The electron beam lifetime problem in HERA

The electron beam lifetime in the HERA storage ring is not only uniformly lower than the positron beam lifetime, but the electron beam lifetime curves display complicated structures not present in the smoothly behaved positron beam lifetime curves. Sum- maries of characterising quantities are presented for large num- bers of runs in 1993 and 1994, enabling the identification of trends in the beam lifetime behaviour.

The lifetime reduction seems to be due to many discrete life- time reduction events. The lifetime achieved at given current de- pends in a complicated (and stochastic) manner on the filled cur- rent, energy and integrated ion getter pump voltage. The proper- ties of the lifetime problem are overwhelmingly consistent with the capture of material by the electron beam (most likely dust particles of size 0.1-1?m) which – being then highly positively charged – scatters beam electrons. The source of this material, or the mechanism of its liberation from the chamber or pump walls, is not entirely understood.

Moves towards curing the electron beam lifetime problem are discussed.