A Monte Carlo study of surface sputtering by dual and rotated ion beams

Several, recently proposed methods of surface manufacturing based on ion beam sputtering, which involve dual beam setups, sequential application of ion beams from different directions, or sample rotation, are studied with the method of kinetic Monte Carlo simulation of ion-beam erosion and surface diffusion. In this work, we only consider erosion dominated situations. The results are discussed by comparing them to a number of theoretical propositions and to experimental findings. Two ion-beams aligned opposite to each other produce stationary, symmetric ripples. Two ion beams crossing at right angle will produce square patterns only, if they are exactly balanced. In all other cases of crossed beams, ripple patterns are created, and their orientations are shown to be predictable from linear continuum theory. In sequential ion-beam sputtering we find a very rapid destruction of structures created from the previous beam direction after a rotation step, which leads to a transient decrease of overall roughness. Superpositions of patterns from several rotation steps are difficult to obtain, as they exist only in very short time windows. In setups with a single beam directed towards a rotating sample, we find a non-monotonic dependence of roughness on rotation frequency, with a very pronounced minimum appearing at the frequency scale set by the relaxation of prestructures observed in sequential ion-beam setups. Furthermore we find that the logarithm of the height of structures decreases proportional to the inverse frequency.     

Nonlinear dynamic analysis of reinforced concrete structures

Dynamic ultimate load calculations mainly for reinforced concrete beams and plates, are discussed. Starting from the corresponding differential equations, the calculations also include the rotational inertia of single beam or plate elements as well as the shear deformations. With actual structural dynamic problems in nuclear power plants, the shear behaviour of reinforced concrete beams and plates is more important than it is usually, as is shown by examples. The finite propagation velocity of bending and shear waves are taken into account. Solution of the equations of motion is obtained by numerical intergration using finite time and space intervals. The calculations are performed using time dependent bending and shear laws for reinforced concrete up to the point of failure with realistic deformations. These latest scientific developments are of great significance for dynamic ultimate load analysis in practice.Elastic-plastic examples of application are compared with corresponding linear-elastic solutions. It is shown that the design of construction members based on elastic-plastic dynamic stress calculations in general is economically advantageous. This important conclusion is proven by numerical results. Also the relation to the approximation of a one-degree-of-freedom dynamic system, including or excluding the plastic ductility of the structural member, is demonstrated.Finally, lumped-mass multi-degree systems calculated by integrating numerically the corresponding equations of motion, are dealt with briefly. A nonlinear dynamic calculation of a foundation of a recently built reactor building is presented as an example for blast resistant analysis.     

On stress-relaxation in bending: For example results applied to Zircaloy-4

The meaning of the different parameters obtained from stress-relaxation measurements in bending is discussed. It is shown that the measured stresses are an average value of the stress distribution in the material. A procedure is given to calculate the stress in the surface of the beam independently from the creep law and the possibility of obtaining the creep constants from the stress-relaxation data is discussed. Finally, the results are applied to some stress-relaxation measurements performed in Zircaloy-4 at 673 K with cold-worked and stress-relieved specimens.     

New Method of Large-Area DC Ion Beam Formation with Multiple Electron Beams, (II)

A new method to form a small divergence dc ion beam using a large-area single-hole electrode instead of a multiaperture electrode is described. A distinctive feature of this method is to make use of multiple electron beams in an ion source to control the spatial ion density distribution at the region of ion beam formation; the electron beams play a role to aid formation of the concave plasma boundary, which is required to extract a convergent ion beam from gaseous plasma according to the experimental facts described in Part (I). In this experiment, it was verified with three slab-like electron beams that formation of the concave boundary was possible in a stable and steady state. This electrode system can be applied to some types of the present high current ion sources to obtain large-area dc ion beams.     

Electromagnetic dispersion characteristics of a high energy electron beam guided with an ion channel

Taking self-fields into consideration,dispersion properties of two types of electromagnetic modes for a high energy electron beam guided with an ion channel are investigated by using the linear perturbation theory.The dependences of the dispersion frequencies of electromagnetic waves on the electron beam radius,betatron frequency and boundary current are revealed.It is found that the electron beam radius and betatron frequency have different influences on the electromagnetic waves dispersion behavior by compared with the previous works.As the boundary current is taken into account,the TM modes will have two branches and a lowfrequency branch emerged as the new branch in strong ion channel case.This new branch has similar dispersion behavior to the betatron modes.For TE modes,there are two branches and they have different dispersion behaviors in strong ion channel case.However,in weak ion channel case,the dispersion behaviors for both of the low frequency and high frequency branches are similar.     

