Transverse instability driven by trapped electrons

An instability of a positron beam can be driven by electrons trapped in the combined average electric field of the beam and the magnetic field of the dipoles. An estimate of the growth rate is given. The growth rate of the instability is smaller than the growth rate of the fast ion instability by a factor 50 for the PEP-II LER parameters. Tendency to use long trains of bunches in storage rings intensified interest of stability of such trains. In the last year, two new instabilities were discovered: fast transverse instability, driven by one-turn ions for an electron storage ring, and a transverse instability driven by interaction of a positron beam with photo-electrons (Ohmi-effect). Cornell instability has been also explained as instability driven by the interaction of a beam with electrons trapped in the combined magnetic field of dipoles and the electrostatic field of distributed ion pumps (DIPs) leaking into the beam pipe. All these instabilities are caused by the effective wake fields defined by the charge density of the beam environment rather than due to geometric or resistive wall wake fields