Dust in accelerator vacuum systems

Many accelerators of electron beams are susceptible to persistent beam lifetime disruptions, with correspondingly reduced performance. One distinguishes between three prevailing explanations of these disruptions: (1) trapping of positive atomic ions in the negatively charged beam; (2) trapping of small highly positively ionised micro-objects ("dust") in the negatively charged beam; (3) disruptions due to stray magnetic objects trapped in the magnetic field of undulators. The lifetime disruption of certain electron storage rings that employ ion-getter pump systems are attributed by most researchers to explanation (2), the trapped dust hypothesis. Systematic experimental studies of HERA, PETRA and DORIS reinforce the suspicion that specifically this type of pump system is the culprit. Examples of beam lifetime disruptions are presented, together with a summary of observations and experiments performed at various afflicted storage rings to investigate dust trapping and the connection between ion getter pumps and dust particle release. Observations of the disrupted beam are found to agree with the dust trapping hypothesis.