Design and simulation of the wire scanner for the injector linac of BEPCⅡ

BEPCⅡ, the upgrade project of Beijing Electron Positron Collider (BEPC), is an accelerator with large beam current and high luminosity, so an efficient and stable injector is required. Several beam diagnostic and monitoring instruments are used. A new diagnostic instrument — wire scanner, has been designed and will be used to measure the profile of the linac beam of BEPCⅡ. This paper describes the prototype of this system and the cause of heat generating of the wire. Some simulation results of the heat and force by using finite element method software—ANSYS^®,2) are presented and discussed.     

Beam experiments with a non-intercepting beam induced f luorescence profile monitor for the ADS LINAC

An accelerator-driven subcritical system(ADS) project was launched in China in 2011, aiming to design and build an ADS demonstration facility with the capability of more than 1000 MW thermal power. The driver linac is defined to be a 10 m A current of high energy protons at 1.5 Ge V in continuous wave operation mode. To meet the extremely high power and intense beam accelerator requirements, non-interceptive monitors for the beam transverse profile are required for this proton linac. Taking advantage of the residual gas as active material, the Beam Induced Fluorescence(BIF) monitor exploits gas-excited fluorescence in the visible spectrum region for transverse profile measurements. The advantages of this non-intercepting method are that nothing is installed in the vacuum pipe,component design is compact and there is no need for expensive signal processing electronics. Beam experiments have been performed under constant beam conditions. The helium spectrum has been verified with different optical filters, showing that a proper optical band-pass filter covering 400–500 nm is necessary for fluorescence experiments with helium. By changing gas pressure, it is shown that gas pressure is proportional to the signal amplitude but has no influence on detected profile width. Finally, a comparison experiment between the BIF monitor and a wire scanner shows that the detected profile width results of both methods agree well.     

A New Low-Impedance High Power Microwave Source

We propose a new low-impedance high power microwave source based on the axial transit time effect, i.e. alow-impedance axial transit-time oscillator. The analysis and simulation are carried out to investigate the char-acteristics of this new configuration. In the numerical simulation, the microwave output peak power of 6 GWis obtained with an electron beam in the electron energy 580keV and in the beam current 20kA. The diodeimpedance is 29Ω The power of the input electron beam is 11.6 GW. The beam-to-microwave power conversionefficiency is 51.7%. The frequency is 3.9 GHz by numerical simulation.     

Beam steering correction of the quarter wave resonator in the HISCL

The Intensity Heavy Ion Superconducting Linear Accelerator as the injector of the High Intensity Heavy-Ion Accelerator Facility, which is a new project proposed in China has been designed. One of the design options in the low energy part is based on Quarter Wave Resonators (QWRs). However, because of the unsymmetrical geometry of the cavity, there are dipole fields near the beam hole, which may steer the beam vertically, thus leading to emittance growth and beam loss. The effect of the dipole mode field is analyzed, and a method to overcome the beam steering effect by placing QWRs with opposite orientation is proposed in this paper. The simulation results show that the beam steering effect is reduced effectively by this method, and the deviation of the beam centroid is decreased from 2.87 mm to 0.1 mm. The emittance growth is also smaller.     

A Novel Nano-Grating Structure of Polarizing Beam Splitters

A metal wire nanograting is fabricated and used as a polarizing beam splitter that reflects TE polarization and transmits TM polarization. The metal wire nanograting is based on a fully optimized design structure that consists of not only the core nanowire metal grid but also the substrate nanograting. The substrate nanograting is designed to provide better performance for both TM and TE polarizations. We fabricate metal-stripe gratings on a glass substrate using nanoimprint lithography and reactive ion etching process. A detailed investigation of the polarization effect at 1550 nm wavelength is carried out with the theoretical analysis and experimental results. The polarizing beam splitter has uniform performance with wide variations in the incident angle （±25） and has high efficiency for both the reflected and the transmitted beams.     

Simulation of plasma doping process by using the localized molecular dynamics method

Plasma doping is the candidate for semiconductor doping. Accurate simulation of the doping technology is needed for the advanced integrated circuit manufacturing. In this paper, the plasma doping process simulation is performed by using the localized molecular dynamics method. Models that involve the statistics of the implanted compositions, angles and energies are developed. The effect of the model on simulation results is studied. The simulation results about the doping concentration profile are supported by experimental data.     

BEPCII丝靶束流截面测量系统

To monitor the beam profile at the end of the linac non-destructively, a wire scanner as a new diagnostic instrument was designed, manufactured and installed in 2007. Since then, several measurements have been carried out using this device. This paper describes the whole system of the wire scanner and the testing results.     

