The Effect of a Partially Depleted Halo on the Criticality and Detectability of Fast Failures in the HL-LHC

In the High Luminosity LHC (HL-LHC) era, the bunch intensity will be increased to 2.2 $\times$ 10$^{11}$ protons, which is almost twice the nominal LHC intensity. The stored energy in each of the two beams will increase to 674 MJ. The HL-LHC will feature beams whose transverse halos are partially depleted by means of a hollow electron lens. The reduced stored energy in the beam tails will significantly change the development of losses caused by failures. This paper reports on beam tracking simulations evaluating the effect of a partially depleted halo on the criticality and detection of failures originating from the superconducting magnet protection systems. In addition, the effect of the transverse damper operating as a coherent excitation system leading to orbit excursions on a beam with a partially depleted halo is discussed. The results in terms of time-dependent beam losses are presented. The margins between the failure onset, its detection, and the time to reach critical loss levels, are discussed. The results are extrapolated to failure cases of different origins that induce similar beam loss dynamics.