LHC: The world's largest vacuum systems being operated at CERN

Chapter 2 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary Design Report. The Large Hadron Collider (LHC) is one of the largest scientific instruments ever built. Since opening up a new energy frontier for exploration in 2010, it has gathered a global user community of about 7,000 scientists working in fundamental particle physics and the physics of hadronic matter at extreme temperature and density. To sustain and extend its discovery potential, the LHC will need a major upgrade in the 2020s. This will increase its luminosity (rate of collisions) by a factor of five beyond the original design value and the integrated luminosity (total collisions created) by a factor ten. The LHC is already a highly complex and exquisitely optimised machine so this upgrade must be carefully conceived and will require about ten years to implement. The new configuration, known as High Luminosity LHC (HL-LHC), will rely on a number of key innovations that push accelerator technology beyond its present limits. Among these are cutting-edge 11-12 tesla superconducting magnets, compact superconducting cavities for beam rotation with ultra-precise phase control, new technology and physical processes for beam collimation and 300 metre-long high-power superconducting links with negligible energy dissipation. The present document describes the technologies and components that will be used to realise the project and is intended to serve as the basis for the detailed engineering design of HL-LHC.

Machine Layout and Performance

NEG coating deposition and characterisation of narrow-gap insertion devices and small-diameter chambers for light sources and particle accelerators

Based on the 2011 experience and Machine Development study results the performance reach of the LHC with 25 and 50 ns beams will be addressed for operation at 3.5 and 4 TeV. The possible scrubbing scenarios and potential intensity limitations resulting from vacuum, heating will be taken into account wherever possible. The paper mainly covers the performance of the two high luminosity regions in IR1 and IR5. 2011 PEAK PERFORMANCE Table 1 summarizes the main machine and peak beam parameters achieved at the end of 2011 and the corresponding peak performance. A peak luminosity of 3.6×10 33 cm -2 s -1 has been exceeded thanks to:  the excellent quality of the beams delivered by the injectors with a brightness exceeding the nominal values by 75% allowing to inject beams with almost 1.5x10 11 p/bunch in transverse emittances of 2 m [1];  the large number of bunches with 50 ns spacing that could be accumulated as a result of the successful scrubbing run [2];  the good control of the collective effects related to beam-beam, electron cloud and impedance [2,3,4];  the reduction of * down to 1 m in the high luminosity regions [5];  the excellent performance of the collimation system allowing to handle safely the corresponding impressive beam energy of up to 115 MJ [6];  last but not least, the good availability and reliability of all the machine systems [7]. The evolution of the transverse emittance in collision (estimated from the average peak luminosity data from ATLAS and CMS and from the average bunch population) as a function of the average bunch population is shown in Fig. 1 for the physics fills with 50 ns beams following the suppression of the controlled transverse emittance blow-up in the