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 well-approximated analytical formula for the low-frequency range of multi-layered beam screens is presented.
Highlights
In the framework of the High-Luminosity Large Hadron Collider (HL-LHC) project, numerous components of the accelerator will be upgraded during the third LHC long shutdown [1,2]
The new beam screen is an octagonally-shaped pipe made of high nitrogen–high manganese stainless steel (P506) [20] whose main function is to shield by means of tungsten-based inserts the superconducting magnets from debris coming from the collisions, screening the cold mass from beaminduced heating, and ensuring the vacuum levels required for the beam lifetime
The results presented in this paper take into account the magnetoresistivity of copper, whose variation of electrical resistivity due to the magnetic field is not negligible in comparison with the other materials of the beam screen
Summary
In the framework of the High-Luminosity Large Hadron Collider (HL-LHC) project, numerous components of the accelerator will be upgraded during the third LHC long shutdown [1,2]. The HL-LHC studies revealed that a tune modulation due to power converter ripple in the quadrupoles in the range of 10−5 to 10−6 for specific frequencies lead to a visible decrease of the dynamic aperture and an expected degradation of the beam lifetime, see Fig. 3. Those studies approximated the magnet circuit with a simple RL-series circuit, but did not include the impact of the beam screen shielding that could change the figures on power converter noise tolerances.
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