![]() A material often used for magnetic shielding is mu-metal. ![]() The proposed mitigation technique for CLIC is to shield the beam from stray magnetic fields. Figure 4: (Left) Power spectral density and (right) correlation with respect to s=30 m of the magnetic fields measured at point 5 of the Large Hadron Collider. ![]() Figure 3: Magnetic field measurement at point 5 of the Large Hadron Collider. Therefore, a dedicated mitigation system for stray magnetic fields is required. With a feedback system that corrects the beam trajectory, the luminosity loss is reduced to 15%, which is still a significant degradation in performance. These simulations show without any mitigation an LHC-like magnetic field in CLIC, would lead to a luminosity loss of 43%, which would be disastrous. Using these measurements, a model was developed which could be used to simulate the impact of stray magnetic fields in CLIC further details can be found in. This measurement contains stray magnetic fields from all possible sources. Figure 4 shows the power spectrum and correlation at different locations. Figure 3 shows a measurement at point 5 in the Large Hadron Collider (LHC) tunnel. To characterise stray magnetic fields from accelerator elements, such as power cables, magnets, RF systems, ventilation systems, etc., measurements were taken in the vicinity of a live accelerator. By operating at this repetition frequency, each beam pulse sees the same stray magnetic field from the electrical grid, which means it appears as if it is static and its impact can be removed. Figure 2: Power spectral density of the magnetic field measured at the Prevessin, CERN site.Ī design choice of CLIC is the repetition frequency, which was chosen to be 50 Hz to minimise the impact of stray magnetic fields from the electrical grid. Figure 1: Magnetic field measurement near the sub-power station on the Prevessin, CERN site. Such a spectrum is typical of stray magnetic fields from technical equipment powered by the European electrical grid. There are clear peaks at harmonics of 50 Hz along with additional smaller peaks due to amplitude modulation of the 50 Hz harmonics. Figure 2 shows the power spectrum of the magnetic field. The Prevessin site contains a sub-power station. To characterise stray magnetic fields from the electrical grid, the ambient magnetic field on the Prevessin, CERN site (shown in Figure 1) was measured. Therefore, the Earth’s magnetic field is not a dangerous source. Stray magnetic fields from the Earth’s magnetic field tend to have a slow temporal variation, which makes it possible to directly correct the beam under the inference of the Earth’s field. Efforts were made to characterise each of these sources through measurements. Stray magnetic field sources in an accelerator environment include the Earth’s magnetic field, nearby electrical infrastructure, such as power lines/power stations, and accelerator elements. The proposed mitigation system for CLIC is a mu-metal magnetic shield, which can prevent the stray magnetic fields from reaching the beam. Therefore, a mitigation system will be essential. The magnetic fields measured were several orders of magnitude greater than the sub-nT tolerance for CLIC. Recently, a measurement campaign was undertaken in collaboration with a Hungarian geological institute to characterise the ambient magnetic field on the CERN site. This leads to a sensitivity to sub-nT external dynamic (stray) magnetic fields, which can deflect the beams and lead to a relative offset at collision. The Compact Linear Collider (CLIC) targets nm beam sizes at collision.
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