Beam Halo in High-Intensity Hadron Linacs

This document aims to cover the most relevant mechanisms for the development of beam halo in high-intensity hadron linacs. The introduction will outline the various applications of high-intensity linacs and it will explain why, in the case of the CERN Superconducting Proton Linac (SPL)study a linac was chosen to provide a high-power beam, rather than a different kind of accelerator. The basic equations, needed for the understanding of halo development will be derived and employed to study the effects of initial and distributed mismatch on high-current beams. The basic concepts of the particle-core model, envelope modes, parametric resonances, the free-energy approach, and the idea of core-core resonances will be introduced and extended to study beams in realistic linac lattices. The approach taken is to study the behavior of beams not only in simplified theoretical focusing structures but to highlight the beam dynamics in realistic accelerators. All effects which are described and derived with simplified analytic models, are tested in realistic lattices and are thus related to observable effects in linear accelerators. This approach involves the use of high-performance particle tracking codes, which are needed to simulate the behavior of the outermost particles in distributions of up to 100 million macro particles. In the end a set of design rules will be established and their impact on the design of a typical high-intensity machine, the CERN SPL, will be shown. The examples given in this document refer to two different design evolutions of the SPL study: the first conceptual design report (SPL I) and the second conceptual design report (SPL II)

The low-power SPL

This paper describes the basic parameters and the machine layout of the Low-Power SPL (LP-SPL), a 4GeV superconducting H− linac. In the first stage this machine is only designed to replace the PS booster (PSB) and to inject into a new proton synchrotron (PS2) at CERN. At the same time the components are designed such that the machine can be upgraded to become a high-power proton driver (5GeV, > 4MW) for future radioactive ion beam (RIB) facilities or for various neutrino production schemes at CERN. The consequences for the low-power design are explained together with the possible upgrade paths

Revised beam dynamics and layout for the superconducting section of the SPL

Since the publication of the SPL conceptual design report [1] the beam dynamics and the layout of the linac were subject to several hanges. As the studies of the room temperature part of the lina [2] show practically no emittance growth, the longitudinal emittance in the SC section of the SPL is reduced from 0.6 to 0.3 p deg MeV. This measure enhances the longitudinal debunching eject in the transfer line between the SPL and the Proton Driver Accumulator Compressor rings (PDAC). A modified layout for this transfer line which stretches the bunches in phase and compensates energy and phase jitter from the linac is presented. A new matching tool for the IMPACT code is used to improve the transitions between the different sections of the linac. Furthermore the results of a study concerned with cavity vibrations [3] is taken into account to readjust the synchronous phases in the linac. Finally a new alternative for the high energy section of the linac is tested, which uses no LEP cavities and which reduces the linac length by almost 90 m. The results of multiparticle simulations with matched and mismatched beams are presented for both versions

Beam Dynamics in the Superconducting Section of the SPL (120 MeV - 2.2 GeV)

After the decomissioning of LEP-2 a considerable amount of RF hardware becomes available and can be used for the construction of a high current superconducting proton linac [1]. The present design of the SPL uses four types of superconducting cavities (beta=0.52, 0.7, 0.8, and LEP) to accelerate a pulsed beam of 70 mA from 120 MeV up to 2.2 GeV. In this paper a beam dynamics layout for the superconducting section is presented. The layout is optimized to keep the overall linac length short and to ensure a stable steering of the beam. Multiparticle simulations have been carried out to analyse the proposed focusing structure, and to investigate emittance growth for the nominal case and for a mismatched input beam

The Effect of Phase Slippage in Multicell Cavities on Longitudinal Beam Dynamics

In modern linear accelerators it is often convenient to use multicell cavities which are designed for a constant particle velocity.These cavities are then employed over a certain velocity range until the next cavity type, designed for a higher ß, further accelerates the beam. For superconducting cavities this approach has become mandatory due to the high R&D costs of such devices. Using multicell cavities at velocitiesdifferent from their design value yields phase slippage in the single cells, thereby reducing energy gain and longitudinal focusing. In this paper we derive some simple analytical formulas in order to demonstrate the implications of phase slippage on longitudinal dynamics. It is shown that for large phase slip values the longitudinal focusing is not only reduced by lower transit time factors but also by an additional "slip factor ", that can drive particles into unstable regions of parameter space

Future High-Intensity Proton Accelerators

This paper provides an overview of currently planned high-intensity proton accelerators. While for high energies (>10GeV) synchrotrons remain the preferred tools to produce high-intensity beams, recent years have seen an impressive development of linac-based lower-energy (<8GeV) high-intensity proton drivers for spallation sources, accelerator driven systems (ADS), production of Radioactive Ion Beams (RIB) and various neutrino applications (beta-beam, superbeam, neutrino factory). This paper discusses the optimum machine types for the various beam requirements and uses a range of projects, which are likely to be realised within the coming decade, to illustrate the different approaches to reach high average beam power with the application-specific time structure. Only machines with a beam power above 100kW are considered

Beam Dynamics of Non-Equipartitioned Beams in the Case of the SPL Project at CERN

The SPL [1] working group at CERN is studying a 2.2 GeV H- linac, which recuperates a large amount of RF hardware from the now decommissioned LEP at CERN. During the ongoing design effort for an optimized layout, it was found that in some cases non-equipartitioned beams tend to exchange energy between the longitudinal and the transverse planes. Strict energy equipartition, however, imposes tight restrictions on such a high energy linac and often contradicts the goal of cost effective design. On the other hand, stability charts derived from 2D Vlasov analysis suggest the existence of stable non-equipartitioned equilibria in certain regions of parameter space. Due to the low bunch current (22 mA) in the SPL, these regions are large enough to ensure stable machine operation for non-equipartitioned beams. Systematic multiparticle simulations with IMPACT [2] are used to apply the stability charts to the beam dynamics design of a realistic high energy linac. Using the example of the SPL, it is shown that designs with stable non-equipartitioned bunches are feasible, and how these designs react to mismatched input beams

A Comparison of pi/2-mode standing wave structures for Linac4

Cell coupled structures at twice the basic frequency provide a higher shunt impedance for proton energies above 90 MeV for Linac4 when compared to drift tube based geometries. For this reason the nominal accelerating structure for the energy range of 90-160 MeV in Linac4 was chosen to be a Side Coupled Structure at 704.4 MHz. High power klystrons will feed this structure, with the consequence that a large number of cells are coupled together in one module. To provide field stability in a long chain of resonators, a pi/2-mode structure will be used. In this report, the three best known and most widely used ones - the Annular Ring Coupled Structure (ACS), the On Axis Coupled Structure (OCS) and the Side Coupled Structure (SCS) - are compared in terms of the electrical parameters Q-value and shunt impedance as well as structure dimensions