at E < 10 keV, elastic energy losses exceed inelastic ones. Profiles of the distribution of channeled ions have been calculated depending on crystal lattice type, kind of ions and their energy.     

Optimization of the collimation system for CSNS/RCS with the robust conjugate direction search algorithm

The Robust Conjugate Direction Search(RCDS) method is used to optimize the collimation system for the Rapid Cycling Synchrotron(RCS) of the China Spallation Neutron Source(CSNS). The parameters of secondary collimators are optimized for a better performance of the collimation system. To improve the efficiency of the optimization, the Objective Ring Beam Injection and Tracking(ORBIT) parallel module combined with MATLAB parallel computing is used, which can run multiple ORBIT instances simultaneously. This study presents a way to find an optimal parameter combination of the secondary collimators for a machine model in preparation for CSNS/RCS commissioning.     

Use of a bent crystal with a decreasing curvature to increase the efficiency of the extraction and collimation of a beam in an accelerator

It has been shown that the use of a bent crystal with a variable curvature radius makes it possible to reduce the fraction of dechanneled particles by an order of magnitude. This effect enables the strong reduction of the particle density at the edge of a collimator or the partition of a septum upon the multi-turn extraction of a beam from a ring accelerator. In particular, the beam extraction efficiency at the U-70 synchrotron and Large Hadron Collider can be increased to 95 and 99.65%, respectively.     

The effect of the rotating double-prism wide-angle laser beam scanner on the beam shape

In this paper, we describe a wide-angle laser beam scanner and the rigorous result of the wide-angle laser beam scanner was obtained with the help of the vector refraction theory. Using the rigorous results, the distortion of the beam shape was discussed. The distortion of the beam shape is varying with the different relative angles of the double prisms. According to the conservation of the energy, the distribution of the laser intensity is changed too.     

准直系统热光学效应对静态傅里叶变换红外光谱仪光谱复原的影响研究

在以多级微反射镜为核心器件的静态傅里叶变换红外光谱仪中, 由于准直系统距离红外光源较近, 光源的热辐射会导致其局部温度升高, 从而引起材料折射率发生改变, 使得由准直系统出射的光束存在一定的发散角, 进而影响光谱仪系统复原光谱所能达到的分辨率水平. 本文研究了光谱仪系统正常工作状态下准直系统各区域的温度分布情况, 由此计算出了相应的离焦量. 通过计算准直光束发散角在光程差采样区域内的分布, 分析了由此引入的附加光程差对光谱复原的影响. 通过计算光谱结构误差随准直系统温度的变化, 得到了准直系统温度控制的合理范围. 最后, 对基于SiC光源的光谱仪进行了实验, 结果显示制冷光源复原光谱的光谱结构误差与非制冷光源的光谱结构误差相比有明显改善. 因此, 降低光源温度对减小准直系统热光学效应的影响是非常有效的. 本文的研究结果将为解决同类问题提供参考.     

Study on the reflectivity properties of spherically bent analyser crystals

Theoretical and experimental studies are presented on properties of spherically bent analyser crystals for high‐resolution X‐ray spectrometry. A correction to the bent‐crystal strain field owing to its finite surface area is derived. The results are used to explain the reflectivity curves and anisotropic properties of Si(660) and Si(553) analysers in near‐backscattering geometry. The results from the calculation agree very well with experimental results obtained using an inelastic X‐ray scattering synchrotron beamline.     

Numerical investigation of beam quality in unstable optical resonators and the effects of practical imperfections

Field characteristics and beam quality of confocal positive branch unstable resonator under some practical imperfections has been investigated. Based on Fresnel-Kirchhoff integral, the near and far field intensity of the resonator was calculated numerically. Using the second order moment method, the beam quality factor, M² of the resonator was also calculated. Variety of practical distorting affects, mainly misalignment, off-axis, and thermal deflection of the mirrors, as well as gain non-uniformity were considered in these calculations. The results show that the beam quality factor is more sensitive to the off-axis and misalignment distortions while the non-uniform gain has significant effect only for unusual conditions. The effects of geometrical parameters, i.e. Fresnel number as well as magnification factor on the beam quality were also investigated.     

Theoretical analysis of non-polarizing beam splitters with appropriate amplitude and phase

When used at oblique angles of incidence, the reflectance and transmittance of thin films exhibit strong polarization effects, particularly for the films inside a glass cube. However, the polarization effects are undesirable in many applications. Novel non-polarizing beam splitter designs are shown. Non-polarizing beam splitters with unique optical thin films are achieved through the combination of interference and frustrated total internal reflection. The non-polarizing condition expressions based on frustrated total internal reflection are derived, and examples of the non-polarizing beam splitters are also presented with the optimization technique and the results of R_p=(50±0.5)%, R_s=(50±0.5)%, and Δ_r=(0±0.3) in the wavelength range of 400-700 nm are obtained.