Abstract
We present the results of numerical simulations and experimental studies about the effects of resonant and random excitations on proton losses, emittances, and beam distributions in the Large Hadron Collider (LHC). In addition to shedding light on complex nonlinear effects, these studies are applied to the design of hollow electron lenses for active beam halo control. In the High-Luminosity Large Hadron Collider (HL-LHC), a considerable amount of energy will be stored in the beam tails. To control and clean the beam halo, the installation of two hollow electron lenses, one per beam, is being considered. In standard electron-lens operation, a proton bunch sees the same electron current at every revolution. Pulsed electron beam operation (i.e., different currents for different turns) is also considered, because it can widen the range of achievable halo removal rates. For an axially symmetric electron beam, only protons in the halo are excited. If a residual field is present at the location of the beam core, these particles are exposed to time-dependent transverse kicks and to noise. We discuss the numerical simulations and the experiments conducted in 2016 and 2017 at injection energy in the LHC. The excitation patterns were generated by the transverse feedback and damping system, which acted as a flexible source of dipole kicks. Proton beam losses, emittances, and transverse distributions were recorded as a function of excitation patterns and strengths. The resonant excitations induced rich dynamical effects and nontrivial changes of the beam distributions, which, to our knowledge, have not previously been observed and studied in this detail. We conclude with a discussion of the tolerable and achievable residual fields and proposals for further studies.
Highlights
In circular accelerators and storage rings, beam quality can be affected by the interplay of external excitations with machine lattice
This work, through calculations and experiments, focuses on how a certain class of resonant excitations influences beam dynamics and to which extent these excitations can cause beam losses, emittance growth, or changes in the particle beam distributions. Besides their general relevance to the topic of complex nonlinear dynamics, these studies were motivated by the need to assess the effects of a pulsed hollow electron lens for active beam halo control
Recent measurements at the Large Hadron Collider (LHC) show that the tails of the transverse beam distribution are overpopulated compared to a Gaussian distribution
Summary
In circular accelerators and storage rings, beam quality can be affected by the interplay of external excitations with machine lattice. Recent measurements at the LHC show that the tails of the transverse beam distribution are overpopulated compared to a Gaussian distribution. This results in a considerable amount of energy being stored in the beam tails. In the case of the LHC, about 5% of the beam population is stored in the tails (i.e., above 3.5σ, where σ is the standard deviation of the Gaussian beam core), compared to 0.22% in an ideal Gaussian distribution, leading to 19 MJ of stored energy for nominal LHC parameters and 34 MJ in the case of HL-LHC [1] This leads to the conclusion that a mechanism is needed to deplete the beam tails in a controlled manner. Further information on the needs for halo control in LHC can be found in Ref. [2]
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