Luminosity Optimization for a Higher-Energy LHC

Journal of Instrumentation

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 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 undergo a major upgrade in the 2020s. This will increase its rate of collisions by a factor of five beyond the original design value and the integrated luminosity by a factor ten. 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 T superconducting magnets, including Nb 3 Sn-based magnets never used in accelerators before, compact superconducting cavities for longitudinal beam rotation, new technology and physical processes for beam collimation. The dynamics of the HL-LHC beams will be also particularly challenging and this aspect is the main focus of this paper.

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By measuring and adjusting the β-functions at the interaction point (IP) the luminosity is being optimized. In LEP (Large Electron Positron Collider) this was done with the two closest doublet magnets. This approach is not applicable for the LHC (Large Hadron Collider) and RHIC (Relativistic Heavy Ion Collider) due to the asymmetric lattice. In addition in the LHC both beams share a common beam pipe through the inner triplet magnets (in these region changes of the magnetic field act on both beams). To control and adjust the β-functions without perturbation of other optics functions, quadrupole groups situated on both sides further away from the IP have to be used where the two beams are already separated. The quadrupoles are excited in specific linear combinations, forming the so-called "tuning knobs" for the IP β functions. For a specific correction this knob is scaled by a common multiplier. The different methods which were used to compute such knobs are discussed: (1) matching in MAD, (2) inversion and conditioning of the response matrix by singular value decomposition, and (3) conditioning the response matrix by multidimensional minimization using an adapted Moore Penrose method. For each accelerator, LHC and RHIC, a set of knobs was calculated and the performance compared. In addition the knobs for RHIC were successfully applied to accelerator. Simultaneously this approach allows us theoretically to measure the beam sizes of both colliding beams at the IP, based on the tuneability provided by the knobs. This possibility was investigated. The standard method for LEP to measure the IP β-functions was adapted and advanced to the asymmetric LHC lattice. First of all I would like to thank my wife Eva and two sons, Maxi and Martin, for their love, support and understating during these years of my thesis. The confidence and patience they spent me to face the sometimes difficult time. I would like to thank my university supervisor Univ. Prof. Dr. Bernhard Schnizer of the Institute fuer Theoretische Physik, TU-Graz, who encouraged me, despite of all circumstances, to start and carry out this thesis, for his help and faith. My CERN supervisor Dr. Frank Zimmermann, of the AB Department, Accelerator and Beam Physics group, for his guidance of this thesis, and the confidence in me and my ideas. With their assistance, patience and many fruitful discussions they made this time a great experience. I would also like to thank other members, especially Andre Verdier and Daniel Schulte, who always had time to discuss sometimes crazy ideas. This work would not have been possible without the help of the many members of this group. CERN CERN, from the French "Conseil Europèen pour la Recherche Nuclaire" is one of the world's largest scientific research facilities. It is located at the border between France and Switzerland close to the city of Geneva. In English referred to as the European Organization for Nuclear Research, it was founded in 1953. The motivation for establishing this laboratory was to drive the nuclear research to smaller dimensions to win a deeper insight into matter. Starting with the Proton Synchrotron (PS) Complex, demands to increase the energy of the accelerated particle led to expansion of the facility. Figure 1.1 shows a sketch of the now existing or currently being constructed accelerators. The machine built after the PS is the SPS (Super Proton Synchrotron). It provided the energy to discover the weak force particles W + , W − , Z 0 earning Carlo Rubbia and Simon Van de Meer the Nobel prize 1984 for their discovery. To push the frontier of energy higher and to obtain more precise data the particle accelerator LEP (Large Electron Positron collider) was built. LHC To step one step higher on the energy ladder, currently the LHC (Large Hadron Collider) is constructed. Using two proton beams, it will provide a top center of mass energy of 2 × 7 TeV (Tera electron Volts = 10 12 eV) to the high energy physics community. It is currently being installed in the tunnel that earlier hosted the LEP collider. This tunnel and the adaptations are depicted in Fig.1.2. Contrary to LEP the LHC is based on superconducting magnets which are needed to provide the field strengths for guiding and focusing a beam at this energy level. The LHC will collide the two beams in four interaction points (IPs) which host four different experiments with their detectors (ATLAS, CMS, ALICE, LHC-B), and a fifth experiment (TOTEM) installed close to LHC-B. Figure 1.3: Acceleration chain of the installation located at Brookhaven National Laboratory. The Relativistic Heavy Ion Collider is the final stage of this chain. In RHIC gold ions and spin polarized protons are brought into collision.

The Higher-Energy LHC (HE-LHC) should collide two proton beams of 16.5-TeV energy, circulating in the LHC tunnel. We discuss the main parameter choices, as well as some optics and beam dynamics issues, in particular the time evolution of emittances, beam-beam tune shift and luminosity, with and without controlled emittance blow up, considering various constraints, and the quadrupole-magnet parameters for arcs and interaction regions.

Journal of Physics: Conference Series, 2018

In the frame of the FCC study we are designing a 27 TeV hadron collider in the LHC tunnel, called the High Energy LHC (HE-LHC). The HE-LHC can be realized by replacing the LHC's 8.33 T niobium-titanium dipole magnets with 16 T niobium-tin magnets developed for FCC-hh. A high-quality beam available from the upgraded LHC injector complex and significant radiation damping allow achieving the challenging target values for both peak and integrated luminosity required by particle physics. Tunnel integration determines the maximum outer size of the magnet cryAPCostat. The HE-LHC arc optics maximizes the dipole filling factor and optimizes the dynamic aperture, while limiting the field strengths of quadrupoles and sextupoles. The low-beta optics for the experimental insertions features a shielded quadrupole triplet even longer than the HL-LHC's, which can support an interaction-point beta function of 25 cm, and survive an integrated luminosity above 10/ab. Other challenges include collimation and extraction. The choice of injection energy and injector is another important element, and so are various collective effects. We here report the HE-LHC design status.

Physical Review Special Topics - Accelerators and Beams, 2015

Colliding bunch trains in a circular collider demands a certain crossing angle in order to separate the two beams transversely after the collision. The magnitude of this crossing angle is a complicated function of the bunch charge, the number of long-range beam-beam interactions, of β Ã and type of optics (flat or round), and possible compensation or additive effects between several low-β insertions in the ring depending on the orientation of the crossing plane at each interaction point. About 15 years ago, the use of current bearing wires was proposed at CERN in order to mitigate the long-range beam-beam effects [J. P. Koutchouk, CERN Report No. LHC-Project-Note 223, 2000], therefore offering the possibility to minimize the crossing angle with all the beneficial effects this might have: on the luminosity performance by reducing the need for crab-cavities or lowering their voltage, on the required aperture of the final focus magnets, on the strength of the orbit corrector involved in the crossing bumps, and finally on the heat load and radiation dose deposited in the final focus quadrupoles. In this paper, a semianalytical approach is developed for the compensation of the long-range beam-beam interactions with current wires. This reveals the possibility of achieving optimal correction through a careful adjustment of the aspect ratio of the β functions at the wire position. We consider the baseline luminosity upgrade plan of the Large Hadron Collider (HL-LHC project), and compare it to alternative scenarios, or so-called "configurations," where modifications are applied to optics, crossing angle, or orientation of the crossing plane in the two low-β insertions of the ring. For all these configurations, the beneficial impact of beam-beam compensation devices is then demonstrated on the tune footprint, the dynamical aperture, and/or the frequency map analysis of the nonlinear beam dynamics as the main figures of merit.