On the Choice of Linac Parameters for Minimal Beam Losses

  • M. Eshraqi
  • Published 2013

Abstract

In high intensity linear accelerators, the tune spreads induced by the space-charge forces in the radial and longitudinal planes are key parameters for halo formation and beam losses. For matched beams they are the parameters governing the number of resonances (including coupling resonances) which affect the beam and determine the respective sizes of the stable and halo areas in phase space. The number and strength of the resonances excited in mismatched beams leading to even higher amplitude halos are also directly linked to the tune spreads. In this paper, the equations making the link between the basic linac parameters (rf frequency, zero-current phase advances, beam intensity and emittances) and the tune spreads are given. A first analysis of the way these linac parameters can be chosen to minimize the tune spreads is presented. The ESS linac parameters are used for this study. INTRODUCTION In high intensity linear accelerators where the beam power is in the order of hundred kilowatts up to few megawatts, activation due to loss of halo particles is a crucial parameter affecting the design as well as the cost of the accelerator. There has been several studies demonstrating that the tune spreads induced by the space charge forces in the two radial and longitudinal planes are key parameters for halo formation and beam losses [1–3]. With σ0t and σt (σ0l and σl) the transverse (longitudinal) phase advances without and with space charge, the transverse and longitudinal relative tune spreads are given by ζt = σ0t − σt σ0t ζl = σ0l − σl σ0l (ζ = 1− η), (1) where η = σ/σ0 is the tune depression. These relative tune spreads are parameters which give a “measure” of the space-charge nonlinear effects on the beam dynamics as a function of the beam current I with lim I→0 ζ = 0 lim I→+∞ ζ = 1. (2) Relative tune spreads are good indicators of the strength of the space charge force, e.g., in a linac with a relative tune spread of 0.6, the space charge force is 84% of the external focusing force. In a beam with space charge, the particle phase advances range from the phase advance with space charge for particles with low amplitudes to the zero current phase advance for large amplitude ones (see Fig. 1). The relative x′

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Cite this paper

@inproceedings{Eshraqi2013OnTC, title={On the Choice of Linac Parameters for Minimal Beam Losses}, author={M. Eshraqi}, year={2013} }