Beam-Induced Electron Cloud in the LHC and Possible Remedies Baglin V Brüning Oliver Sim Calder R Caspers Friedhelm Collins I R Gröbner Oswald Hilleret Noël CERN Laurent Jean Michel Morvillo M Pivi M Ruggiero F 1998 Synchrotron radiation from proton bunches in the LHC creates photoelectrons at the beam screen wall. These photoelectrons are accelerated towards the positively charged proton bunch and drift across t he beam pipe between successive bunches.When they hit the opposite wall, they generate secondary electrons which can in turn be accelerated by the next bunch if they are slow enough to survive. We sum marize the results of an intensive research program set up atCERN and discuss recent multipacting tests as well as the importance of several key parameters, such as photon reflectivity, photoelectron and secondary electron yield.Then, based on analytic estimates and simulation results, we discuss possible solutions to avoid the fast build-up of an electron cloud with potential implications for bea m stability and heat load on the cryogenic system. research-article

Measurements of the LHC Corrector Magnets at Room and Cryogenic Temperatures Ang Z Arn A Bottura L Giloux C Sievers P Smirnov N Vincent S Walckiers L 1998 The superconducting twin aperture main dipole magnets of the LHC accelerator are equipped with pairs of sextupole and decapole correctors at their ends. Similarly, octupole correctors are aligned at t he end of the main quadrupole magnets. Dedicated stations have been built for tests of these correctors at room temperature as well as superfluid helium temperature. Measurements of the training behav iour and of the magnetic field quality are routinely performed. The search for the magnetic axis and the transfer of its position to fiducials are performed at room temperature. A description and the performances obtained with these two benches are also presented. research-article

Multipacting Tests with Magnetic Field for the LHC Beam Screen Brüning Oliver Sim Caspers Friedhelm Laurent Jean Michel Morvillo M Ruggiero F 1998 In connection with electron-cloud induced heating of the LHC beam screen, multipacting tests with a resonant coaxial cavity have been successfully performed in presence of a solenoid and a dipole magn etic field. We have developed a simple and reliable technique, based on amplitude modulation of the input signal, to detect electronically the onset of multipacting and to monitor the field and power level in the resonator. Several multipacting patterns have been systematically investigated under the effect of a variable DC-bias applied to the inner conductor of the coaxial setup. The results at r oom temperature are qualitatively similar to those obtained during cold tests (below 20 K in a cryostat) with a dipole magnetic field up to 7.5 T. A weak solenoid field of about 50 Gauss is usually su fficient to stop the multipacting, but the same longitudinal field is ineffective in presence of a strong vertical dipole field (up to 1.5 T). We have also measured the rise time of the multipacting v ersus the intensity of the solenoid field. Moreover, a substantial decrease of the multipacting threshold is observed when the dipole magnetic field has an intensity such that the electron cyclotron f requency is equal to the resonant frequency of the coaxial cavity. research-article

Mechanical and Vacuum Stability Design Criteria for the LHC Experimental Vacuum Chambers Collins I R Gröbner Oswald Lepeule P Veness R J M 1998 Four colliding beam experiments are planned for the Large Hadron Collider (LHC) requiring experimental vacuum chambers in the interaction region. The beam pipe should be as transparent as possible to scattered particles and detectors should be as close as possible to the interaction point, resulting in small diameter beam pipes. This, together with the bunched beam structure, makes ion induced pre ssure bump instability, well known from the Intersecting Storage Rings (ISR) at CERN, a potential problem. Adequate conductance, cleanliness of the beam pipes and efficient pumping are required to avo id this instability. Suppression of electron multipacting requires appropriate surface coatings and cleaning procedures. Small beam pipe diameters must provide the required beam stay clear and still a llow margin for alignment and stability inside detectors. Design criteria to ensure both local and global stability under static and dynamic mechanical loads are defined. research-article

Photoelectron Yield and Photon Reflectivity from Candidate LHC Vacuum Chamber Materials with Implications to the Vacuum Chamber Design Baglin V Collins I R Gröbner Oswald 1998 Studies of the photoelectron yield and photon reflectivity at grazing incidence (11 mrad) from candidate LHC vacuum chamber materials have been made on a dedicated beam line on the Electron Positron A ccumulator (EPA) ring at CERN. These measurements provide realistic input toward a better understanding of the electron cloud phenomena expected in the LHC. The measurements were made using synchrotro n radiation with critical photon energies of 194 eV and 45 eV; the latter corresponding to that of the LHC at the design energy of 7 TeV. The test materials are mainly copper, either, i) coated by co- lamination or by electroplating onto stainless steel, or ii) bulk copper prepared by special machining. The key parameters explored were the effect of surface roughness on the reflectivity and the pho toelectron yield at grazing photon incidence, and the effect of magnetic field direction on the yields measured at normal photon incidence. The implications of the results on the electron cloud phenom ena, and thus the LHC vacuum chamber design, is discussed. research-article

