Synchrotron light sources like the Advanced Photon Source (APS) (as well as SPring-8 and the ESRF) are typically mentioned as accelerating electrons up to the GeV range to produce extremely high brilliance X-ray beams. For instance, the APS's site says:
Electrons are injected into the booster synchrotron, a racetrack-shaped ring of electromagnets, and accelerated from 450 MeV to 7 billion electron volts (7 GeV) in one-half second. (By comparison, the electron beam that lights a TV screen is only 25,000 electron volts.) The electrons are now traveling at >99.999999% of the speed of light. [emph added]
However, I came across this paper about the APS's list of parameters and it constantly mentions positrons, not electrons, and even gives a rationale:
The Advanced Photon Source (APS) is a third-generation synchrotron radiation source that stores positrons in a storage ring. The choice of positrons as accelerating particles was motivated by the usual reason: to eliminate the degradation of the beam caused by trapping of positively charged dust particles or ions.
So, what does it use? Can it use both? And why does the charge of the residual(?) or otherwise unintended charges ("trapped dust particles or ions") matter if they're positive or negative (wouldn't either an attractive force or repulsive force between some errant charge and the beam cause problems)?