Seismic behaviour and design of steel coupling beams in a hybrid coupled shear wall systems

The hybrid coupled shear wall system is more efficient and economical than individual structural walls because the steel coupling beams connected shear walls significantly increase the strength, stiffness and energy dissipation capacity of the system. In this study, experimental studies on the steel coupling beam were carried out. The main test variables were the ratios of the coupling beam strength to the connection strength. In addition, the seismic design methods are presented for steel coupling beam–wall connection and shear critical and flexure critical steel coupling beams in hybrid coupled shear wall system consisting of steel coupling beams and reinforced concrete shear walls. Finally, this paper provides background for design guidelines in hybrid coupled shear walls that include steel coupling beam–wall connections and steel coupling beams.     

Elasto-plastic behavior of pipe subjected to steady axial load and cyclic bending

The elasto-plastic behavior of a pipe subjected to a steady axial force and a cyclic bending moment is studied. By using two parameters c and d, which describe the elasto-plastic interfaces of beam cross-section, the boundary curve equations between various types of elasto-plastic behavior, such as shakedown, plastic fatigue, ratcheting, and plastic collapse, are derived. The results are applicable for beams of any cross-section with two orthogonal axes of symmetry. As a result, the load regime diagram for a pipe is obtained, which gives an intuitive picture of the elasto-plastic behavior of the pipe under a given combination of constant axial load and cyclic bending moment.     

尼尔逊离子源重离子束流的发射度

尼尔逊离子源已广泛地应用于重离子加速器、同位素分离器及研究用离子注入机。用于C-600离子注入机的尼尔逊离子源,目前已获得达100μA的三十余种重离子束,其中有近二十种固体元素。     

Dynamics and generation of stress waves in cracked graphite moderator bricks

In the present study, static and dynamic analyses have been performed for graphite moderator bricks with and without key grooves under the different initial loading conditions of pure bending and slot rotation by using the finite element method. The possibilities of the occurrence of multiple cracking due to internal stresses have been investigated in terms of the (1) initial loading condition; (2) vibration mode shapes; and (3) damping of the bricks. Firstly, modal dynamic analysis has been carried out, and the effect of the initial loading conditions, such as pure bending and slot rotation, on the dynamic stress responses was investigated. It was found that stress enhancement was more significant under the condition of pure bending than that of slot rotation. For pure bending, the key groove which was opposite to the primary cracking site had the maximum dynamic circumferential stress. This implied that secondary cracking might occur at this position under pure bending. Secondly, in order to investigate the dominant mode shape for the stress enhancement, natural frequency analysis has been conducted. Fast Fourier Transformation (FFT) has also been performed to convert time–domain stress responses into frequency–domain stress responses. The initial loading conditions determined which mode shape was dominant for the stress enhancement. It was also suggested that a fundamental mode shape principally contributed to the occurrence of the multiple cracking. Thirdly, modal damping analysis has been performed, and the effect of damping on the stress enhancement was studied. It was found that damping had little effect on the initial peak of the dynamic stress response. This suggested that secondary cracking might still occur at the key groove opposite the primary cracking site, even if damping was present. The results of the analysis give a fundamental insight into the mechanism that may lead to multiple cracking in graphite components subject to internal stresses. However, the effects of 3-D, crack propagation speed and external restraint have not been taken into consideration in the present study. These effects may mitigate the possibility of secondary cracking, and are parts of further investigation.     

