Transverse profile expansion and homogenization at target for the injector Scheme-I test stand of China-ADS

For the injector Scheme- test stand of the China-ADS(Accelerator Driven subcritical System), a beam with the maximum power of 100 k W will be produced and transported to the beam dump. To solve the very high thermal load problem at the dump, two measures are taken to deal with the huge power density at the target.One is to enlarge the contact area between the beam and the target, and this is to be accomplished by expanding the beam profile at the target and using slanted target plates. The other is to produce a more homogenous beam profile at the target to minimize the maximum power density. Here the beam dump line is designed to meet the requirement of beam expansion and homogenization at 3 different energies(3.2 Me V, 5 Me V and 10 Me V), and the step-like field magnets are employed for the beam spot homogenization. Taking into account the fact that the space charge effects are very strong at such low beam energies, the simulations have included space charge effects and errors which show that the beam line can meet the requirements very well. In the meantime, the alternative beam design using standard multipole magnets is also presented.     

Study on space charge effects of the CSNS/RCS

The Rapid Cycling Synchrotron (RCS) is a key component of the China Spallation Neutron Source (CSNS). The space charge effect is one of the most important issues in the CSNS/RCS, which limits the maximum beam intensity, as well as the maximum beam power. Space charge effects are the main source of emittance growth and beam loss in the RCS. Space charge effects have been studied by simulation for the CSNS/RCS. By optimizing the painting orbit, the optimized painting distribution was obtained. The space charge effects during the acceleration are studied and dangerous resonances, which may induce emittance growth and beam loss, are investigated. The results are an important reference for the design and commissioning of the CSNS/RCS.     

Arc Power Limit and Distribution on the Large Negative Ion Source Based on JT-60 NNBI

The target of the large negative ion source based on neutral beam injection (N-NBI) is to produce neutral beam current of 22 A and beam power of 10 MW at 500 keV with duration time of 10 s. Since it was successfully operated in 1996, the 5.3 MW neutral beam power at 380 keV with duration time of 3 s has been achieved. In recent years improving and enhancing the beam power are going on. Several reasons such as plasma non-uniformity, higher beam density at the upper region in the beam profile,     

Electron beam density study using a portable slit imaging system at the Shanghai Electron Beam Ion Trap

In this work,a portable slit imaging system is developed to study both the electron beam diameter and the profile of the newly developed Shanghai Electron Beam Ion Trap (Shanghai EBIT).Images are detected by a charge coupled device (CCD) sensitive to both X rays and longer wavelength photons (up to visible).Large scale ray tracings were conducted for correcting the image broadening effects caused by the finite slit width and the finite width of the CCD pixels.A numerical de-convolution method was developed to analyse and reconstruct the electron beam density distribution in the EBIT.As an example of the measured beam diameter and current density,the FWHM (full width at half maximum) diameter of the electron beam at 81 keV and 120 mA is found to be 76.2 μm and the density 2.00 × 10 3 A·cm 2,under a magnetic field of 3 T,including all corrections.     

Electron beam density imaging system at Beam study using a portable slit the Shanghai Electron Ion Trap

In this work, a portable slit imaging system is developed to study both the electron beam diameter and the profile of the newly developed Shanghai Electron Beam Ion Trap （Shanghai EBIT）. Images are detected by a charge coupled device （CCD） sensitive to both X rays and longer wavelength photons （up to visible）. Large scale ray tracings were conducted for correcting the image broadening effects caused by the finite slit width and the finite width of the CCD pixels. A numerical de-convolution method was developed to analyse and reconstruct the electron beam density distribution in the EBIT. As an example of the measured beam diameter and current density, the FWHM （full width at half maximum） diameter of the electron beam at 81 keV and 120 mA is found to be 76.2 μm and the density 2.00 × 10^3 A.cm-2, under a magnetic field of 3 T, including all corrections.     

Beam Line Design of Compact Laser Plasma Accelerator

A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44 MeV,energy spread within ±5% and 10~(6-8) protons per pulse.It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 109 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0-±5%.     

PIC mode simulation of solenoid collimation channel

A new SSC （Separated Sector Cyclotron）-Linac is being designed to serve as an injector for the SSC at the HIRFL （Heavy Ion Research Facility Lanzhou）. The beam intensity at the LEBT （Low Energy Beam Transport） for the heavy ions after the selection is typically low and the space charge effects are inconspicuous. The space charge effects become obvious when the beam current increases to a few hundred microamperes. The emittance growth deriving from the space charge effects may be particularly troublesome for the following linac and cyclotron. An optical system containing three solenoids has been designed for the LEBT to limit the beam emittance and to avoid the unnecessary beam loss in the cyclotron, as well as for the purpose of immunizing the LEBT emittance growth due to the space charge effects. The results of the PIC （Particle- In-Cell） mode simulation illustrate that this channel could limit the beam emittance growth and increase the beam brightness.     