Development and Test Results of a low-$\beta$ Quadrupole Model for the Large Hadron Collider Ajima Y Burkhardt E E Doi Y Haruyama T Higashi N Iida M Kawamata H Kimura N Makida Y Nakamoto T Ogitsu T Ohhata H Ohuchi N Shintomi T Tanaka K Terashima A Tsuchiya K Yamamoto A Kovachev V Minemoto K Kirby G A Ostojic R Taylor T M 1998 A 1-m model of the high gradient 70 mm aperture superconducting low-b quadrupoles for the Large Hadron Collider (LHC) has been developed. A field gradient of 250 T/m at 1.9 K has been achieved with a peak field of 10 T in the coil. This paper describes development of the first model magnet and presents the test results. research-article

Genetic Algorithms for the Optimal Design of Superconducting Accelerator Magnets Ramberger S Russenschuck Stephan 1998 The paper describes the use of genetic algorithms with the concept of niching for the optimal design of superconducting magnets for the Large Hadron Collider, LHC at CERN. The method provides the designer with a number of local optima which can be further examined with respect to objectives such as ease of coil winding, sensitivity to manufacturing tolerances and local electromagnetic force distribution. A 6 block dipole coil was found to have advantages compared to the standard 5 block version which was previously designed using deterministic optimization methods. Results were proven by a short model magnet recently built and tested at CERN. research-article

ROXIE Russenschuck Stephan 1998 The paper describes the ROXIE software program package which has been developed for the design of the superconducting magnets for the LHC at CERN. The software is used as an approach towards the integrated design of superconducting magnets including feature-based coil geometry creation, conceptual design using genetic algorithms, optimization of the coil and iron cross-sections using a reduced vector-potential formulation, 3-D coil end geometry and field optimization using deterministic vector-optimization techniques, tolerance analysis, production of drawings by means of a DXF interface, end-spacer design with interfaces to CAD-CAM for the CNC machining of these pieces, and the tracing of manufacturing errors using field quality measurements. research-article

Experience with different constructions of superconducting corrector magnets for the LHC Salminen J Ijspeert Albert 1998 research-article

Performance of the 1-meter models of the 70 mm aperture quadrupole for the LHC low-beta insertions Bottura L Giloux C Kirby G A Lamm M J Milward S R Ostojic R Rival F Rodríguez-Mateos F Schloss S Siemko A Smith K D Taylor T M Townsend M C Treadgold J R Wiatrzyk J M 1998 Following the successful testing of the first 1-metre model of the 70 mm aperture quadrupole for the LHC low-beta insertions, two further 1-metre magnets have been built. All magnets feature a four-layer coil wound from two 8.2 mm wide graded NbTi cables and a four-way split yoke supporting structure. In this paper we review the training history of the three magnets performed at 4.3 K and 1.9 K in several tests. All magnets surpassed the operating gradient required for the LHC, with the third magnet reaching 260 T/m, its short-sample gradient at 1.9 K. The peak temperatures in the superconductor at various operating conditions are reported and a summary of magnetic field measurements is given. research-article

Testing of the large bore single aperture 1-meter superconducting dipoles made with phenolic inserts Boschmann H Bottura L Dubbeldam R L Kirby G A Lucas J Ostojic R Russenschuck Stephan Siemko A Taylor T M Vanenkov I Weterings W 1998 Two identical single aperture 1-metre superconducting dipoles have been built in collaboration with HMA Power Systems and tested at CERN. The 87.8 mm aperture magnets feature a single layer coil wound using LHC main dipole outer layer cable, phenolic spacer type collars, and a keyed two part structural iron yoke. The magnets are designed as models of the D1 separation dipole in the LHC experimental insertions, whose nominal field is 4.5 T at 4.5 K. In this report we present the test results of the two magnets at 4.3 K and 1.9 K. research-article

Long term stability in hadron colliders in presence of synchrotron oscillations and tune ripple Scandale Walter Todesco Ezio 1998 The problem of long-term losses in hadron colliders such as the Large Hadron Collider (LHC) is considered. A previous formula that provides the reduction of dynamic aperture with the number of turns is generalized to include also the relevant cases of off-momentum and tune ripple. The dynamic aperture turns out to shrink with a power of the inverse logarithm of the number of turns. Long-term tracking data of the LHC are analysed in this framework. The formula proves to hold in all cases, and the possibility of using its extrapolation to predict long-term losses are explored. research-article

Long term estimates for sorting strategies of the LHC dipoles Scandale Walter Bartolini R Giovannozzi Massimo Todesco Ezio 1998 Sorting strategies are investigated in view of improving the dynamic aperture of the CERN-LHC. Local and quasi-local compensation of the random field shape imperfections are discussed and applied to simplified model of the LHC lattice. The most promising strategies are further investigated on a realistic LHC model with particular emphasis on the analysis of the robustness of the dynamic aperture improvements including long term effects. First results on the application of the recently developed extrapolation law for the prediction of the dynamic aperture to the sorting problem are presented. research-article