SPS. A part from a few physics fills (red markers in Fig. 1) with larger transverse emittance coming from the injectors (resulting from the blow-up in the transfer from PSB and PS) the emittance of the beams in collision follows a linear trend as a function of the bunch population (blue markers). The transverse emittance in collision for a bunch population of 1.5×10 11 p is 2.6 m to be compared with a transverse emittance of approximately 1.8 m at extraction from the SPS. An important blow-up (35 to 40 %) is therefore taking place in the LHC. This is partly occurring at the injection plateau and during the ramp and squeeze [8]. The origin of the observed blow-up is not known. It must be noted that in the two fills (2030 and 2032) a reduction of the blow-up by almost 20% has been measured but no motivation could be found for that. The linear fit to the data (blue markers in Fig. 1) is given by:  * coll [m]=1.900×10 -11 Nb 0.2956 (1) Momentum [TeV/c] 3.5 * [m] IP1/2/5/8 1/10/1/3 coll*(start fill) [m] 2.6 Half Ext. Crossing angle cross IP1/2/5/8 [rad] 120/80/120/250 Max. Bunch Population Nb [10 11 p] 1.49 Max. Number of bunches 1380 Max. Brightness [10/m] 0.64 Max. Number of colliding pairs 1331/0/1331/1320 Full bunch length (4 )[ns]/ (r.m.s.) [cm] 1.25 / 9.4 Max. Beam Current [A]/population[10 p] 0.37 / 2.05 Max. Stored energy [MJ] 115 Peak luminosity Lpeak [10 33 cms] in IP1/5 3.6 Lpeak/coll. pair [10 30 cms] in IP1/5 2.7 Beam-beam tune shift  (start fill)/IP ~0.007 Min. beam-beam separation dsep [] 9.3 Measured Luminosity lifetime [h] 20 Average pile-up at IP1/5 (start fill) <PU>peak 17/17 Table 1. Peak performance reached in 2011 with 50 ns beam. Figure 1. Transverse emittance in collision vs. bunch population for the physics fills with 50 ns beams from number 1986 to 2267 (included). 2011 INTEGRATED PERFORMANCE Figure 2 shows the evolution of the peak luminosity and of the number of colliding bunches in IR1/5 during the run 2011. Figure 2. Evolution of the peak luminosity and of the number of the colliding bunches in IR1/5 during the 2011 run. The following operational phases can be identified during the run 2011: i. physics with 75 ns beams with up to 200 bunches (194 colliding in IP1/5) with a * of 1.5 m in IP1/5; ii. operation with 50 ns beams (* of 1.5 m) and an increasing number of bunches (up to 768, 700 of which colliding in IP1/5) with population of ~1.2×10 11 p following the successful scrubbing run with 50 ns beams proposed in Chamonix 2011 [9]; iii. operation at high total intensity (up to 1380 bunches, 1318 of which colliding in IP1/5) and bunch population of ~1.2×10 11 p; iv. adiabatic reduction of the transverse emittance by suppression of the controlled transverse blow-up applied in the SPS and increase of the bunch population up to ~1.3×10 11 p; v. reduction of * to 1 m in IP1/5 and increase of the bunch population up to ~1.5×10 11 p. The integrated luminosity during a given period is determined by the peak luminosity but also by other parameters like the efficiency for physics (i.e. the fraction of time spent in stable beams with respect to the corresponding scheduled physics time, in the following called Stable Beam fraction) and by the luminosity lifetime. The convolution of the stable beam fraction and of the effect of the luminosity lifetime can be represented in the so-called Hübner factor. A “sliding” Hübner factor HFi can be defined for every fill leading to stable beams as:

