Abstract
Micrometer-sized dust particles present in the Large Hadron Collider (LHC) beam pipe are believed to have caused many thousands of sporadic beam loss events around the LHC. These so-called unidentified falling objects (UFOs) have been under continuous study since the start of high intensity beam operation in the LHC due to their impact on the LHC availability: 139 beam dumps and 12 magnet quenches during Run II (2015--2018) alone. To mitigate the impact of UFOs on future accelerators such as the High Luminosity LHC and the Future Circular Collider, it is fundamental to foster a better understanding of these beam loss events. In this paper, key observations made since the start of LHC operation are summarized and the prevailing UFO hypothesis is confronted by a compilation of observations acquired during Run II. In particular, it is shown that UFOs must carry an initial negative charge before entering the proton beam, or that they are by some other means accelerated toward the beam not only by gravity. A simulation model for the dynamics of the dust particles and their interaction with the beam was developed over the years. This model is improved and validated by measurements. It is however also shown that a subset of observed beam losses, which contain a positive time profile skewness, cannot be explained by it.
Highlights
Spurious beam loss spikes distributed throughout the Large Hadron Collider (LHC) have been present ever since high intensity beam operations began [1]
In-depth analysis of the losses revealed that the initial spike was due to the LHC bunches interacting with unidentified falling objects (UFOs)-like solid matter, while the following beam losses were caused by transverse beam instabilities [23]
Out of the events detected by the diamond BLMs (dBLMs), one was of significantly better quality, with sufficient length and signal-to-noise ratio to perform a detailed analysis of the dynamics of the dust particle
Summary
Spurious beam loss spikes distributed throughout the LHC have been present ever since high intensity beam operations began [1] These events, believed to be caused by micrometer-sized dust particles, came to be known as unidentified falling objects (UFOs). The different types of UFOs that have been observed are discussed This is followed by an explanation of the current hypothesis, which is tested using the UFO dynamics simulation tool together with beam loss measurements of overall time profiles, as well as losses arising from individual bunches. The latter allows calculating the dust particle position in relation to the beam center on a turn-by-turn basis. This is consistent with the dust inspection done on an MKI, removed from the accelerator in 2010, where more than five million particles with radii up to 100 μm were found in a full MKI tank [12]
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