Magnetic frequency response of High-Luminosity Large Hadron Collider beam screens

Abstract

Magnetic fields used to control particle beams in accelerators are usually controlled by regulating the electrical current of the power converters. In order to minimize lifetime degradation and ultimately luminosity loss in circular colliders, current-noise is a highly critical figure of merit of power converters, in particular for magnets located in areas with high beta-function, like the High-Luminosity Large Hadron Collider (HL-LHC) insertions. However, what is directly acting upon the beam is the magnetic field and not the current of the power converter, which undergoes several frequency-dependent transformations until the desired magnetic field, seen by the beam, is obtained. Beam screens are very rarely considered when assessing or specifying the noise figure of merit, but their magnetic frequency response is such that they realize relatively effective low pass filtering of the magnetic field produced by the system magnet-power converter. This work aims at filling this gap by quantifying the expected impact of different beam screen layouts for the most relevant HL-LHC insertion magnets. A well-defined postprocessing technique is used to derive the frequency response of the different multipoles from multiphysics finite element method (FEM) simulation results. In addition, a wellapproximated analytical formula for the low-frequency range of multi-layered beam screens is presented.

Fast failures in the LHC and the future high luminosity LHC

“…It should, thus, play a role for both QH and CLIQ discharges, delaying their effects on the beam. The attenuation solely due to the beam screen for frequencies at 1 kHz has been calculated to be 15 dB [38]. These calculations were based on periodically varying the current in a magnet, whereas the QH field rises in ∼40 μs and then decays with a time constant of ∼10 ms. Measurements [36] of the kick seen by the beam due to QH discharges in the LHC, imply a significantly weaker shielding than expected from [38].…”

“…The attenuation solely due to the beam screen for frequencies at 1 kHz has been calculated to be 15 dB [38]. These calculations were based on periodically varying the current in a magnet, whereas the QH field rises in ∼40 μs and then decays with a time constant of ∼10 ms. Measurements [36] of the kick seen by the beam due to QH discharges in the LHC, imply a significantly weaker shielding than expected from [38]. In the measurements, the field seen by the beam reached 75% of the externally applied field in two to three turns, followed by a slower rise of about 20 turns to reach the maximum.…”

Modeling of the emittance growth due to decoherence in collision at the Large Hadron Collider

“…On the other hand, the contribution from the rest of the machine remains unclear. Since the LHC's first betatron line is at approximatively 3 kHz, the ground motion is naturally suppressed [26] and most of the contributions from power converter ripple are expected to be heavily attenuated by the colaminated copper layer of the beam screen [27]. Studies of the mechanical vibrations of the beam screen due to the helium flow in the cooling capillaries and the resulting field variations in the vacuum chamber suggest that this contribution could yield a noise amplitude of the right order of magnitude [28].…”

Noise effects in the large hadron collider (LHC) and its high-luminosity upgrade (HL-LHC)

“…If both clusters emerge from a common source, the question that arises is what is the mechanism that allows these high-frequency components to excite the beam. In fact, oscillations at such high frequencies are expected to be signicantly attenuated by the shielding eect of the beam screen in the dipole magnets [101]. A review of the power converter's spectrum in sector 1-2 (Fig.…”

“…4.23a shows that the noise in the power converter voltage spectrum attenuates with increasing frequency. Furthermore, high-frequency perturbations such as the high-frequency cluster strongly exceed the cuto frequency of the LHC main dipoles due to the shielding eect of the beam screen [101]. To this end, if the high-frequency cluster is driven by a direct excitation due to power Additionally, it should be mentioned that the increase of the noise by a factor of two in Fill 7035 did not lead to an increase of losses or emittance growth compared to the rest of the lls.…”