Preliminary design of a laser accelerator beam line

A compact laser plasma accelerator(CLAPA) is being built at Peking University, which is based on an RPA-PSA mechanism or other acceleration mechanisms. The beam produced by this laser accelerator has the characteristics of short duration, high pulse current, large divergence angle, and wide energy spectrum. The beam cannot be produced by a normal ion source and accelerator. The space charge field in the initial is very strong.According to the beam parameters from preparatory experiments and theoretical simulations, a compact beam line is preliminarily designed. The beam line mainly consists of common transport elements to deliver proton beam with the energy of 1–50 MeV, energy spread of 0–±1% and current of 0–108 proton per pulse to satisfy the requirement of different experiments. The simulation result of a 15 MeV proton beam with an energy spread of ±1%, current of 400 m A, and final spot radius of 9 mm is presented in this paper.     

Simulation of Resonant Interaction between Energetic Electrons and Whistler-Mode Chorus in the Outer Radiation Belt

We construct a realistic model to evaluate the chorus wave-particle interaction in the outer radiation belt L = 4.5. This model incorporates a plasmatrough number density model, a field-aligned density model and a realistic wave power and frequency model. We solve the 2D bounce-averaged momentum-pitch-angle Fokker-Planck equation and show that the Whistler-mode chorus can be effective in the acceleration of electrons, and enhance the phase space density for energies of -1 MeV by a factor from 10 to 10^3 in about two days, consistent with the observation. We also demonstrate that ignorance of the electron number density variation along field line and magnetic local time in the previous work yields an overestimate of energetic electron phase space density by a factor 5-10 at large pitch-angle after two days, suggesting that a realistic plasma density model is very important to evaluate the evolution of energetic electrons in the outer radiation belt.     

ADS注入器II的5 MeV高能传输线的设计与搭建

ADS先导专项的注入器Ⅱ计划在将质子束通过一个Cryomodule加速到5 MeV后,进行10 mA连续束流的调试,以验证ADS低能段的强流超导直线加速器技术。为了将50 kW的束流功率沉积到束流垃圾站,需要搭建一条高能束流传输线,从超导段开始传输束流。采用两套三组合四极透镜控制束流包络及垃圾站的束流尺寸,利用诊断真空室进行水平和垂直发射度的测量。为避免束流产生的真空管道损伤,该传输线必须确保束流无损失地传输到垃圾站。A CW 10 mA,5 MeV beam commissioning of CADS Injector Ⅱ is planed recently to test the high power superconduction linac techniques.To transport 50 kW beam from linac to the beam dump,a high energy beam line (HEBT) line is designed and setup.Two Triplet are used to control the beam size along HEBT and at the beam dump.One diagnostics box is used for horizontal and vertical emittance measurement.To avoid damages to the vacuum pipe,beam should be transported to the beam dump without losses.The details of the HEBT design will be described in the paper.     

Studies of dual-harmonic acceleration at CSNS-Ⅱ

The Rapid Cycling Synchrotron(RCS) of the China Spallation Neutron Source(CSNS) complex is designed to provide 1.56×1013 protons per pulse(ppp) during the initial stage,and it is upgradeable to 3.12×1013 ppp during the second stage and 6.24×1013 ppp during the ultimate stage.The high beam intensity in the RCS requires alleviation of space charge effects to reduce beam losses,which is key in such high beam power accelerators.With higher intensities in the upgrading phases,a dual-harmonic RF system is planned to produce flat-topped bunches that are useful to reduce the space charge effects.We have studied different schemes to apply the dual-harmonic acceleration in CSNS-,and have calculated the main parameters of the RF systems,which are presented in this paper.     

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.     

CSNS-II双谐波加速研究

The Rapid Cycling Synchrotron （RCS） of the China Spallation Neutron Source （CSNS） complex is designed to provide 1.56×10^13 protons per pulse （ppp） during the initial stage, and it is upgradeable to 3.12× 10^13 ppp during the second stage and 6.24× 10^13 ppp during the ultimate stage. The high beam intensity in the RCS requires alleviation of space charge effects to reduce beam losses, which is key in such high beam power accelerators. With higher intensities in the upgrading phases, a dual-harmonic RF system is planned to produce flat-topped bunches that are useful to reduce the space charge effects. We have studied different schemes to apply the dual-harmonic acceleration in CSNS-Ⅱ, and have calculated the main parameters of the RF svstems, which are presented in this Darter.     

