摘要:

A halo generation mechanism in the non‐periodic lattices such as the SNS linac MEBT (Medium‐Energy Beam‐Transport between RFQ and DTL) is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6‐m‐long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the Front End (FE) emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radio activation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the CCL, which is a preferred scheme for mitigating halo. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638321

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In high intensity proton linacs, beam halo can be observed as early as the very first stages of acceleration. The inital population of halo particles found in the output distribution of the RFQ, increases in chopper lines, which interrupt the regular focusing pattern of RFQ and DTL. The resulting emittance growth and halo development contributes to beam loss in the low‐energy accelerator sections and partly defines the beam quality in the high‐energy sections. Using the examples of the ESS and Linac4 (CERN) front ends (RFQ to 20 MeV) we investigate emittance growth and beam halo due to statistically distributed field and gradient errors. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638322

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The influence of space charge on emittance growth is studied in simulations of a coasting beam exposed to a strong octupolar perturbation in an otherwise linear lattice, and under stationary parameters. We explore the importance of self‐consistency by comparing results with a non‐self‐consistent model, where the space charge electric field is kept “frozen‐in” to its initial values. For Gaussian distribution functions we find that the “frozen‐in” model results in a good approximation of the self‐consistent model, hence coherent response is practically absent and the emittance growth is self‐limiting due to space charge de‐tuning. For KV or waterbag distributions, instead, strong coherent response is found, which we explain in terms of absence of Landau damping. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638323

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

An advantage of a proton Super‐Conducting Linac (SCL) is that RF cavities can be operated independently, allowing easier beam transport and acceleration. But cavities are to be separated by drifts long enough to avoid they couple to each other. Moreover, cavities are placed in cryostats that include inactive insertions for cold‐warm transitions; and interspersed are warm insertions for magnets and other devices. The SCL is then an alternating sequence of accelerating elements and drifts. No periodicity is present, and the longitudinal motion is not adiabatic. This has the consequence that the beam bunch ellipse will tumble, dilute and create a halo in the momentum plane because of inherent nonlinearities. When this is coupled to longitudinal space‐charge forces, it may cause beam loss with latent activation of the accelerator components. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638324

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A benchmarking experiment with a high intensity bunched beam stored for 1.2 s in a nonlinear lattice has been performed at the CERN Proton Synchrotron (PS). Beam emittance and beam intensity have been measured for several working points at different distances from a lattice‐induced resonance. We found a regime of emittance growth for machine tunes far from the resonance and a regime of emittance shrink/beam loss for working point close the resonance. We compare the observations with 3D simulations and in the blow‐up regime we find good agreement. We interpret these results in terms of space charge induced trapping and de‐trapping on the lattice resonance. We show that this mechanism is responsible of halo formation which depends on the distance from the resonance and discuss the interplay of dynamic aperture and halo size as main mechanism to explain beam loss in the experiment. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638325

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Collective beam dynamics will play a major role in determining losses in high intensity rings. We demonstrate here, using both analytic and computational models, that beam halo can form under the influence of transverse impedances, even for stable cases well below the instability threshold. It is shown for cases above the instability threshold that rms beam size and halo develop more rapidly than the beam centroid. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638326

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

For a KV beam the halo is created by test particles, initially out of the core, via nonlinear resonances with the betatron motion. The number of resonances and the related diffusive processes are enhanced by the presence of mismatch oscillations of the core. Numerical (PIC) or physical (collisions) noise modifies the core of a KV beam creating tails on time scales relevant for storage rings. We consider a 2D model defined byNpseudo line‐charges per meter, whose limitN→ ∞, at fixed perveance, is the 2D mean field theory for a coasting beam in a constant focusing channel. Using an algorithm of optimal computational complexity we find that the relaxation time scales linearly withNin agreement with Landau’s kinetic theory we have developed for the 2D model. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638327

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

BNL is conducting a design study of a 1.0 MW super neutrino beam facility. It requires 230 turns charge exchange injection from a 1.2 GeV superconducting linac with 28 mA current for 0.72 msec. This report studies the impact of halo distribution of the linac beam on the efficiency of injection and the final beam distribution in the AGS as functions of the injection orbit bump and the foil thickness. Another important consideration is the residual radiation generated on the accelerator components near the injection area. If necessary, radiation hardened components and local shielding have to be provided. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638328

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Systems where the beam has a rotation angle relative to the plane of the quadrupoles can support a larger number of envelope modes. Since beam halos can be produced by parametric resonance between the oscillations of the core and individual particle orbits, many more classes of such resonances are to be expected in rotated beams. Early simulation studies are reviewed in which halos are formed by random quadrupole rotation errors. The injection of a rotated beam into a perfect lattice is also explored. The results are compared to a preliminary experiment on the University of Maryland Electron Ring (UMER). © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638329

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Recent code improvements and some simulation results of halo particles with Simpsons will be presented. We tried to identify resonance behavior of halo particles by looking at tune evolution of individual macro particle. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1638330

出版商:AIP    

年代:1903

数据来源: AIP