Studies of beam halo became unavoidable feature of high-intensity machines where uncontrolled beam loss should be kept to extremely small level. For a well controlled stable beam such a loss is typically associated with the low density halo surrounding beam core. In order to minimize uncontrolled beam loss or improve performance of an accelerator, it is very important to understand what are the sources of halo formation in a specific machine of interest. The dominant mechanisms are, in fact, different in linear accelerators, circular machines or Energy Recovering Linacs (ERL). In this paper, we summarize basic mechanisms of halo formation in high-intensity beams and discuss their application to various types of accelerators of interest, such as linacs, rings and ERL.

In space-charge dominated beams the nonlinear space-charge forces produce a filamentation pattern, which in projection to the 2-D phase spaces results in a 2-component beam consisting of an inner core and a diffuse outer halo. The beam-halo is of concern for a next generation of cw, high-power proton linacs that could be applied to intense neutron generators for nuclear materials processing. The author describes what has been learned about beam halo and the evolution of space-charge dominated beams using numerical simulations of initial laminar beams in uniform linear focusing channels. Initial results are presented from a study of beam entropy for an intense space-charge dominated beam.

Progress was made during the past decade towards a better understanding of halo formation caused by beam mismatch in high-intensity beams. To test these ideas an experiment was carried out at Los Alamos with proton beams in a 52-quadrupole focusing channel. Rms emittances and beam widths were obtained from measured beam profiles for comparison with the maximum emittance growth predictions of a free-energy model and the maximum haloamplitude predictions of a particle-core model. The experimental results are also compared with multiparticle simulations. In this paper we will present the experimental results and discuss the implications with respect to the validity of both the models and the simulations. Keywords: beam halo, emittance growth, beam profiles, simulations, space charge, mismatch.

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This book is devoted to the quickly developing area of high intensity particle beam physics. Beam emittance growth, halo formation and chaotic particle motion are the main areas of research in the new intense particle accelerators. Knowledge of those phenomena is crucial for the design of particle accelerators with space-charge dominated beams. This important book provides a new, self-consistent description of high brightness particle beams with essentially nonlinear space-charge forces. The emphasis is on the proper matching of the beam with focusing and accelerating structures to suppress beam emittance growth and halo formation.The book will be useful for researchers and engineers dealing with space-charge dominated beams and for graduate and undergraduate students who are starting to work in this field.

An rms-mismatched beam with space-charge forces can evolve rapidly, and during the course of evolution its emittance grows and a diffuse halo forms. The halo is of particular concern for long-term operation of linacs such as those envisioned for accelerator transmutation of waste (ATW) because too much particle impingement on the accelerating structures would cause enough radioactivation to prohibit routine hands-on maintenance. Thus, halo suppression is a design consideration for these linacs. In this paper, the authors summarize both the physical processes associated with the rapid evolution of the rms-mismatched beam and a semianalytic formalism which was recently developed to account for them. They also provide representative results derived from the formalism.