/pdf/performance-reach-in-the-lhc-for-2012-90txpvgpl6.pdf

Performance Reach in the LHC for 2012

/pdf/space-charge-compensation-and-electron-cloud-effects-in-2jzbfglo22.pdf

Space Charge Compensation and Electron Cloud Effects in Modern High Intensity Proton Accelerators

The bake-out and activation of the TiZrV NEG coatings of the 6 km Long Straight Sections (LSS) of the Large Hadron Collider (LHC) is in its final step. After the bakeout and the NEG activation, the average ultimate pressure, over more than one hundred vacuum sectors, is below 10 -10 Pa. Therefore, the nominal requirement for the four experimental insertions is fulfilled. The nominal performances are also ensured for all the insertions housing the collimator systems, the RF cavities and the beam dumping systems. The main difficulties encountered during the bake-out and the activation of the NEG coated chambers is presented and discussed. In particular, the acceptance test and the limiting factors of the reached ultimate pressures are addressed. Furthermore, the influence on the ultimate pressures of the beam components (collimators, beam instrumentation, etc.) is discussed. Finally, preliminary results obtained from a NEG vacuum pilot sector installed in the laboratory and dedicated to the evaluation of the NEG performance is presented.

/pdf/achievement-and-evaluation-of-the-beam-vacuum-performance-of-n7ed9o5mj9.pdf

Achievement and Evaluation of the Beam Vacuum Performance of the LHC Long Straight Sections

Beam-induced RF heating has been observed in several LHC components when the bunch/beam intensity was increased and/or the bunch length reduced. In particular eight bellows, out of the ten double-bellow modules present in the machine in 2011, were found with the spring, which should keep the RF fingers in good electrical contact with the central insert, broken. Following these observations, the designs of all the components of the LHC equipped with RF fingers have been reviewed. The lessons learnt and mitigation measures are presented in this paper.

/pdf/lessons-learnt-and-mitigation-measures-for-the-cern-lhc-3pxqbz0b6u.pdf

Lessons learnt and mitigation measures for the cern lhc equipment with rf fingers

During the LHC run 2011, a tremendous progress has been made towards the machine operation with design parameters. In the same time, the run confirmed the sensitivity of the beam vacuum system to the machine parameters. As expected, a successful scrubbing period allowed mitigating the effects of the electron cloud giving room to an entire filling of the ring with 50 ns beams. In parallel issues such as the impact of the beam screen regulation, pressures spikes and local outgassing were observed during the year. On-line mitigations and immediate compensatory measures implemented during the winter technical stop are reviewed together with their efficiencies. The expected limitations while waiting for LS1 consolidation or when running with 25 ns beams are addressed. Lessons for 2012 are discussed.

/pdf/vacuum-performance-and-lessons-for-2012-2jd9wdfeyr.pdf

Vacuum performance and lessons for 2012

The room temperature vacuum system of the Large Hadron Collider (LHC) at CERN has been designed to ensure vacuum stability and beam lifetime of 100 h with nominal current of 0.56 A per beam at 7 TeV of energy. The requirements for the interaction regions are moreover driven to minimize the background noise of the experiments, to keep the equivalent hydrogen gas density below 10 13 molecules of H2 per m 3 . During the last two years, the LHC operated with proton beams at a maximum energy of 4 TeV, coasting for several hours each time, causing vacuum pressure increase owing to different effects: synchrotron radiation, electron cloud and localized temperature increase due to high order modes (HOMs). All these phenomena liberated an important gas load from the vacuum chamber walls, which led in some cases to a partial or a total saturation of the NEG coating. To match the design vacuum performances and hence to schedule technical interventions for NEG vacuum reactivation, it is necessary to take into account all these aspects and to regularly evaluate the saturation level of the NEG coating. This study analyses the saturation level of the NEG coated beam pipes in the LHC accelerator. Pressure reading variation of the Bayard-Alpert gauges without circulating proton beams are analysed and combined with laboratory studies of the NEG saturation behaviour. In addition, Vacuum Stability Code (VASCO) simulations are used to define the gas density profile change due to the saturation process in the NEG coated beam pipes.

/pdf/evaluation-of-the-neg-coating-saturation-level-after-3-years-3122nw68yx.pdf

Evaluation of the neg coating saturation level after 3 years of lhc beam operation

At the beginning of 2013 the LHC accelerator stopped for the Long Shutdown (LS1) by the need to consolidate the magnets interconnects. During this period of time, despite the very good performances of the beam vacuum system during the 2010-2012 physic run, different activities were held in parallel by the VSC group so as to consolidate, improve and upgrades some dedicated area of the LHC accelerator. As example a campaign aiming the consolidation of some RF bridges was conducted, NEG coated inserts were installed as a permanent electron cloud multipacting suppressor in critical locations and boosting of pumping speed by the introduction of compact NEG cartridges were performed in special devices. In addition consolidation of different beam equipment such as collimators, BGI, BSRT, BQS, installation of news TCDQ and MKB to name some, were carried out. In this paper a review of the main consolidations carried out during the LS1 in the beam vacuum system of the LHC are presented and discussed. Their impacts for the future operation are presented and finally a restart expected scenario for the LHC beam vacuum system is described.

/pdf/lhc-vacuum-system-upgrade-during-long-shutdown-1-and-vacuum-hg1yvhdeyx.pdf

LHC vacuum system upgrade during long shutdown 1 and vacuum expectation for the 2015 operation restart

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