Overview of the beams from the injectors

The injectors have delivered different beam types for luminosity production in the LHC during the 2017 run. Besides the nominal beam with 25 ns spacing and 72 bunches at PS extraction, the batch-compression-merging-splitting (BCMS) beam with multiples of 48 bunches at extraction from the SPS has been produced. The reduced number of bunches per batch from the PS is compensated by almost twice as smaller transverse emittance. The vacuum related issues in the LHC (16L2 cell) could be mitigated by switching to the so-called 8b4e beam, where mini-batches of 8 bunches are followed by 4 empty bunch positions in between. Thanks to the flexibility of the injectors, a higher brightness version of the 8b4e has been prepared to quickly react to the needs of the LHC. In this paper, an overview of the beams from the injector complex is given, describing how the beams are produced and summarizing their characteristics, achieved performance and specific limitations. In view of the operation in 2018, the expected beam parameters are presented, as well as a reminder of possible alternative beam types from the injectors. BEAM PRODUCTION SCHEMES All the accelerators in the LHC injector chain contribute to the definition of the beam parameters. The transverse emittance is initially defined at injection in the PS Booster (PSB) and increases linearly with the bunch intensity (brightness curve [1]). The beam pattern is then defined in PS, where rf manipulations are performed to split, merge and compress the beam. The versatility of the rf systems in the PS allows to produce various beam patterns and the rf manipulations used during the 2017 run are shown in Fig. 1. At extraction from the PS, the bunch spacing is 25 ns with the longitudinal emittance adjusted to εL = 0.35 eVs per bunch as a compromise for low capture losses and beam stability in the SPS. The nominal bunch intensity at PS extraction is Nb = 1.3 × 1011 protons per bunch (p/b). Finally, 1 to 4 batches are extracted from the PS to the SPS to maximize the number of bunches per injection into the LHC. An important limitation for beam brightness occurs at the transfer from the PSB to the PS. The longitudinal emittance extracted from the PSB should bemaximized to reduce space charge effects on the PS flat bottom [2]. However, the maximum bunch length for extraction from the PSB to the PS is limited by the rise time of the recombination kickers [3]. In addition, too large momentum spread leads to transverse emittance blow-up due to a known, and unavoidable with ∗ alexandre.lasheen@cern.ch h = 9 .. 1 4 m 7 1 4 2 1 h = 9 .. 1 4 .. 2 1 h = 7 → 2 1 25 ns BCMS 8b4e BC 8b4e standard h = 7 1 4 2 1 Standard h = 2 1 → 4 2 → 8 4