Trapped Macroparticles in Electron Storage Rings

Sudden drops of the beam lifetime ascribed to the capture of positively charged macroparticles (‘dust’) are observed at many high-energy electron storage rings. A trapped macroparticle has a certain thermal lifetime due to energy transfer from the beam; the interplay of different ionizing and discharging processes gives rise to an equilibrium charge. The beam lifetime is reduced by bremsstrahlung both in the field of the atomic nuclei and in the macroscopic field of the highly charged dust particle. Recent observations at some storage rings can be explained by the capture of a single macroparticle of about micron size, possibly made from silicon dioxide. In the two B factories under construction, PEP-II and TRISTAN-II, trapped macroparticles will be thermally unstable and explode after a few tens of microseconds, thanks to the much higher beam current as compared with other storage rings.