Abstract
Micrometer sized particles entering the LHC beam (the so called Unidentified Falling Objects or UFOs) are a known cause of localized beam losses since the beginning of high intensity beam operation, however the origin of these particles is not fully known. Their effect limits LHC availability by causing premature dumps due to excessive beam losses and occasionally even magnet quenches. This could become an important limitation for future accelerators such as the High Luminosity upgrade of the LHC (HL-LHC) and the Future Circular Collider (FCC). The dynamics of these UFOs was investigated in two dedicated experiments. In the first experiment, it was shown that the transverse movements of these particles can be studied by observing bunch-by-bunch losses from bunches with different horizontal and vertical emittances. In the second experiment, UFO-like events around the 16L2 interconnect in the LHC, which has seen intense UFO activity in 2017, were studied with the above method. This paper summarizes the results of both experiments.
Highlights
Unidentified Falling Objects (UFOs), believed to consist of micrometer sized macroparticles, have been known to cause localized beam losses resulting in premature dumps and magnet quenches since the beginning of high intensity beam operation of the LHC [1], their origin is yet to be understood
The UFO type 2 experiment showed that the macroparticle enters the beam in the vertical plane, but whether from the top or the bottom can not be distinguished by the chosen method
The LHC performance already suffered from UFO type 1 and type 2 events
Summary
Unidentified Falling Objects (UFOs), believed to consist of micrometer sized macroparticles, have been known to cause localized beam losses resulting in premature dumps and magnet quenches since the beginning of high intensity beam operation of the LHC [1], their origin is yet to be understood. Extrapolating from the current LHC beam energy, 6.5 TeV, to the nominal energy, 7 TeV, the expected number of UFO induced quenches could increase by up to a factor four [3]. Their origin, the dependence of the UFO rate on beam parameters and the mechanism for how they enter the beam is not well understood. To estimate the criticality of UFOs for the HL-LHC and future hadron accelerators, it is vital to understand their dynamics and the mechanism leading to the conditioning observed in the LHC This will allow countermeasures to be taken in future machines
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