Beam Mismatch and Halo

Abstract

In the presence of space charge perfect matching of the beam is a desirable goal, in particular in high current and high average power linear accelerators. However, discontinuities in focusing geometry, rf frequency or phase etc. are often inevitable sources of mismatch, which result in emittance growth, beam halo and loss. This chapter discusses some basic aspects of the role of rms mismatch with regards to beam halo. Interpreting mismatch and halo is a complex interplay between coherent beam motion, single particle parametric resonance with the core and chaotic motion. A further mechanism – although quantitatively less significant – is density profile mismatch, which is also outlined. Application of this chapter is primarily to linear accelerators, whereas in circular accelerators beam halo is normally understood as a result of nonlinear dynamics driven by external nonlinearities, which are not considered here.