Preliminary Research Results for the Generation and Diagnostics of High Power Ion Beams on FLASH II Accelerator

The preliminary experimental results of the generation and diagnostics of highpower ion beams on FLASH II accelerator are reported, The high-power ion beams presently are being produced in a pinched diode, The method for enhancing the ratio of ion to electron current is to increase the electron residing time by pinching the electron flow, Furthermore, electron beam pinching can be combined with electron reflexing to achieve ion beams with even higher efficiency and intensity. The anode plasma is generated by anode foil bombarded with electron and anode foil surface flashover. In recent experiments on FLASH II accelerator, ion beams have been produced with a current of 160 kA and an energy of 500 keV corresponding to an ion beam peak power of about 80 GW. The ion number and current of high power ion beams were determined by monitoring delayed radioactivity from nuclear reactions induced in a ^12C target by the proton beams, The prompt γ-rays and diode bremsstrahlung X-rays were measured with a PIN semi-conductor detector and a plastic scintillator detector, The current density distribution of ion beam were measured with a biased ion collector array. The ion beams were also recorded with a CR-39 detector.     

Nano-scale quasi-melting of alkali-borosilicate glasses under electron irradiation

Quasi-melting of micro- and nano-samples during transmission electron microscope irradiation of glassy materials is analysed. Overheating and true melting by the electron beam is shown not to be an explanation due to the ultra-sharp boundary between transformed and intact material. We propose that the observed fluidisation (quasi-melting) of glasses can be caused by effective bond breaking processes induced by the energetic electrons in the electron beam. The bond breaking processes modify the effective viscosity of glasses to a low activation energy regime. The higher the electron flux density the lower is the viscosity. Quasi-melting of glasses at high enough electron flux densities can result in shape modification of nano-sized particles including formation of perfect beads due to surface tension. Accompanying effects, such as bubble formation and foil bending are revisited in the light of the new interpretation.     

Simulation studies on two-frequency RF gun

Photocathode RF gun is widely used for particle accelerators as an electron source.When driving an RF electron gun at the fundamental frequency and a higher harmonic frequency simultaneously with proper field ratio and relative phase,it generates electron beams of ultralow emittance and a linear longitudinal phase space distribution.Such a gun provides high quality electron beam with low energy spread,small traverse emittance and high brightness.In this paper,the RF design of a 1.5 cell cavity is presented.Simulation results of beam dynamics for the two-frequency gun and a standard single-frequency RF gun are also shown in this paper.In addition,bunch compression with a two-frequency gun is explored.     

Fluid-structure interaction in non-rigid pipeline systems

Fluid-structure interaction in non-rigid pipeline systems is modelled by water hammer theory for the fluid coupled with beam theory for the pipe. Two different beam theories and two different solution methods in the time domain are studied and compared. In the first method, the fluid equations are solved by the method of characteristics and the pipe equations are solved by the finite element method in combination with a direct time integration scheme. In the second method, all basic equations (fluid and pipe) are solved by the method of characteristics. The solution methods are applied to a straight pipeline system subjected to axial and lateral impact loads and to a one-elbow pipeline system subjected to a rapid valve closure. In comparing the beam theories, the effects of rotatory inertia and shear deformation for practical pipe geometries and loading conditions are investigated. The significance of fluid-structure interaction is demonstrated. The fluid-structure interaction computer code , developed by and which solves the acoustic equations using the method of characteristics (fluid) and the finite element method (structure), enables the user to determine dynamic fluid pressures, structural stresses and displacements in a liquid filled pipeline system under transient conditions. To validate , experiments are performed in a large scale 3D test facility consisting of a steel pipeline system suspended by wires. Pressure surges are generated by a fast acting shut-off valve. Dynamic pressures, structural displacements and strains (in total 70 signals) are measured under well determined initial and boundary conditions. The experiments are simulated with . The agreement between experiments and simulations is shown to be good: frequencies, amplitudes and wave phenomena are well predicted by the numerical simulations. It is demonstrated that the results obtained from a classical uncoupled water hammer computation, would render completely unreliable and useless results.     

Extending Bragg peak of heavy ion beam and melanoma cell inactivation measurement

A rotating range modulator was designed and manufactured.which is applied to extend Bragg peak of heavy ion beam.Bragg curves of 75MeV/u ^16O and 75MeV/u ^12C ion beams through this range modulator were measured respectively and two evident spread-out Bragg peaks corresponding to the modulated beams above are shown.In addition,inactivation effect of the modulated 75MeV/u ^16O ion beam at nine different penetration depths on melanoma cells(B16) was measured.Results indicate that lethal effects at the spread-out Bragg peak region are larger than at the plateau of the particle beam entrance.     

