Does the Advanced Photon Source use electrons or positrons? 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)?
 A: From reading various sources, it seems that the main synchrotron at APS is capable of carrying either an electron or positron beam (for example, the introduction of this paper)
I gather that the positrons are created by colliding an electron beam from a LINAC with a target, but the target can also be withdrawn to allow the electrons to go directly into the booster synchrotron, then into the main synchrotron.
I haven't found any recent literature that mentions positrons, which makes me think that they mainly use electrons - I don't know why.

As for your other question, I think they are mostly concerned with dust directly impinging on the beam, thus scattering the beam.  Most dust will have a slightly positive charge, so an electron beam will attract the dust into the middle of the beamline, which is undesirable.  A positron beam would do the opposite, providing a repulsive force keeping dust away from the beam.
A: Light sources usually store an electron beam. The reason is very simple: getting electrons is extremely easy! You can just heat a piece of metal (thermionic guns), or you can shoot a laser on a photo-cathode. If you want a positron beam you need to produce and capture it converting a primary beam. The yield of this process is not very big and you end up with a much weaker beam.
Could we store positrons in an electron machine? The answer is yes: we can just inject them in the opposite direction (but we need a dedicated transfer line and all the light lines are now in the wrong direction!) or we can switch the polarity of the magnets. A positron beam radiates exactly in the same way as an electron beam, so the physics is mostly the same except for one effect: the ion/electron cloud.
In the chamber there are always some residual gas molecules. When the beam hits them we get ions which are very heavy and slow, and free electrons, witch are light and fast. If we have a negatively charged beam, the electrons are repelled and the ions are captured, on the other hand, a positive beam will expel the ions and attract the electrons. Since the ions are slower they persists more and in principle have more chances to build up, so a negatively charged beam looks worse.
However note that this is not necessary true in all the conditions. Low energy electrons can also be emitted by the surface of the vacuum chamber, when this is hit by photons or low energy ions/electrons, and this may make the electron cloud more severe than the ion cloud! For instance Da$\Phi$ne, at the INFN Laboratories of Frascati, is an $e^+\;e^-$ collider with two separated beam pipes. There the positron current is limited by the electron cloud, more than the electron current is limited by the ion cloud. The LHC also had quite many issues with the electron cloud in the past run. However while it is generally possible to mitigate the electron cloud by adding weak solenoids or cleaning electrodes, not much can be done for the ions.
This is not trivial at all and a detailed study should be performed in order to confirm the rationale that you have found. Since all the literature that I have found about positrons in the APS is quite dated, my guess is that they initially planned to operate with positrons, but then came back to electrons.
