Simulation of a low energy beam transport line

A 2.45 GHz electron cyclotron resonance intense proton source and a low energy beam transport line with dual-Glaser lens were designed and fabricated by Institute of Modern Physics for a compact pulsed hadron source at Tsinghua. The intense proton beams extracted from the ion source are transported through the transport line to match the downstream radio frequency quadrupole accelerator. Particle-in-cell code BEAMPATH was used to carry out the beam transport simulations and optimize the magnetic field structures of the transport line. Emittance growth due to space charge and spherical aberrations of the Glaser lens were studied in both theory and simulation. The results show that narrow beam has smaller aberrations and better beam quality through the transport line. To better match the radio frequency quadrupole accelerator, a shorter transport line is desired with sufficient space charge neutralization.     

Transport of Intense Ion Beams

The possibility of using intense bursts of heavy ions to initiate an inertially confined fusion reaction has stimulated interest in the transport of intense unneutralized heavy ion beams by quadrupole or solenoid systems. We have examined this problem in somne detail, using numerical integration of the coupled envelope equations for the quadrupole case. The general relations which emerge are used to develop examples of high energy transport systems and as a basis for discussing the limitations imposed by a transport system on achievable intensities for initial acceleration.     

Limitations of Heavy Ion Synchrotron Acceleration for Inertial Fusion

The potential benefits from heavy ion inertial fusion motivate the rapid development of a program to test the principle.1,2,3 To define the program, accelerator parameters which have not hitherto been commonly considered must be studied interactively with basic questions of space charge limitations and charge exchange. Beam lifetime and power output efficiency may ultimately lead to a linear accelerator as the choice for an ignition device. For proof of principle, however, at power levels way beyond present inertial fusion experience, synchrotrons may have applicability at lower cost. The power and energy which can be delivered by the accelerating system to the reaction chamber are limited by space charge defocussing and intra beam charge exchange scattering, both of which are beam density dependent. These put constraints on linac injector energy, synchrotron aperture, synchrotron magnetic rigidity, acceleration time, ion species and charge to mass ratio. The accelerator system considered is classical. A linear accelerator injects into a synchrotron which accelerates the ion beam to the full energy delivered to the target. The maximum energy deliverable by a synchrotron is treated in section I. The targetting parameters and the energy gained through synchrotron acceleration completely determine the synchrotron aperture. These are discussed in sections II and III. The ion range in material is treated in section IV. The problem of intrabeam scattering is considered in section V. Finally, in section VI is a discussion of examples to meet specified goals.     

High Intensity Multi-GeV Proton Accelerators

For performing many high-energy physics experiments, it is necessary to have high-energy particle beams of very high intensity. The intensity of particle beams in circular proton accelerators in the multi-GeV energy range depends, first, on the number of protons accelerated per pulse and, second, on the number of pulses per second. The number of protons which can be accelerated per pulse is limited by the space charge effects, and the number of pulses per second is limited by the design or type of the accelerator. These intensity limitations and methods of pushing back these limits are discussed. It is now feasible both technologically and economically to construct accelerators in the tens of GeV energy range which will accelerate more than 1014 protons per second.     

在束流输运网络中强流离子束的复杂性及其控制研究进展

讨论了在束流输运网络中强流离子束在国防与民用等许多方面极其重要的应用潜力和诱人的发展前景。指出在束流输运网络中强流离子束形成的束晕-混沌的复杂性已经成为强流离子束应用中的关键问题之一;必须深入研究这类束晕-混沌的复杂特性及其产生的物理机制,并实现对束晕-混沌的有效控制。考虑到这是一个特殊的复杂的时空混沌的控制问题,解决这一难题已经成为强流离子束涉及的高科技领域、非线性-复杂性科学及复杂网络交叉领域中极富挑战性的一个新课题。最后,对上述课题当前的最新进展进行了评论,指出了该课题今后的研究方向。     

400 kV强流中子发生器的物理设计

对400 kV强流中子发生器进行了物理设计。采用Poisson/Superfish软件对中子发生器高压电极和加速管的电场分布进行了模拟,结果显示,各关键区域的空间电场最大值远低于击穿电场限值。以强流束旁轴包络方程为基本模型,发展了强流束传输系统束包络的计算机模拟程序IONB1.0,模拟了中子发生器传输系统中40 mA的D束流包络。结果显示,设计方案中所采取的两间隙高梯度加速结构有较强的聚焦性能,能有效抵消强流束空间电荷效应造成的束流发散,加速管出口处的束包络半径约3 cm,由加速管出口处的空间电荷透镜和三重四极磁透镜组成的传输系统能将束流聚焦在约140 cm处的靶上,且束斑直径小于2 cm。     

PIC方法在RFQ加速器z-code和t-code模拟程序中的应用研究

本文介绍了二维PIC(Particle in cell)方法,这种方法常用于粒子动力学模拟中空间电荷作用的计算。并比较了以时间为自变量(t-code)和以纵向位置为自变量(z-code)的两种动力学模拟程序;针对“国家重点基础研究发展规划”洁净核能项目中的射频四极(RFQ)加速器结构参数,给出了单束加速和正、负离子束同时加速两种情况下,t-code和z-code模拟得出的传输效率。结果表明,当束团的相位宽度大或能散大时,z-code在计算空间电荷作用时会引入相对较大的误差,从而应该使用t-code来进行动力学模拟,以获得更准确的结果。     

