Emittance growth from space-charge forces

Space‐charge‐induced emittance growth has become a topic of much recent interest for designing the low‐velocity sections of high‐intensity, high‐brightness accelerators and beam‐transport channels. In this paper we review the properties of the space‐charge force, and discuss the concepts of matching, space‐charge and emittance‐dominated beams, and equilibrium beams and their characteristics. This is followed by a survey of some of the work over the past 25 years to identify the mechanisms of this emittance growth in both ion and electron accelerators. We summarize the overall results in terms of four distinct mechanisms whose characteristics we describe. Finally, we show numerical simulation results for the evolution of initial rms‐mismatched laminar beams. The examples show that for space‐charge dominated beams, the nonlinear space‐charge forces produce a highly chaotic 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. In the examples we have studied the halo contains only a few percent of the particles, but contributes about half of the emittance growth.