摘要:

We have explored the formation of longitudinal and transverse halos in 3-D axisymmetric beam bunches by starting with a self-consistent 6-D phase space distribution. Stationary distributions allow us to study the halo development mechanism without being obscured by beam redistribution and its effect on halo formation. The beam is then mismatched longitudinally and/or transversely, and we explore the rate, intensity and spatial extent of the halos which form, as a function of the beam charge and the mismatches. We find that the longitudinal halo forms first because the longitudinal tune depression is more severe than the transverse one for elongated bunches and conclude that it plays a major role in halo formation. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56777

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A self-consistent set of equations for the azimuthal variation of rms betatron oscillation amplitudes, including the effects of dispersion and space charge, is derived. These effective envelope equations can be integrated over the beam energy distribution to provide space charge forces in a particle core model for rings. The derivation of the envelope equations involves an accelerator ordering scheme for the beam dynamics and a statistical moments analysis of the canonical distribution function in the six-dimensional phase space of the beam Hamiltonian. The azimuthal variation of the second moments of the transverse canonical coordinates,x&bgr;andz&bgr;,integrated over the kinetic distribution function of the beam, provides the rms equations. These equations, at fixed beam energy, are integrated over the beam energy distribution to provide the overall space charge distribution and force. Because the envelope equations and dispersion function both depend upon and determine the space charge forces, the consistency of the particle core model requires either analytic closure assumptions or numerical iteration. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56760

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The standard definition for an achromat is a transport line having zero values for the spatial dispersion (R16) and the angular dispersion (R26). In the presence of linear space charge this definition of achromaticity does not hold. The linear space charge in the presence of a bend provides coupling between (a) bunch spatial width and bunch length (R15) and (b) bunch angular spread and bunch length (R25). Therefore, achromaticity should be redefined as a line having zero values of the spatial dispersion (R16), the angular dispersion (R26), and matrix elements R15 & R25. These additional conditions(R15=R25=0)can be achieved, for example, with two small RF cavities at appropriate locations in the achromat, to cancel space charge effects. An example of the application of this technique to the Spallation Neutron Source (SNS) high energy beam transport line will be presented. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56753

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The presence of space charge affects the value of the dispersion function. On the other hand dispersion has a role in shaping the beam distribution and therefore in determining the resulting forces due to space charge. In this paper we present a framework where the interplay between space charge and dispersion for a continuous beam can be simultaneously treated. We revise the derivation of a new set of rms envelope-dispersion equations we have recently proposed in [1]. The new equations generalize the standard rms envelope equations currently used for matching to the case where bends and a longitudinal momentum spread are present. We report a comparison between the solutions of the rms envelope-dispersion equations and the results obtained using WARP, a Particle in Cell (PIC) code, in the modeling of the Maryland Electron Ring. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56748

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We studied the electrostatic fields due to the longitudinal and transverse perturbations of a charged particle beam with a uniform distribution propagating inside an rf-shielding cage constructed from evenly-spaced conducting wires. Simple formulae are derived for estimating the space-charge impedances. Numerical examples are given for illustration. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56754

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Betatron tune shift due to space charge effect is investigated by solving the equation of motion of particles including total space charge (linear and non-linear part). We introduce a binomial representation for 4-dimensional phase space distribution, which can characterize the important factors to tune shift while largely reducing the mathematical difficulties. With this representation, a numerically solvable equation of motion is obtained. From its solution, betatron tune is deduced by FFT (Fast Fourier Transformation). We derive the tune shift as function of beam current, amplitude of betatron oscillation and the particle distribution. We also present the differences between tune shifts calculated by including total space charge effect and including only its linear term. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56755

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

High brightness beam transport in a non-linear focusing channel, while avoiding beam emittance growth and halo formation, is considered. Invariance of beam distribution function is treated as a problem of proper matching of the beam with focusing channel. Matched conditions for intense beam are obtained from the stationary Vlasov’s equation for beam distribution function and Poisson’s equation for electrostatic beam potential. Two approaches to self-consistent problem for matched beam distribution are treated: (i) focusing field is determined by given beam distribution, and (ii) matched beam distribution is defined by focusing potential of the structure. Attained solutions provide theoretical basis for choosing parameters of the space charge dominated beam transport with suppressed halo. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56756

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Two papers presented at the Shelter Island workshop are very briefly summarized here, in view of recent publications elsewhere. The WARP code, developed for Heavy-Ion beam-driven inertial confinement Fusion (HIF) accelerator studies, combines features of a particle-in-cell plasma simulation and an accelerator tracking program. Its methods and architecture have been developed for efficiency both in detailed simulation of individual machine sections and in long-time beam tracking. The transverse “slice” model in the code has been applied to the study of transverse resonance effects associated with quadrupole strength errors. These simulations confirm that rapid passage through a resonance can reduce the associated mismatch and emittance growth. References to published details and to other sources of information are supplied. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56757

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The Spallation Neutron Source, SNS, is the next US neutron source for condensed matter physics. The SNS consists of a 35 mA peak currentH−front end, a 1.0 GeV proton linac operating at 6&percent; duty factor, and a 220.67 m circumference accumulator ring with injection, HEBT, and extraction, RTBT, beam transport lines. The SNS Ring injection process is facilitated by painting the Linac beam phase space into the ring beam phase space by dynamically bumping the ring equilibrium orbit at the injection foil. We find that inclusion of transverse space charge effects has a large impact on the final transverse beam distributions. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56749

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This paper presents the results of calculations of the transverse effects of space charge of coasting beams in rings for three globally comparable lattice configurations: uniform focusing, FODO, and doublet. The parameters of the lattice and theH−beam are chosen similar to those of the SNS accumulator ring. Three models for space charge are considered: 1) a particle core model based on rms beam parameters, 2) a self-consistent particle-in-cell (PIC) model, and 3) a phase-averaged PIC model. In all cases both matched and mismatched K-V distributions are considered by randomly initializing, and then tracking, collections of macroparticles representing the beam. In the particle core model the initial rms values of the macroparticle distributions are used as initial values for solving the envelope equations, including space charge forces and dispersion effects. For matched beams the calculations, performed using a modified version of the injection and tracking code, ACCSIM, reveal only a slight emittance growth and no halo generation with the particle core model. However, the self-consistent PIC model yields greater emittance growth and halo generation, particularly for the doublet lattice in the vertical plane. When the calculations are performed with the phase-averaged particle-in-cell (PIC) model, the results agree substantially with the particle core model, suggesting that the observed self-consistent PIC results are not a consequence of numerical truncation. As further confirmation, we have performed numerical convergence studies using the doublet lattice, and have observed the behavior of the self-consistent PIC model to persist. The tendency for emittance growth and halo generation in the doublet lattice is not surprising, as the fluctuations in beam area, which are an excellent indicator of space charge forces, are larger and more rapid than for the other lattices. For mismatched beams, only the particle core model has been applied at present, and good beam transport is obtained with up to 25&percent; envelope oscillation amplitudes for all three lattices. ©1998 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.56758

出版商:AIP    

年代:1998

数据来源: AIP