R Tomas

HOW TO MAXIMIZE THE HL-LHC PERFORMANCE *

This contribution presents an overview of the parameter space for the HL-LHC [1] upgrade options that would maximize the LHC performance after LS3. The analysis is assuming the baseline HL-LHC upgrade options including among others, 25ns spacing, LIU [2] parameters, large aperture triplet and matching-section magnets, as well as crab cavities. The analysis then focuses on illustrations of the transmission efficiency of the LIU beam parameters from the injection process to stable conditions for physics, the minimization of the luminous region volume while preserving at the same time the separation of multiple vertices, the luminosity control mechanisms to extend the duration of the most efficient data taking conditions together with the associated concerns (machine efficiency, beam instabilities, halo population, cryogenic load, and beam dump frequency) and risks (failure scenarios, and radiation damage). In conclusion the expected integrated luminosity per fill and year is presented.

LIU: EXPLORING ALTERNATIVE IDEAS

The baseline upgrade scenarios for the injector complex cover the connection of Linac4 to the PSB, the increase of the PSB-PS transfer energy from 1.4 GeV to2 GeV and the major SPS RF upgrade during LS2. The achievable beam characteristics will nonetheless remain below the expectation of the HL-LHC project. Therefore, alternative or additional options like, e.g., special bunch distributions, the use of injection optics optimized for high space charge or extra RF systems will be discussed. The expected beam parameters, possible implementation and impact on beam availability for these more exotic options will be analysed and compared to the LIU baseline plan. Moreover, the potential interest of further batch compression schemes will be evaluated.

PICs: what do we gain in beam performance

The beam parameters in the LHC resulting from the Performance Improvement Consolidation (PIC) activities presented in (1)(2) will be briefly recalled and motivated assuming that LINAC4 will be operational as PS-Booster Injector. The corresponding limitations in the LHC are outlined. Based on the above performance an estimate of the LHC yearly integrated luminosity will be provided. The evaluation of the need and extent of the performance and reliability improvement for some of the PIC items might imply additional information: the necessary machine studies and the specific operational experience required during Run 2 will be summarized.

HL-LHC Alternative Scenarios

The HL-LHC parameters assume unexplored regimes for hadron colliders in various aspects of accelerator beam dynamics and technology. This paper reviews the possible alternatives that could potentially improve the HLLHC baseline performance or lower the risks assumed by the project. The alternatives under consideration range between using flat beams at thexa0 IP, compensate the long-range beam-beam encounters with wires and adding new RF cavities with larger or lower frequencies with respect to the existing RF system.

HL-LHC alternatives

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

IOP : Experimental validation of the Achromatic Telescopic Squeezing (ATS) scheme at the LHC

The Achromatic Telescopic Squeezing (ATS) [1] scheme offers new techniques to deliver unprecedentedly small beam spot size at the interaction points of the ATLAS and CMS experiments of the LHC, while perfectly controlling the chromatic properties of the corresponding optics (linear and non-linear chromaticities, off-momentum beta-beating, spurious dispersion induced by the crossing bumps). The first series of beam tests with ATS optics were achieved during the LHC Run I (2011/2012) for a first validation of the basics of the scheme at small intensity. In 2016, a new generation of more performing ATS optics was developed and more extensively tested in the machine, still with probe beams for optics measurement and correction at β∗ = 10 cm, but also with a few nominal bunches to establish first collisions at nominal β∗ (40 cm) and beyond (33 cm), and to analysis the robustness of these optics in terms of collimation and machine protection. The paper will highlight the most relevant and conclusive results which were obtained during this second series of ATS tests.

The HL-LHC Machine

The performance of the HL-LHC machine is boxed in between the request for a high integrated luminosity (ca. 3000 fbuf02d1 by the end of the HL-LHC exploitation over ca. 10 years of operation and translating to an annual integrated luminosity of ca. 250 fbuf02d1 assuming scheduled 160 days for proton physics production per year and that the HL-LHC exploitation starts with an integrated luminosity of ca. 300 fbuf02d1 at the end of the LHC Run III in 2022) and a maximum number of 140 events per bunch crossing. While the request for maximum integrated luminosity asks for the largest possible peak luminosity, the request for limited number of events per bunch crossing limits the peak luminosity to a maximum value of ca. 5·1034 cmuf02d2suf02d1. Operating the HL-LHC with the maximum number of bunches and utilizing luminosity leveling provides the best compromise for satisfying both requests. Table 1 shows the resulting baseline parameters approved by the HL-LHC Layout and Parameter Committee [1] for the standard 25 ns bunch spacing configuration together with the parameters for the nominal LHC configuration and two alternative scenarios which might become interesting in case the LHC operation during Run II reveals problems either related to the

Correction of multipolar field errors in insertion regions for the phase 1 LHC upgrade and dynamic aperture

The Phase 1 upgrade of the LHC interaction regions aims at increasing the machine luminosity by reducing the beam size at the interaction point. This requires an in-depth review of the full insertion region layout and a large set of options have been proposed with conceptually different designs. This paper reports on a general approach for the compensation of the non-linear field errors of the insertion region magnets by means of dedicated correctors. The goal is to use the same correction approach for all the different layouts. The correction algorithm is based on the computation of the high orders of the polynomial transfer map using MAD-X and Polymorphic Tracking Code, while the actual performance of the method is estimated by computing the dynamic aperture of the layouts under study.