Angular momentum density of a Gaussian vortex beam

The Gaussian vortex beam is assumed to be linearly polarized.The analytical expression of the electric field of a linearly polarized Gaussian vortex beam propagating in free space is derived by using the vectorial Rayleigh-Sommerfeld integral formulae.The propagating magnetic field of the linearly polarized Gaussian vortex beam is presented by taking the curl of the electric field.By employing the electromagnetic field of the linearly polarized Gaussian vortex beam beyond the paraxial approximation,the analytical expression of the angular momentum density of the linearly polarized Gaussian vortex beam is derived.The three components of the angular momentum density of a linearly polarized Gaussian vortex beam are demonstrated in the reference plane.The effects of the linearly polarized angle and the topological charge on the three components of the angular momentum density are investigated.To acquire the more longitudinal angular momentum density requires such an optimal choice that the linearly polarized angle is set to be zero and the topological charge increases.This research is useful to the optical trapping,the optical guiding,and the optical manipulation.     

Beam Energy Dependence of the π-/π+ Ratio

Using the isospin- and momentum-dependent hadronic transport model IBUU04, the effects of symmetry energy on the π^-/π^＋ ratio are studied. Our investigations are based on the calculations of the ^132 Sn＋^124Sn semi-central collisions at beam energies of 400/AMeV, 600/A MeV and 800/A MeV. It is found that both the transverse momentum and kinetic energy distributions of the π^-/π^＋ ratio are rather sensitive to the symmetry energy, especially around the Coulomb peaks. The position of the Coulomb peak is shown to be nearly independent of beam energy. The sensitivity of the π^-/π^＋ ratio to the symmetry energy decreases as the beam energy increases from 600/A MeV to 800/A MeV.     

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.     

X-ray beam hardening correction for measuring density in linear accelerator industrial computed tomography

Due to X-ray attenuation being approximately proportional to material density,it is possible to measure the inner density through Industrial Computed Tomography（ICT） images accurately. In practice,however,a number of factors including the non-linear effects of beam hardening and diffuse scattered radiation complicate the quantitative measurement of density variations in materials. This paper is based on the linearization method of beam hardening correction,and uses polynomial fitting coecient which is obtained by the curvature of iron polychromatic beam data to fit other materials. Through theoretical deduction,the paper proves that the density measure error is less than 2% if using pre-filters to make the spectrum of linear accelerator range mainly 0.3 MeV to 3 MeV. Experiment had been set up at an ICT system with a 9 MeV electron linear accelerator. The result is satisfactory. This technique makes the beam hardening correction easy and simple,and it is valuable for measuring the ICT density and making use of the CT images to recognize materials.     

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.     

Thermal analysis and optimization of proton beam window for the CSNS

The proton beam window (PBW) is one of the key devices of China Spallation Neutron Source (CSNS). It is the boundary between transport line and target. This paper will present a new PBW structure and detailed thermal-stress analysis. The energy deposition and scattering effect need to be low when the beam passes through the PBW, so proper selection of material and structure is important. According to the study of energy deposition, A5083-O is selected as the PBW material. A single-double layer structure is first proposed based on the study of cooling structures. Thermal analysis and structural optimization are discussed, and transient analysis is done to show the effect of the beam pulse. Besides, safety is confirmed for cases of cooling tunnel blockage, beam profile shrinkage, or centroid orbit offset. All these analyses show the newly designed PBW structure can meet the requirements of the CSNS well.     

Fragmentation Cross Sections of 12C on Different Targets at Beam Energies from 50 to 100MeV/Nucleon

The fragmentation cross sections of reactions ^12C＋^2H, ^12C, ^14N, ^16O at beam energies from 50 to 100 MeV/nucleon are investigated using the isospin-dependent Boltzmann-Langevin equation model. It is found that fragment species increase approximately with the increasing target mass. The fragment species and some fragments production cross sections in reactions of ^12C＋^12C, ^14N, ^16O show an obvious variation at the beam energies from 50 to 80 MeV/nucleon. However the calculated fragment production cross sections do not change much when the incident energy increases from 80 to 100 MeV/nucleon.