BEPC中电子云导致的束团横向尺寸增长研究

多束团正电子储存环中可能发生电子云不稳定性. 由于电子云导致的束团横向尺寸增长已经成为提高对撞机对撞亮度的主要限制因素之一. 介绍了在BEPC储存环中, 利用条纹相机直接测量由于电子云导致的束团横向尺寸增长结果, 并与模拟计算进行了比较.     

Head-tail instability caused by electron clouds in positron storage rings

In positron or proton storage rings with many closely spaced bunches, an electron cloud can build up in the vacuum chamber due to photoemission or secondary emission. We discuss the possibility of a single-bunch two-stream instability driven by this electron cloud. Depending on the strength of the beam-electron interaction, the chromaticity and the synchrotron oscillation frequency, this instability either resembles a linac beam breakup or a head-tail instability. We present computer simulations of the instabilities, and compare the simulation results with analytical estimates.     

The fluid theory of steady-state magnetically confined electron clouds sustained by ionization and emission

Summary The fluid theory is applied to study the axisymmetrical steady-state magnetically confined electron clouds sustained by ionization or emission. These electron clouds can be obtained by means of low-pressure Penning discharge, thermoelectronic emission, ion beam ionization, etc. In the electron clouds the property of motion of electrons can be described by the fluid equations: the continuity equation, the momentum equation, the energy equation, the heat transfer equation and the electrostatic field equation. These equations are used to discuss the equilibrium between supplement and escape of electrons in the clouds and the distributions of the physical quantities of some long electron clouds. The authors of this paper have agreed to not receive the proofs for correction. The work is supported by National Natural Science Foundation of China.     

Secondary electron yield from stainless steel surface coated with titanium nitride

The experiment on measurement of secondary electron yield from surface of a stainless steel Kh189 sample covered with titanium nitride is performed at stand “Recuperator”. This work is related to known problem of electron clouds formation in a vacuum chamber by a propagating charge particle beam. An original method of secondary electron yield measurement was developed in this experiment. The obtained results allow one to estimate efficiency of coating nitride titanium.     

Stochastic particle acceleration in multiple magnetic islands during reconnection

A nonthermal particle acceleration mechanism involving the interaction of a charged particle with multiple magnetic islands is proposed. The original Fermi acceleration model, which assumes randomly distributed magnetic clouds moving at random velocity V(c) in the interstellar medium, is known to be of second-order acceleration of O(V(c)/c)(2) owing to the combination of head-on and head-tail collisions. In this Letter, we reconsider the original Fermi model by introducing multiple magnetic islands during reconnection instead of magnetic clouds. We discuss that the energetic particles have a tendency to be distributed outside the magnetic islands, and they mainly interact with reconnection outflow jets. As a result, the acceleration efficiency becomes first order of O(V(A)/c), where V(A) and c are the Alfvén velocity and the speed of light, respectively.     

高斯光束在双层云中传输的蒙特卡罗模拟

基于辐射传输理论, 利用蒙特卡罗方法模拟了无限窄(冲击函数)准直光束入射到典型水云以及冰水双层云时的后向散射特性, 进而将得到的冲击响应与高斯光束卷积, 得到高斯光束在云层中传输的多次散射特性. 文中给出了两种波束入射时水云以及冰水双层云的反射函数随径向r和天顶角α的变化关系, 并给出了光强在云层内部的二维分布图. 计算结果表明, 高斯光束入射时, 云层反射函数的特点与无限窄准直光束入射时有较大区别. 因此在利用激光雷达进行云层探测时需要考虑激光的散斑, 文中的方法可以为此提供理论依据.     

