Photoemission Instabilities: Theory and Experiment

Synchrotron radiation in electron-positron colliders and synchrotron light sources ejects photoelectrons from the beam chamber walls. The resulting cloud of slow photoelectrons interacts with the beam, and may generate a transverse coupled bunch instability. In some high current storage rings a photoemission effect causes the dominant transverse instability, with a sub-millisecond risetime. Two types of photoemission instabilities have been observed. In CESR, photoelectrons are trapped by the combination of a quadrupole electrostatic field from the distributed ion pumps and the field of the bending magnets. This "trapped photoelectron instability" has a very long range, is nonlinear in beam current, and is predominantly horizontal. In the Photon Factory and BEPC, photoelectrons moving freely across the chamber interact with the positron beam to produce a transverse instability. This "free photoelectron instability" occurs for bunch-tobunch intervals smaller than the transit time (a few tens of nanoseconds) of photoelectrons across the chamber, occurs in the absence of external fields, is approximately linear in bunch current, and is predominantly vertical. Numerical simulations of both effects reproduce the observed behavior of the beam, as does an analytical calculation of the trapped photoelectron instability. Experiments to observe these instabilities in other storage rings are underway. There is both theoretical and observational evidence that other types of instabilities due to photoemission may be important. We will discuss the consequences of the photoemission instabilities for the design and operation of high current storage rings. 1 ELECTRON EMISSION PROCESSES The two main sources of electrons in the beam chamber of a high energy electron or positron storage ring are photoemission and secondary emission. Ionization of the residual gas is negligible source of electrons in comparison to these.