Charging mechanisms and orbital dynamics of charged dust grains in the LHC

Dust grains interacting with the beam of particle accelerators are believed to be the cause of several detrimental effects such as beam losses, emittance growth, pressure bursts, and even quenches of superconducting magnets. Experimental observations suggest that these grains are positively charged in electron storage rings and negatively charged in the Large Hadron Collider (LHC). In this paper, the charging mechanisms for dust grains in the LHC are discussed and a possible explanation for the observed polarity is presented. Electron collection, secondary electron emission, and photoelectric emission are considered because of the presence of electron clouds and synchrotron radiation. It is found that the same mechanisms can explain both the positive grain polarity observed in electron storage rings and the negative polarity in the LHC. As a consequence of the charge acquired, the possibility of grains orbiting the beam is discussed. The orbital dynamics in a logarithmic potential is analyzed and critical parameters for describing such orbits are introduced. Finally, LHC beam losses attributed to beam-dust interactions with multiple loss peaks are presented. It is shown that they have an amplitude and a peak separation consistent with what can be expected for orbiting grains.