I am not sure but in 1996 someone accidentally left two beer bottles in the Large Electron Positron collider (LEP) at CERN, which was the largest particle accelerator at the time. The bottles blocked the beam so the particles could not go around the ring, but as far as know the bottles were not damaged.
particle accelerators have to kept in a very clean environment, they have to be kept under vacuum (all the air molecules have been sucked out) at all times to keep them as contaminat-free as possible or else the experiments won’t work properly, so if you put crisps in there you’d have like the whole of the facility on your back ha! but in real terms, the particle accelerators only have an effect on charged particles (like electrons or protons) and as far as I know, crisps are fairly neutral, insulators (non-conductive) so they won’t be affected by the electric fields of the accelerator (i don’t think so anyway)
When I was a student at the Tevatron our competitor experiment, CDF, managed to move part of their detector very close to the Tevatron beam; close enough that the beam was deflected and hit a tungsten collimator. The beam pretty much instantly burnt a hole through the tungsten (vaporised it), shutting down the accelerator. So it would make short work of a bag of crisps.
Like Arttu said, someone left a beer bottle inside LEP after construction. I do not think it was an accident – my understanding is there had been some issues over pay, and the beer itself was Heineken, which at the time had the slogan “reaches the parts other beers cannot reach.”