Dust Analysis from LHC Vacuum System to Identify the Source of Macro-Particle-Beam-Interactions

Since in 2010 the first sub-millisecond beam losses were observed at varying locations all along the LHC, it is well known that dust can interact with high-intensity proton beams and cause significant beam losses. Initially the sudden localized losses were enigmatic and coined the phrase “unidentified falling objects” (UFOs), which is still widely used. These very fast beam losses have resulted in hundreds of premature beam dumps and even magnet quenches since the start of LHC. So far, the only mitigation strategy involved an optimization of dump thresholds and the beneficial conditioning effect which leads to a reduction of the UFO rate over time. To understand the physics involved in these events and to allow an active diminution, it is essential to know the chemical composition and the size of the dust particulates interacting with the protons. The exchange of a dipole magnet offered the unique opportunity to collect dust samples from inside the LHC vacuum system. They were extracted from the various components and analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reveal size distribution and abundant elements. The results of this investigation will optimize the existing UFO models and the improved understanding of the phenomenon may help to prevent future performance limitations. This is also of relevance for future projects, in particular for the Future Circular Collider (FCC) under study. INTRODUCTION TO MACRO PARTICLE-BEAM-INTERACTIONS Two types of macro-particle interactions with the LHC proton beams were observed. The most common interaction happens when a solid macro-particle (between 1 and 100 μm in radius) gets into the proton beam. This leads to elastic and inelastic scattering, ionization and finally to the ejection of the macro-particle from the beam [1, 2]. The corresponding beam losses can exceed the beam dump thresholds and even cause magnet quenches. A second type of macro-particle beam interactions was identified in 2017, when frozen gas caused localized beam losses in one location (16L2), which were accompanied by a rapidly developing beam instability [3–6]. The source is believed to be successfully removed. This is not the case for the first and most common type of UFO events. The hypothesis of solid macro-particles interacting with the high intensity proton beam was already confirmed by the observations at an injection kicker magnet ∗ laura.grob@cern.ch Figure 1: The available UHV components included the plugin modules (PIMs), beam screen (BS) and cold bore (CB) which are visible in the left part. Liquid-based particle extraction was used to prepare the samples suitable for electron microscopy (middle and right part). PROCEDURE OF DUST EXTRACTION AND ANALYSIS At the end of 2016, a dipole magnet was removed from the LHC. This gave the opportunity to collect dust samples from inside the LHC UHV system. By extracting and analysing the particulate contamination, two crucial parameters can be analysed, chemical composition and size distribution for UFO modelling. This information can help to understand the source of the contamination and prevent similar issues in future accelerators such as FCC. Before the mechanical removal of the dipole magnet, an RF ball test [8] was performed to verify the absence of obstructions in the vacuum chamber. A parasitic dust collection was performed during this procedure (results see Fig.2). Once the dipole magnet was removed, the open apertures of the neighbouring magnets were accessible for a positionsensitive wipe test. Dust extraction from the dipole’s vacuum components (four connecting plug-in modules (PIMs), two beam screens (BS) and cold bores(CB)) required a more complex sample preparation [9] as illustrated in Fig. 1. The dust analysis was performed by a dedicated software employing scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDX). A representative fraction of the collected dust was stamped onto a standardized carbon sticker (see Fig.1), which can be directly analyzed without additional treatment. To avoid conglomerates, the surface coverage of the filter substrate was inspected for its homogeneity by optical microscopy before the transfer. findings [7]. The origin of the macro-particles in all other parts of the LHC still lacks an explanation. Dust Collection with Vacuum Filter unit (VFU) After warm-up of sector 12, an RF ball travels through the vacuum chamber by an airflow transporting dust. A special filter unit (VFU) was installed in front of the vacuum pumps. 10th Int. Partile Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-MOPTS094