Electron Cloud Instability Simulation Studies for BEPCⅡ

Electron cloud instability (ECI) may take place in a storage ring when the machine is operated with multi-bunch positively charged beam. According to the actual shape of vacuum chamber in BEPCⅡ, a computer simulation program has been developed. With the code, the electron density in the chamber with the different widths of antechamber and the different secondary electron yields, respectively, can be obtained. The possibility to put clearing electrodes in the normal chamber or ante-chamber to reduce the electron density in the central region of the chamber is also investigated. In the simulation, the coherent oscillation of bunches and the formation of electron cloud (EC) are tracked simultaneously. The sideband distribution and growth rate can be obtained through tracking and dealing the bunch oscillation amplitude with FFT. Based on the head-tail model, the single bunch instability induced by EC is also studied. By the simulation, for BEPCⅡ the threshold density for the single bunch instability is estimated to be～10 12 m -3 and the restraining measures that might be adopted in BEPCⅡ can control the EC density under the threshold. According to the simulation results, a relatively big positive chromaticity should be used to suppress the bunch blow up.     

Estimation of electron-cloud effect on NICA collider

The preliminary results from simulating the formation of electron clouds in the NICA collider using the ECLOUD program package are presented. Requirements for the acceleration chamber of the NICA collider for a gold nuclei beam are discussed.     

Review of collective beam instabilities in electron-positron storage rings

The review of studies of collective beam instabilities in electron-positron storage rings is presented. The processes of excitation and suppression of the longitudinal microwave instability, transverse mode coupling instability, longitudinal and transverse multi-bunch instabilities, and instabilities induced by an interaction of a beam with ions or electron clouds are discussed. Important equations for estimation of the threshold beam currents and the rise time of instabilities, as well as the references to the major original works are given. The methods for diagnostics and suppression of instabilities are considered using specific examples.     

A Direct Convertor Based upon Space Charge Effects

A device capable of converting directly the kinetic energy of charged particles into electrical energy is considered. The device differs from earlier ones (such as Post's periodic focus electrostatic direct convertor) in that it makes use of the space charge repulsion in a high density charged particle beam. The beam is directed into a monotonic decelerating electrostatic field of a several stage planar finned structure. The collector fins coincide with vacuum equipotential surfaces. Space charge blowup of the beam directs particles onto various collector fins. The energy efficiency of a 4-stage device has been determined using a numerical simulation approach. We find that efficiencies approaching 75 percent are possible. An approximate scaling law is derived for the space charge based direct convertor and a comparison is made to the periodic focus direct convertor. We find the space charge based direct convertor to be superior in a number of ways.     

强激光与等离子体相互作用中沿靶表面发射的高能超热电子

在强激光与等离子体相互作用研究中，文章作者从实验上首次观测到沿靶面方向发射的高能超热电子束．该电子束只有在等离子体电子密度标长较短的条件下才会出现。数值模拟表明，靶表面电磁场的约束作用是产生这束电子的主要原因。这一结果有助于加深对激光惯性约束聚变快点火实验中的锥靶物理过程的理解，并有潜在的应用前景。     

HIRFL-CSR电子冷却系统空心电子束性能研究

 电子冷却系统中冷却力的大小与电子束的温度密切相关。由于强流电子束自身产生的空间电荷场,使得电子束的速度离散,增加了电子束温度,降低了冷却效率。为了减小空间电荷效应,HIRFL CSR的电子冷却系统将首次采用空心电子束对储存环中的重离子束流进行冷却。通过分析实心电子束和空心电子束的空间电荷场,研究了其对电子束速度和温度的影响。     

Chaotization of the virtual cathode oscillations in the external magnetic field created by a ring magnet

The physical mechanisms leading to the chaotization of the virtual cathode oscillations in a low-voltage vircator system at an increase in the amplitude of the external inhomogeneous magnetic field created by a ring magnet have been studied within a two-dimensional numerical model. It has been established that the chaotization of the virtual cathode oscillations in a strongly inhomogeneous external magnetic field is due to the formation of the secondary electronic structure (electron beam) in the electron flow resulting from the magnetic trap in the outer layers of the electron beam.     