Influence of Monomer Types on the Designability of a Protein—Model Chain

In a three-dimensional off-lattice model,the method of Shakhnovich and Gutin for minimizing the Hamiltonian is applied to the design of a protein-model chain.The effect of the number of hydrophobic and hydrophilic monomer types on the designability of a protein-model chain is investigated.The simulation results reveal that the number of hydrophobic monomer types is a much more important factor than that of the polar monomer types in the design of a protein-model chain.     

Synergistic Effect of Triple Ion Beams on Radiation Damage in CLAM Steel

The synergistic effect of triple ion beams is investigated by simultaneous and scquential irradiations of gold, hydrogen and helium ions on the low activation martensitie steel （CLAM） developed in China. The depth profile measurements of the positron annihilation Doppler broadening S parameter are carried out as a function of slow- positron beam energy to examine the produced radiation damage. The synergistic effect of displacement damage and hydrogen and helium on the formation of radiation damage is clearly observed. In the preset case ,this effect suppresses the radiation damage in the CLAM steel due to the helium and/or hydrogen filling of vacancy clusters.     

Preliminary design and simulation of a 162.5 MHz high-intensity proton RFQ for an accelerator driven system

A new procedure for the design and simulation of a Radio Frequency Quadrupole (RFQ) accelerator has been developed at the Argonne National Laboratory. This procedure is integrated with the beam dynamics design code DESRFQ and the simulation code TRACK, which are based on three-dimensional field calculations and the particle-in-cell mode beam dynamics simulations. This procedure has been applied to the development of a 162.5 MHz CW RFQ which is capable of delivering a 10 mA proton beam for the Accelerator Driven System (ADS) of the CAS. The simulation results show that this RFQ structure is characterized by the stable values of the beam acceleration efficiency for both the zero current beam and space charge dominated beam. For an average beam current of 10 mA, there is no transverse rms emittance growth, the longitudinal rms emittance at the exit of RFQ is low enough and there is no halo formation. The beam accelerated in the RFQ could be accepted easily and smoothly by the following super-conducting linear accelerator.     

Alignment techniques for DRAGON-Ⅰ LIA

DRAGON-I designed and manufactured by CAEP is a linear induction accelerator which can produce a 20 MeV-3 kA-60 ns electron beam. The high performance required for the machine is determined by the beam quality and thus is greatly dependent on the accelerator alignment. In order to reduce the chromatic effect of the beam, the stretched wire technique has been developed to measure magnetic axes of the cells precisely, and the dipole steering magnets have been equipped into each cell to correct its magnetic axis misalignment. Finally, the laser tracker has been used to examine the installation error of the accelerator. In this paper, different alignment techniques and the primary results are presented and discussed.     

Hofstadter＇s Butterfly and Phase Transition of Checkerboard Superconducting Network in a Magnetic Field

We study the magnetic effect of the checkerboard superconducting wire network. Based on the de Gennes- Alexader theory, we obtain difference equations for superconducting order parameter in the wire network. Through solving these difference equations, we obtain the eigenvalues, linked to the coherence length, as a function of magnetic field. The diagram of eigenvalues shows a fractal structure, being so-called Hofstadter＇s butterfly. We also calculate and discuss the dependence of the transition temperature of the checkerboard superconducting wire network on the applied magnetic field, which is related to up-edge of the Hofstadter＇s butterfly spectrum.     

Analysis and experimental concepts of the vibrating wire alignment technique

The vibrating wire alignment technique is a method which, by measuring the spatial distribution of a magnetic field, can achieve very high alignment accuracy. The vibrating wire alignment technique can be applied to fiducializing magnets and the alignment of accelerator straight section components, and it is a necessary supplement to conventional alignment methods. This article gives a systematic summary of the vibrating wire alignment technique, including vibrating wire model analysis, system frequency calculation, wire sag calculation, and the relation between wire amplitude and magnetic induction intensity. On the basis of this analysis, this article outlines two existing alignment methods, one based on magnetic field measurement and the other on amplitude and phase measurements. Finally, some basic experimental issues are discussed.     

A novel solution-based self-assembly approach to preparing ultralong titanyl phthalocyanine sub-micron wires

Ultralong titanyl phthalocyanine （TiOPc） sub-micron wires have been synthesized by a novel solution-based self- assembly method. By using different solvents, changing the mass concentration and the solvent vapor pressure, the length and the shape of the wires can be adjusted. The mixed-phase properties of the TiOPc sub-micron wires were investigated by the ultraviolet-visible （UV-vis） absorption spectrum and X-ray diffraction. Organic transistors based on these wires were studied, which show the typical p-channel characteristics.     