Field-shape imperfections of the CERN-LHC dipole arising from mechanical deformations and component tolerances Scandale Walter Bartolini R Ferracin P Fessia P Todesco Ezio 1998 The stability of the geometry of the superconducting coils is essential to the field homogeneity of the LHC dipole magnets. Mechanical stresses during coil assembly, thermal stresses during cool-down and electromagnetic stresses during operation are the source of deformations of the coil geometry. Additional sources of field-shape errors are the dimensional tolerances of the magnet components and of the manufacturing and assembly tooling. To provide a realistic evaluation of the field-shape imperfections of the LHC dipoles arising from the above effects, appropriate finite-element computations were carried out to model the dipole cross-section in presence of stresses and a first statistical simulation of the effect of the manufacturing tolerances was performed as well. research-article

Manufacturing features and performances of long models and first prototype for the LHC project Billan J Bóna M Bottura L Pagano O Perin R Perinet-Marquet J L Perini D Rossi L Savary F Spigo G Siemko A Vlogaert J Walckiers L 1998 This paper reports about the 10-m-long models and one 15-m-long prototype. Their main design features are a 5-block coil cross section, an intra-beam distance of 194 mm at room temperature and a 15-mm-wide superconducting cable. The collared coil of the 10-m-long models were built in Industry and the assembly of the magnetic circuit and cold mass was done at CERN while the 15-m-long prototype was entirely made in Industry. Manufacturing features, assembly steps and quench performances of each magnet are presented. Results of magnetic measurements taken in the course of magnet assembly, during and after the cold test campaigns are also given. research-article

Magnetic Performance of First low-$\beta$ dipole corrector prototype, MCBX Ang Z Bottura L Ijspeert Albert Hauge N Karppinen M Nielsen B R Walckiers L 1998 The LHC low-beta triplets require a short and strong dipole corrector. The MCBX is a superconducting single aperture magnet, which features a horizontal dipole nested inside a vertical dipole. The cold tests of the first prototype, which was designed by CERN and built by Danfysik A/S, have been carried out at CERN. This paper presents the results of the magnet training and quench propagation studies at 4.3 K and 1.9 K. The magnetic measurements carried out at constant field and in ramped conditions are compared to the expected figures from the calculations. research-article

Training Study on Superconducting Coils of the LHC Sextupole Corrector Magnet Bajko M Calero J García-Tabarés L Ijspeert Albert Pérez J C Toral F 1998 A study on a single sextupole coil, working under the same conditions as the full magnet, has been made to evaluate the effect of the azimuthal pre-compression and the longitudinal pre-tension on the training of superconducting coils. A testing device has been used that allows to test individual sextupole type coils in a cryostat at 4.2 K by exerting variable pre-stresses in situ. The paper describes the tests made with this device and discusses the results obtained for different pre-stress conditions and for different central island materials, in particular G-10 and stainless steel. research-article

State of the Short Dipole Model Program for the LHC Andreyev N I Artoos K Kurtyka T Oberli L R Perini D Russenschuck Stephan Siegel N Siemko A Tommasini D Vanenkov I Walckiers L 1998 Superconducting single and twin aperture 1-m long dipole magnets are currently being fabricated at CERN at a rate of about one per month in the framework of the short dipole model program for the LHC. The program allows to study performance improvements coming from refinements in design, components and assembly options and to accumulate statistics based on a small-scale production. The experience thus gained provides in turn feedback into the long magnet program in industry. In recent models initial quenching fields above 9 T have been obtained and after a short training the conductor limit at 2 K is reached, resulting in a central bore field exceeding 10 T. The paper describes the features of recent single aperture models, the results obtained during cold tests and the plans to ensure the continuation of a vigorous model program providing input for the fabrication of the main LHC dipoles. research-article

Experience with the LEP vacuum system at energies above 90 GeV and future expectations Billy J C Bojon J P Hilleret Noël CERN Jiménez M Laugier I 1998 The LEP storage ring has been operated at energies above 90 GeV for more than 1000 hours during 1997. Because of the rapid increase with the beam energy of the power radiated as synchrotron light, the vacuum system has been submitted to very stringent conditions as far as power evacuation and photon stimulated gas desorption are concerned. The operational experience acquired under these, up to now unexplored, conditions will be reviewed together with an outline of the limitations which were experienced at these high levels of radiation in the use of the available vacuum instrumentation. Based o n the available data detailed predictions concerning the beam lifetime, gas desorption and beam cleaning of the vacuum system under the impact of photons with a critical energy approaching 1 MeV will be formulated. research-article

A 20 kA Test Bench for High-Precision Current Measurements Dahlerup-Petersen K CERN Halvarsson Bjorn CERN Valbuena R CERN 1998 The d.c. currents in the LHC dipole and quadrupole chains will require settings and adjustments with a precision of a few ppm. For an ultimate current level of 13 kA this represents an unprecedented accuracy. Compared to the requirements of previous accelerators at CERN, such as the LEP, this is a factor of ten better in accuracy at more than twice the current. State-of-the-art, zero-flux current transducers from Industry will be used for the precision measurements. As no existing laboratory would be capable of performing the calibrations of these transducers to the required precision, a major upgrading of the current Standards laboratory at CERN was decided. The paper describes the various phases of the project, from field calculations and design to construction and final commissioning of this unique test bench. The highly automated facility allows determination of off-sets, linearity and drift of transducers up to 20 kA but provides equally the means to study the sensitivity of the transducers to external stray fields as generated by currents in adjacent busbars. research-article