Monte Carlo analysis of pion contribution to absorbed dose from Galactic cosmic rays

Accurate knowledge of the physics of interaction, particle production and transport is necessary to estimate the radiation damage to equipment used on spacecraft and the biological effects of space radiation. For long duration astronaut missions, both on the International Space Station and the planned manned missions to Moon and Mars, the shielding strategy must include a comprehensive knowledge of the secondary radiation environment. The distribution of absorbed dose and dose equivalent is a function of the type, energy and population of these secondary products. Galactic cosmic rays (GCR) comprised of protons and heavier nuclei have energies from a few MeV per nucleon to the ZeV region, with the spectra reaching flux maxima in the hundreds of MeV range. Therefore, the MeV-GeV region is most important for space radiation. Coincidentally, the pion production energy threshold is about 280 MeV. The question naturally arises as to how important these particles are with respect to space radiation problems. The space radiation transport code, HZETRN (High charge (Z) and Energy TRaNsport), currently used by NASA, performs neutron, proton and heavy ion transport explicitly, but it does not take into account the production and transport of mesons, photons and leptons. In this paper, we present results from the Monte Carlo code MCNPX (Monte Carlo N-Particle eXtended), showing the effect of leptons and mesons when they are produced and transported in a GCR environment.     

Experimental verification of an APF linac for a proton therapy facility

Alternating phase focusing (APF) is known as a beam focusing method; it was applied to an interdigital H-mode structure and successfully accelerated high current proton beams up to the desired energy for a medical synchrotron injector. A high-current APF linac was achieved by the optimal design of the cavity and the drift tubes themselves, as well as drift tube arrangement based on the co-iteration of a precise electromagnetic field and space charge beam dynamics.A proton injector for a medical accelerator complex was fabricated with the newly developed APF linac. The injector consists of an electron cyclotron resonance ion source, a radio-frequency quadrupole linac and the APF linac. The experimental results showed that over 10 mA proton beams were accelerated up to 7.4 MeV.     

Design of an RFQ for direct plasma injection scheme

A high current radio frequency quadrupole (RFQ) is being studied at the Institute of Modern Physics,Chinese Academy of Sciences (IMP,CAS) for the direct plasma injection scheme (DPIS).Because of the strong space charge of beams from laser ion source,the beam dynamics design of the RFQ has been carried out with a new code,which can deal with space charge effectively.The design of the RFQ structure is performed with an electromagnetic simulation code and the determination of parameters of the structure has been done to maximize the shunt impedance when the frequency is kept fixed.The influences of dipole mode effect and flatness on beams were also discussed.     

Influence of shape effect on dynamic surface charge transport mechanism of cellular electret after corona discharge

Surface charge accumulation and transport on cellular polypropylene play an important role in nanogenerators, which could have a potential impact on energy harvesting and wearable devices for zero carbon energy systems and the internet of things. Different shapes have different charge accumulation and decay characteristics of the polymer. Therefore, we studied the influence of the sample's shape on the surface charge decay by experiment and modeling. The surface potential of square and circular cellular polypropylene was measured by a two-dimensional surface potential measurement system with electrostatic capacitive probe. The experimental result shows that the surface potential distribution of the square sample dissipates non-uniformly from the bell shape to a one-sided collapsed shape, while that of the circular sample dissipates uniformly from the bell shape to the crater-like shape. Moreover, the simulated results of the initial surface potential distributions of the square and circular cellular polypropylene are consistent with the experimental results. The investigation demonstrates that the charge transport process is correlated with the shape of the sample, which provides significant reference for designing electret material used for highly efficient nanogenerators.     

Dosimetry during Space Missions

The United States' astronauts have been exposed to a complex radiation environment that poses unique problems in high-energy radiation dosimetry. Since the astronauts were exposed to natural radiation sources with a potential for delivering very large radiation exposures, exposure limits in excess of occupational limits were adopted. These limits represent a balancing of the risk from injury due to radiation exposure against other very high risks inherent in space flight and against the benefits to be derived from the space missions. The limits adopted for Apollo, Skylab, and Apollo/Soyuz are given along with the details of the radiation monitoring activities of the mission. The radiation exposures experienced in the United States' space missions are also given.     

Vacuum Ultraviolet Radiation Effects in SiO2

Charging effects observed in MOS structures which have been exposed to sputtering plasmas or electron beam deposition suggest that Vacuum Ultraviolet (VUV) or soft X-radiation is important in producing these effects. Our experiments show that under positive gate bias VUV irradiation produces large positive charging effects for photon energies above 8.8 eV, the threshold for electron-hole pair creation in SiO2. This charging appears to be accompanied by an increase in interface state density. VUV radiation proves to be more useful than higher energy quanta or particles in studying radiation charging. This is true because one can control the depth of radiation absorption into the oxide. Etching experiments show that positive charge is induced near the Si-SiO2 interface even when radiation is absorbed near the gate electrode. This result is strong evidence in support of the hole transport and trapping model. We present evidence that under irradiation with positive bias, positive space charge is formed near both interfaces. We also show how a large positive space charge can be introduced into the oxide without a gate electrode.     

