M. Lamont

Accelerator physics at LEP

Accelerator physics issues and their influence on performance are presented for the Large Electron-Positron (LEP) storage ring at CERN in Geneva, Switzerland. After several years of operation on the Z boson resonance at beam energies around 45 GeV, the beam energy was increased in steps to over 100 GeV. The major power loss to synchrotron radiation and its consequences on the maximum beam energy are discussed. The subjects of luminosity optimization, beam-beam effect, instabilities, detector backgrounds and beam lifetime are addressed. The precise beam energy calibration, which is of particular importance for the determination of standard model parameters, is described.

A Brief History of the LEP Collider

Abstract The Large Electron Positron collider LEP at CERN was commissioned in 1989 and finished operation in November 2000. During this period it was operated in different modes, with different optics, at different energies, and with varied performance. In the end, LEP surpassed all relevant design parameters. It has provided a large amount of data for the precision study of the standard model, first on the Z 0 resonance, and then above the W ± pair threshold. Finally, with beam energies above 100 GeV, a tantalizing glimpse of what might have been the Higgs boson was observed. A brief history of the main modes of operation, associated performance, the highlights and the challenges met over the 12 years of running is presented.

Experiments with bunch trains in LEP

During 1994 tests were made to evaluate the possibility of operating LEP with four equidistant bunch trains in each beam during 1995. A train will consist of up to four bunches with a bunch spacing of some 75 m. The bunch trains collide head-on. They are separated at the parasitic collision points on either side of the interaction points by vertical electrostatic separator bumps. The accumulation of bunch trains in a single beam was studied with magnetically simulated bumps. Following the installation of additional separators late in 1994, the parasitic encounters of the bunch trains near the interaction points were studied for one train of electron bunches colliding with one train of positron bunches in two diametrically opposite interaction points. The higher order mode losses in the superconducting RF cavities were measured for several arrangements of beams, trains and bunches. The synchrotron radiation background, enhanced by the off-centred beams in the quadrupoles inside the separator bumps, is reduced by super-imposing asymmetric magnetic bumps such that the offset of the incoming beams is reduced.

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.

Cern Neutrinos to Gran Sasso (CNGS): results from commissioning

The CNGS project (CERN Neutrinos to Gran Sasso) aims at directly detecting vmu - vtau oscillations. An intense vmu beam is generated at CERN and directed towards LNGS (Laboratori Nazionali del Gran Sasso) in Italy where vtau will be detected in large and complex detectors. An overview of the CNGS beam facility is given. Results from the primary and secondary beam line commissioning performed in summer 2006 are presented. Measurements of proton beam parameters are compared with expectations.

Modification of the LEP electrostatic separator systems for operation with bunch trains

To meet the LEP2 luminosity requirements for W-pair production, it is planned to operate LEP with bunch trains from 1995 onwards. This new mode of operation entails significant modification both to the existing separator hardware and its control system. The changes have been implemented so as to provide maximum flexibility for the realisation of the bunch train scheme, and also make a return to operation with Pretzel separation possible during 1995. Two LEP Interaction Points (IP) were equipped with new separators in late 1994, enabling first tests with the collision of one train of four e/sup +/ bunches with one train of four e/sup -/ bunches. During the 1994/95 shutdown, four separators have been installed in the two remaining experimental IPs, and eight separators in the non-experimental IP have been displaced to new positions. Details are given of optics requirements for the separator installations, the polarity of the closed orbit separator bumps, system modifications, and performance considerations. Results are presented of investigations into the effects of separator polarity on high voltage performance and on the commissioning of the new hardware and software systems during tests of the bunch train scheme in 1994.

Chapter 16 Commissioning and Operation

At the point of commissioning and subsequent operation of the HL-LHC, the LHC itself will have been operational for over 10 years and a wealth of knowledge and experience will have been built up. The key operational procedures and tools will have been well established. The understanding of beam dynamics will be profound and refined by relevant measurement and correction techniques. Key beam-related systems will have been thoroughly optimized and functionality sufficiently enhanced to deal with most challenges up to that point. Availability will have been optimized significantly across all systems. This collected experience will form the initial operational basis following the upgrade.

LHC on-line model

The LHC machine will be a very demanding accelerator from a beam control perspective. There are tight constraints on the key beam parameters in the presence of large non-linearities and dynamic persistent current effects. Particle loss in the LHC must be actively minimized to avoid damage to the machine. Therefore any adjustment to the machine parameters would ideally be checked beforehand with a proper modeling tool. The LHC On-Line Model is an attempt to provide such an analysis tool based mainly on the MAD-X code. The goal is not to provide a real-time interactive system to control the LHC, but rather a way to speed up interaction with the power of MAD-X and to facilitate off-line analysis to give results within appropriate time constraints. There will be a rich spectrum of potential applications such as closed orbit correction, beta-beating analysis, optimization of non-linear correction and knob settings. We report the status of the on-line model software which is at present being developed for the beginning of the LHC commissioning.

Performance of the high level application software during LEP operation

Abstract After the first year of operating LEP, it was clear that a new generation of application software would be required to effectively exploit the accelerator. In response a new system of application software was developed. During 1992 and 1993 this software has been used exclusively to drive LEP in many different operational modes including polarization runs and 8 bunch pretzel operation. The software has performed well and has clearly enhanced the performance of the machine. For example, the turn around time has been significantly reduced, contributing an increase of around 20% to the integrated luminosity for 1992. The software has also made the accelerator accessible to less experience operators. After outlining the functionality of the system the impact of the software on various aspects of LEP performance is discussed. Comparative data from the last 3 years is presented.

Nominal Cycle and Options

During Run 2 the LHC operation will be based on the experience gained in Run 1. However the LHC will be operated near to its design energy. Many operational configurations can be considered to improve efficiency and reduce the impact of the longer time required by each operational phase. The expected changes in the magnetic model and the impact of the data updates with the corrections calculated during LS1 are presented together with a general overview of the operational cycle, including time, challenges and possible improvements of each phase.