Abstract
In the past, transverse coherent instabilities have been observed at the Hadron-Electron Ring Accelerator (HERA) proton ring that were instigated by the presence of linear coupling. Linear coupling can also potentially explain some transverse instabilities that were observed in the Large Hadron Collider (LHC) in both run I and run II, however a detailed description of the destabilizing mechanism of linear coupling was not known at the time. A study into the effect of linear coupling on transverse beam stability was carried out, and a new mechanism that could incite transverse instabilities by causing a loss of Landau damping has been found. The study includes time domain simulations with PyHEADTAIL and frequency domain computations based on analytical approaches, and was then verified by measurements with a single proton bunch in the LHC.
Highlights
In run I of the Large Hadron Collider (LHC), many transverse coherent instabilities were observed at high energy (3.5 and 4 TeV) at the end of the betatron squeeze that were not fully explained [1]
Simulations have been performed with both particle tracking in the time domain and analytic computations in the frequency domain, which show that linear coupling can strongly perturb the tune spread acquired from the Landau octupoles
This can lead to transverse instabilities due to the loss of Landau damping
Summary
In run I of the LHC, many transverse coherent instabilities were observed at high energy (3.5 and 4 TeV) at the end of the betatron squeeze that were not fully explained [1]. In 2015, during run II of the LHC, instabilities were observed at injection energy (450 GeV) while the machine was being filled with around 2000 proton bunches for physics operation [2] During this process the horizontal and vertical tunes were drifting closer together due to the Laslett tune shift [3], and once the tune separation became too small emittance blowup was occurring. PYSSD requires as input the transverse amplitude detuning due to the lattice which is computed with single particle tracking using MAD-X [19] The results of this simulation study were able to be tested with controlled measurements with a single bunch in the LHC at an energy of 6.5 TeV.
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