Design and simulation of a wire position monitor for cryogenic systems in an ADS linac

This paper introduces the design and simulation of a Wire Position Monitor （WPM） used in the cryogenic system of an Accelerator Driven System （ADS）. The WPM is designed to monitor the contraction of cold masses during the cooling-down operation. In this paper, POISSON-2D electrostatic field software is used to calculate the best characteristic impedance for the WPM. Furthermore, the time domain signal of different end structures is theoretically analyzed and simulated. The coupling of electrodes and the influence of signal carrier size, which may influence the induced signal, are also discussed. Finally, the linearity of the induced voltage and the sensitivity of the WPM are analyzed. The time domain simulation results are consistent with the theoretical analysis. The influences of the coupling and carrier size are very small, and the linearity of the normalized voltage is good within r/2.     

Stark Effect Dependence on Hydrogenic Impurities in GaAs Parabolic Quantum-Well Wires

The ground-state and lowest excited-state binding energies of a hydrogenic impurity in GaAs parabolic quantum-well wires （Q WWs） subjected to external electric and magnetic fields are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. We define an effective radius Pen of a cylindrical QWW, which can describe the strength of the lateral confinement. For the ground state, the position of the largest probability density of electron in x-y plane is located at a point, while for the lowest excited state, is located on a circularity whose radius is Pen. The point and circularity are pushed along the left haft of the center axis of the quantum-well wire by the electric field dire ted along the right half. When an impurity is located at the point or within the circularity, the ground-state or lowest excited-state binding energies are the largest; when the impurity is apart from the point or circularity, the ground-state or lowest excited-state binding energies start to decrease.     

Spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling

Using the perturbation method,we theoretically study the spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling.The heat generated by the spin current is calculated.With the increase of the width of the quantum wire,the spin current and the heat generated both exhibit period oscillations with equal amplitudes.When the quantum-channel number is doubled,the oscillation periods of the spin current and of the heat generated both decrease by a factor of 2.For the spin current j s,xy,the amplitude increases with the decrease of the quantum channel;while the amplitude of the spin current j s,yx remains the same.Therefore we conclude that the effect of the quantum-channel number on the spin current j s,xy is greater than that on the spin current j s,yx.The strength of the Rashba spin-orbit coupling is tunable by the gate voltage,and the gate voltage can be varied experimentally,which implies a new method of detecting the spin current.In addition,we can control the amplitude and the oscillation period of the spin current by controlling the number of the quantum channels.All these characteristics of the spin current will be very important for detecting and controlling the spin current,and especially for designing new spintronic devices in the future.     

Measurement of the slope parameter for the neutral pion spectrum in KL → πππ decay

The π⁰ spectrum in the K_L⁰ → 3π decay was measured using a wire chambers magnetic spectrometer. In the usual approximation, the matrix element can be expressed as: M² ≈ 1 + 2a₀(M_K/M_π²)(2T_π⁰−T_π⁰max) + a₁(M_K²/M_π⁴)(2T_π⁰−T_π⁰max)². We obtained a₀ = −0.282 ± 0.011 and a₁ consistent with zero.     

Magnetic quantum oscillations in nanowires

Analytical expressions for the magnetization and the longitudinal conductivity of nanowires are derived in a magnetic field, B. We show that the interplay between size and magnetic field energy-level quantizations manifests itself through novel magnetic quantum oscillations in metallic nanowires. There are three characteristic frequencies of de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations, F = F(0)/(1 + gamma)(3/2), and F(+/-) = 2F(0)/|1 + gamma +/- (1 + gamma)(1/2)|, in contrast with a single frequency F(0) = S(F)plankc/(2pie) in simple bulk metals. The amplitude of oscillations is strongly enhanced in some magic magnetic fields. The wire cross-section area S can be measured using the oscillations as S = 4pi(2)S(F)plank(2)c(2)/(gammae(2)B(2)) along with the Fermi-surface cross-section area, S(F).     

First-Principles Calculations of Atomic and Electronic Properties of T1 and In on Si（111）

The atomic and electronic structures of T1 and In on Si（111） surfaces are investigated using the firstprinciples total energy calculations. Total energy optimizations show that the energetically favored structure is 1/3 ML T1 adsorbed at the T4 sites on Si（111） surfaces. The adsorption energy difference of one T1 adatom between （√3 × √3） and （1 × 1） is less than that of each In adatom. The DOS indicates that TI 6p and Si 3p electrons play a very important role in the formation of the surface states. It is concluded that the bonding of TI adatoms on Si（111） surfaces is mainly polar covalent, which is weaker than that of In on Si（111）. So T1 atom is more easy to be migrated than In atom in the same external electric field and the structures of T1 on Si（111） is prone to switch between （√3 × √3） and （1 × 1）.