Optical cavitation probe using light scattering from bubble clouds

To understand the behaviour of systems containing clouds of bubbles (multibubble system) in real sonochemical reactors, a new diagnosis method, i.e., optical cavitation probe (OCP), has been proposed. When a laser beam is introduced into the cavitation bubble cloud, the scattered light intensity changes by the collective oscillation of cavitation bubbles. The frequency domain spectrum of the scattered light contains rich information on the cavitation bubble clouds, comparable with the acoustic emission spectra detected by a hydrophone. The significant merits of OCP, such as capability for spatially resolved, non-invasive measurement of the cavitation bubble clouds, robustness even in a violent cavitation field have been experimentally demonstrated.     

有限引导磁场下相对论环形电子注色散特性的研究

采用等效媒质处理方法来研究有限引导磁场下沿纵向运动的相对论环形电子注.首先建立运动坐标系以电子注纵向速度匀速运动,在运动坐标系中电子注可以被考虑成静止的磁化等离子体,再通过四维空间的洛伦兹变换得到电子注在静止的实验室坐标系下可以被等效为双各向异性媒质,其不仅具有张量形式的电导率和磁导率,还具有手征特性.在此基础上同时考虑了由于电子注表面波动所引起的表面电流密度.采用该方法研究了有限引导磁场下圆柱波导中沿纵向运动的相对论环形电子注,推导出该模型的色散方程,并进行了数值计算.计算结果表明该研究方法能够得到更准 关键词： 相对论环形电子注 磁化等离子体 色散特性     

Structure formation and tearing of an MeV cylindrical electron beam in a laser-produced plasma

The stability of a cylindrical, solid hot electron beam propagating in a high density plasma has been studied using a two-dimensional, hybrid Darwin code. The initially solid beam evolves into a hollow, annular beam due to the Weibel instability and generates strong magnetic fields on both sides of the annular ring. The annular structure subsequently breaks up into several beamlets via a mechanism similar to a tearing instability. It is found that the magnetic fields parallel to the direction of beam propagation also grow due to the tearing process.     

The Energy Spectrum of the Electron Beam of a Betatron

The process of extraction of an electron beam through the glass wall of a sealed-off acceleration chamber has been investigated. It has been shown that the electron beam extracted from the MIB-6E small-sized betatron has a nonuniform cross-sectional distribution of the particle flux density. This nonuniformity is due to the imperfection of the beam extraction devices and to the flaws in design of the extraction window of the accelerator chamber. The electrons extracted through the glass wall of the window lose a major portion of their energy. At the outlet of the chamber of the small-sized betatron, the spectrum of the electrons is not line. The half-width of the spectral line of the electron beam is, at best, 6% for a window glass thickness of 0.5 mm and about 15% for a glass thickness of 4 mm. The results of this study may find practical implementation in developing extracted electron beam formation systems.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 51–55, January, 2005.     

Chlorine migration under electron bombardment observed by aes

During heating, a TiAl alloy becomes covered by elemental sulphur and chlorine which remain segregated at the surface after cooling. Auger electron spectroscopy has shown that the chlorine disappears progressively with time at room temperature, while the sulphur is not removed. Experiments have been performed which show that the disappearance of chlorine is due to electron bombardment and that chlorine is expelled only locally from a zone around, and considerably larger than, the area of beam impact. We suggest that chlorine escapes from the impact area by superficial diffusion and not by electron-stimulated desorption. The influence of various parameters, such as the properties of the electron beam and the temperature of the sample, have been studied in an attempt to explain the observed electron beam effect.     

Charge trapping in NiO surfaces during LEED observations

Time dependent LEED intensities have been observed for a NiO(100)-Cl surface. Measurements of this effect as a function of temperature, beam current and beam energy are reported. Associated observations of the current to ground and the electron loss spectrum are used to suggest that electron trapping is occurring in the crystal surface due to the adsorbed Cl. Excitation from these trapped states could result in decaying LEED beam intensities.