HL-LHC alternatives

Abstract

The HL-LHC parameters assume unexplored regimes for hadron colliders in various aspects of accelerator beam dynamics and technology. This paper reviews three alternatives that could potentially improve the LHC performance: (i) the alternative filling scheme 8b+4e, (ii) the use of a 200 MHz RF system in the LHC and (iii) the use of proton cooling methods to reduce the beam emittance (at top energy and at injection). The alternatives are assessed in terms of feasibility, pros and cons, risks versus benefits and the impact on beam availability. ALTERNATIVES AND MERITS This section introduces three alternatives to the HL-LHC baseline considered in this report together with their merits and weak points. Electron cloud effects in the HL-LHC era could seriously hamper the luminosity upgrade. Therefore special attention is put in the evaluation of electron cloud effects for the different alternatives. Filling scheme 8b+4e By performing a double splitting instead of triple splitting in the PS it is possible to generate fewer and more intense bunches. Basically a PSB bunch is split into 8 bunches rather than 12. The usual 12 bunch structure is preserved keeping 4 empty bucktes in between the microbatches of 8 bunches. For details on the generation of this scheme see [1]. Following the upgrade of the injector chain, the 8b+4e scheme would allow 1840 bunches to be injected into the LHC with 2.4e11 ppb if the LHC is filled without further changes to the bunch pattern. The outstanding merit of this alternative is the huge reduction of electron cloud effects plus the fact that this filling scheme can be implemented from 2015 without any cost (8b+4e bunch population in 2015 might be 1.6×1011 ppb). Figure 1 shows simulations of the heat load due to electron cloud per aperture in the LHC dipoles using the parameters as expected in 2015 for the baseline and for the 8b+4e scheme. A measurement of heat load during 2012 is shown in the figure as a pessimistic reference for tolerable levels of heat load. A large reduction factor in heat load thanks to the 8b+4e scheme is observed, allowing, in principle, operation with secondary emission yields as large as δmax ≈ 1.6. Considering HL-LHC parameters the 8b+4e scheme also generates considerably lower heat load than the nominal 25 ns scheme, yet it requires δmax 1.4, as illustrated in Fig. 2. 0.1 1 10 1.3 1.4 1.5 1.6 1.7 1.8 1.9 H ea t l oa d (W /m ) δmax 25ns (LHC post LS1) 25ns (LHC post LS1 with 4-bunch gaps) Measured HL at LHC (Fill #3429) Figure 1: Heat load versus maximum secondary emission yield due to electron cloud per aperture in the LHC dipoles using the parameters as expected in 2015 for the baseline and for the 8b+4e scheme. The inferred heat load from measurements in 2012 is also shown. During discussions in the RLIUP workshop on how to maximize the number of bunches in the LHC, a proposal was made to inject 7 instead of 6 PSB bunches into the PS. In the nominal filling scheme this would imply losing few (three or four) bunches at the end of the batch while extracting to the SPS, with the consequent transfer of trains made of 80 or 81 bunches. However, this option turned out to fit particularly well into the 8b+4e scheme, as 7 injections can be made from the PSB to the PS and no bunches would need to be removed at extraction thanks to the four empty buckets [2]. The SPS would be filled with the following bunch train structure: 4× (7× (8b + 4e) + 4e) + 572e (1) This optimized scheme produces more luminosity thanks to the larger number of bunches but also yields slightly larger heat load due to electron cloud, see Fig. 2. A filling pattern in the LHC has been prepared using this scheme [3] yielding 1960 colliding bunches in the main interaction points (120 more than for the initial 8b+4e). This optimized scheme is used in the rest of the paper. The feasibility and performance of the 8b+4e scheme should be experimentally assessed via beam tests starting in the LHC injector chain already in 2014. Published by CERN in the Proceedings of RLIUP: Review of LHC and Injector Upgrade Plans, Centre de Convention, Archamps, France, 29–31 October 2013, edited by B. Goddard and F. Zimmermann, CERN–2014–006 (CERN, Geneva, 2014) 978-92-9083-407-6, 0007-8328 – c © CERN, 2014. Published under the Creative Common Attribution CC BY 4.0 Licence. http://dx.doi.org/10.5170/CERN-2014-006.119 119