Muon radiography of large industrial structures

To date there have been a few sporadic attempts to exploit the immense penetrating power of cosmic ray muons to radiographically probe the internal structure of very large objects ranging from ancient pyramids to active volcanoes. Recently, application to security problems has been described, but the very low muon flux limits more general use. However muons have potential uses for examining very large industrial structures where long measurement times are acceptable, and such possibilities are being investigated.A cosmic ray telescope and recording system has been constructed and is being employed on simple model systems to validate Monte Carlo predictions, and results obtained so far will be described.     

Theoretical prediction of the impact of Auger recombination oncharge collection from an ion track

The theoretical analysis presented indicates that Auger recombination can reduce charge collection from very dense ion tracks in silicon devices. It is of marginal importance for tracks produced by 270-MeV krypton, and therefore it is of major importance for ions exhibiting a significantly larger loss. The analysis shows that recombination loss is profoundly affected by track diffusion. As the track diffuses, the density and recombination rate decrease so fast that the linear density (number of electron-hole pairs per unit length) approaches a nonzero limiting value as t→∞. Furthermore, the linear density very nearly equal to this limiting value is a few picoseconds or less. When Auger recombination accompanies charge transport processes that have much longer time scales, it can be simulated by assigning a reduced linear energy transfer to the ion     

Electric quadrupole transition probabilities for atomic lithium

Electric quadrupole transition probabilities for atomic lithium have been calculated using the weakest bound electron potential model theory (WBEPMT). We have employed numerical non-relativistic Hartree–Fock wavefunctions for expectation values of radii and the necessary energy values have been taken from the compilation at NIST. The results obtained with the present method agree very well with the Coulomb approximation results given by Caves (1975). Moreover, electric quadrupole transition probability values not existing in the literature for some highly excited levels have been obtained using the WBEPMT.     

Ionizing Radiation Effects in Inversion Layer Silicon Solar Cells

Ionizing irradiation experiments on a new inversion layer solar cell have been carried out to determine its potential for space applications. A scanning electron microscope was used as a source of low energy (2-12 keV) electrons in doses up to 1014 e/cm2. Phototopographic scanning and C-V analysis were used to determine the effects of the radiation on the Si02-Si system. The electron irradiation results are validated by comparison with those of a conventional X-ray experiment. The operation, fabrication and electrical characteristics of the cell are also discussed. Large increases in both the oxide charge and surface state density were observed, but little change occurred in the cells' output characteristics below 105 rads (Si). A drop-off in power at higher doses is explained in terms of surface recombination and series resistance increases. The results of isochronal and UV annealing experiments on these effects are also presented. The cells are concluded to have potential for use in space.     

电荷束团非均匀密度分布引起的发射度增长

直线加速器中电荷束团的非线性效应是导致束流发射度增长的一个重要原因。文中给出了直线加速器中几种常见的非均匀密度分布的有限长空间电荷束团所具有的非线性自场能，即纵向非均匀电荷密度分布分别为水袋型，抛物线型，而横向密度分布为均匀型，水袋型，抛物线型及高期型等各种组合非均匀密度分布的自场能公式，并得到了由束团非线性的均匀电荷密度分布引起的束流发射度增长。     

Nonneutral Plasma Science Issues for Heavy Ion Drivers

A review of nonneutral plasma science issues for heavy ion drivers is presented. The requirements on transverse and longitudinal focusing at the target lead to constraints on the 6D phase space. Mechanisms which act to prevent focusability, including emittance growth, space charge and instabilities are discussed. Experiments which have explored and validated our understanding of beam transport and focusability of space-charge dominated heavy ion beams are described.     

Numerical Solution of the Spencer-Lewis Equation for Kilovolt Electron Transport in Plane Slabs

We have developed a computer program BOLT for the numerical solution of the Spencer-Lewis equation for kilovolt electron transport in finite slabs. The method of solution used is a discrete ordinates method utilizing Gauss-quadrature angles equivalent to a 20th order Legendre polynomical expansion of the electron flux distribution function. The ultimate goal of the program is to compute x-ray photoelectron emission from thin targets and energy and charge deposition profiles near solid-vacuum boundaries and at interfaces between materials of differing atomic number. In this paper results are presented for a uniformly distributed monoenergetic electron source function, anisotropic in angle with form (1 + ? cos ?). The values for the charge and energy flow and deposition profiles agree within a few percent with analytic solutions characteristic of an infinite medium and with Monte Carlo transport calculations using simulated electron trajectories. The equilibrium value for the net flow or "Compton current" for 30 kilovolt electrons is found to be about 7 percent higher than the analytic value while all other quantities are believed to be accurate to within 2 percent. The principal source of error appears to be in the evaluation of the collision integral involving the exceedingly narrow deflections characteristic of kilovolt electron scattering. The discretization error due to finite grid sizes in the discrete ordinates scheme used was found to be very small.