 This effect is especially a problem in positron accelerations, where electrons are attracted and slingshot into the walls at variable incident angles.Negatively charged electrons liberated from the accelerator walls are attracted to the positively charged beam, and form a "cloud" around it.The effect is most pronounced for electrons with around 300eV of kinetic energy - with a steep drop-off of the effect at less than that energy, and a gradual drop-off at higher energies, which occurs because electrons "bury" themselves deep inside the walls of the accelerator tube, making it difficult for secondary electrons to escape into the tube.