Electron Clouds

Abstract

The term ‘electron cloud’ refers to an accumulation of electrons inside the vacuum chamber of a particle accelerator, which is sufficiently strong to produce undesired effects on the accelerator operation, e.g., by causing beam loss, emittance growth, increase in the vacuum pressure, or unacceptable heat load on cold surfaces. Electrons in the beam chamber can primarily be generated by a number of processes, e.g., ionization of the residual gas. Their number, however, can exponentially increase via a beam-induced multipacting mechanism, which relies on acceleration of electrons in the field of the particle beam and efficient secondary emission from their impact on the chamber wall. Several machines running with high-intensity positively charged beams, made of trains of closely spaced bunches, suffer severe effects from electron clouds, and in some cases their performance is even limited by it. Techniques of electron cloud suppression or mitigation exist; the most popular ones are based on the
reduction of the secondary electron yield of the chamber inner surfaces. This can be achieved passively through the so-called process of machine scrubbing, or actively by coating the inner pipe walls with appropriate low secondary electron yield materials.