HIRFL-CSR频谱现象:中心频率突变分析研究

HIRFL-CSR调试过程中,利用频谱分析仪实时监测束流,根据其上测的数据进行相关参数调整,改善束流的品质.本文对分析仪上观测到的现象一束流的中心频率发生突变进行了研究,主要就电子束的高压不稳定性和中性化因子的突变做了理论分析,并进行了相关的模拟计算,根据模拟结果初步断定该现象的出现由电子束的中性化因子导致.     

Effects of Boundary Current on Electromagnetic Dispersion Characteristics for a Relativistic Electron Beam

With betatron oscillation characteristics of the electron beam and ion channel effect taken into account,dispersion characteristics of electrostatic modes and TM modes for a relativistic electron beam guided by ion channel are studied.Dispersion relations are derived and solved numerically to investigate the dependence of the dispersion characteristics for electrostatic modes and TM modes on the betatron oscillation frequency and the ratio of the relativistic electron beam radius to the waveguide radius.The effects of the boundary current on the dispersion characteristic of the TM modes and the interaction between the betatron modes and TM modes are analyzed.When considering the boundary current,for a strong ion channel,a new low-frequency branch of the TM modes arises and the interaction frequency between the betatron modes and the TM01 modes is increased with the same parameters.     

Transmission characteristics of terahertz Bessel vortex beams through a multi-layered anisotropic magnetized plasma slab

The transmission of terahertz (THz) Bessel vortex beams through a multi-layered anisotropic magnetized plasma slab is investigated by using a hybrid method of cylindrical vector wave functions (CVWFs) and Fourier transform. On the basis of the electromagnetic boundary conditions on each interface, a cascade form of expansion coefficients of the reflected and transmitted fields is obtained. Taking a double Gaussian distribution of the plasma density as an example, the influences of the applied magnetic field, the incident angle and polarization mode of the incident beams on the magnitude, OAM mode and polarization of the transmitted beams are analyzed in detail. The results indicate that the applied magnetic field has a major effect upon the polarization state of the transmitted fields but not upon the transmitted OAM spectrum. The incident angle has a powerful influence upon both the amplitude profile and the OAM spectrum of the transmitted beam. Furthermore, for multiple coaxial vortex beams, an increase of the maximum value of the plasma density causes more remarkable distortion of both the profile and OAM spectrum of the transmitted beam. This research makes a stable foundation for the THz OAM multiplexing/demultiplexing technology in a magnetized plasma environment.     

Deflection of high energy particles during multiple scattering by atomic strings of a bent crystal

Simulation results of high energy particle passage through thick bent crystals near the crystal axis are reported. The bending of positively and negatively charged particles by a bent crystal is shown to occur due to multiple scattering by the atomic strings. It is established that bending is possible for regular as well as chaotic motion of particles in the periodic field of atomic strings of a crystal. The beam is shown to split into several beams separated in angle.     

放射性核束在生物医学中的应用

不同衰变方式和半衰期的放射性核束在生物医学领域有不同应用。本文评述了自20世纪70年代中期美国LBL实验室首次产生高能放射性核束以来，世界范围内利用放射性核束开展的生物医学应用研究情况，并提出了一些可在兰州放射性束流线（RIBLL）和将要建成的兰州重离子加速器冷却储存环（HIRFL－CSR）装置上进行的生物医学应用研究的设想。     

BRISOL钾同位素放射性核束的产生

北京放射性核束装置在线同位素分离器（BRISOL）采用一台100 MeV回旋加速器提供的最大200 μA的质子束打靶在线产生放射性核束,其最高质量分辨率好于20 000。2015年,BRISOL装置建成并使用05 μA质子束轰击氧化钙靶产生了37K+、38K+放射性核束,其中38K+的产额为1×106 pps。为了提高氧化钙靶产生钾放射性核束的产额以满足物理用户需求,BRISOL于近期开展了氧化钙靶的在线实验。实验中使用氧化钙靶产生了36～38K+、43K+、45～47K+等多种放射性核束,同时将38K+的最大产额提高到了112×1010 pps。本文详细介绍氧化钙靶的研制及在线实验结果。