20354147
Possible Phase Loop for the Global Decoupling
The two eigentunes Q<inf>I</inf>and Q<inf>II</inf>, two eigenmode amplitude ratios R<inf>I</inf>and R<inf>II</inf>, and two eignmode phase differences Δφ<inf>I</inf>and Δφ<inf>II</inf>, are defined as the coupling observables for the linear weak difference betatron coupling. Simulations were carried out to investigate their behaviors in global decoupling scans. It was found that the amplitude ratios R<inf>I</inf>,<inf>II</inf>are more sensitive than the tune split when the decoupling scan is approaching the global uncoupled point, and that the phase differences Δφ<inf>I</inf>,<inf>II</inf>tell the right global decoupling direction, the right strength combination of the skew quadrupoles or families. The analytical solution to these six coupling observables is calculated through both the strict matrix approach and the perturbation Hamiltonian approach. The constant phase differences in the right decoupling direction hint a possible global decoupling phase loop. Dedicated beam experiments were carried out at the Relativistic Heavy Ion Collider (RHIC). The preliminary results from the beam experiments are presented. These six parameters can be used for the global decoupling in feedback mode, especially on the non-stop energy ramp.
56094879
First years experience of LHC Beam Instrumentation
The LHC is equipped with a full suite of sophisticated beam instrumentation which has been essential for rapid commissioning, the safe increase in total stored beam power and the understanding of machine optics and accelerator physics phenomena. This paper will comment on all of these systems and on their contributions to the various stages of beam commissioning. It will include details on: the beam position system and its use for realtime global orbit feedback; the beam loss system and its role in machine protection; total and bunch by bunch intensity measurements; tune measurement and feedback; synchrotron light diagnostics for transverse beam size measurements, abort gap monitoring and longitudinal density measurements. Issues and problems encountered along the way will also be discussed together with the prospect for future upgrades.
54665441
APERTURE RESTRICTION LOCALISATION IN THE LHC ARCS USING AN RF MOLE AND THE LHC BEAM POSITION MEASUREMENT SYSTEM
Ensuring that the two 27km beam pipes of the LHC do not contain aperture restrictions is of utmost importance. Most of the ring is composed of continuous cryostats, so any intervention to remove aperture restrictions when the machine is at its operating temperature of 1.9K will require a substantial amount of time. On warming-up the first cooled sector, several of the sliding contacts which provide electrical continuity for the beam image current between successive sections of the vacuum chamber were found to have buckled into the beam pipe. This led to a search for a technique to verify the integrity of a complete LHC arc (~3km) before any subsequent cool-down. In this paper the successful results from using a polycarbonate ball fitted with a 40MHz RF transmitter are presented. Propulsion of the ball is achieved by sucking filtered air through the entire arc, while its progress is traced every 54m via the LHC beam position measurement system which is auto-triggered by the RF transmitter on passage of the ball. Reflectometry at frequencies in the 4-8 GHz range can cover the gaps between beam position monitors and could therefore be used to localise a ball blocked by an obstacle.
110214727
THE LHC SECTOR TEST
The proposal to inject beam into a sector of the partially completed LHC is presented. The test will provide an important milestone, force preparation of a number of key systems, and allow a number of critical measurements with beam. The motivation for the test is discussed, along with the proposed beam studies, the radiation issues and the potential impact on ongoing installation. The demands on the various accelerator systems implicated are presented along with the scheduling of the preparatory steps, the test itself and the recovery phase.
54822292
Commissioning and Initial Performance of the LHC Beam-Based Feedback Systems
The LHC deploys a comprehensive suite of beam-based feedbacks for safe and reliable machine operation. This contribution summarises the commissioning and early results of the LHC feedback control systems on orbit, tune, chromaticity, and energy. Their performance – strongly linked to the associated beam instrumentation, external beam perturbation sources and optics uncertainties – is evaluated and compared with the initial feedback design assumptions.
28953173
Influence of varying tune width on the robustness of the LHC tune PLL and its application for continuous chromaticity measurement
Tune and chromaticity measurement is an integral part of safe and reliable LHC operation. Tight tolerances on the maximum transverse beam excursions allow oscillation amplitudes of less than 30 mum. This leaves only a small margin for transverse beam and momentum excitations required for measuring tune and chromaticity. This contribution discusses a robust tune phase-locked-loop (PLL) operation in the presence of non-linearities and varying chromaticity. The loop design was tested at the SPS, using the LHC PLL prototype system. The system was also used to continuously measure tune width and chromaticity, using resonant transverse excitations of the tune side-slopes.
119965767
A combination by the LEP QCD Working Group of αs values derived from event shape variables at LEP
The LEP QCD Working Group has produced a combination of α s values derived from event-shape distributions. The distributions are measured at scales below the mass of the Z boson up to 206 GeV. Values of α s are all extracted using the consistent application of O ( α s 2 )+ NLLA predictions with the modified Log(R) matching scheme. A combined α s is obtained for each experiment and observable using at least two experiments in each case for energies above at ad above the mass of the Z boson. A consistent treatment and controlled breakdown is made of the sources of uncertainty on the input values. The combined best value for α s (M Z ) = 0.11 ± 0.003 ± 0.003. The dominant uncertainty is theoretical, due to the effects of missing higher order contributions.
178487761
The ganga user interface for physics analysis and distributed resources
60501245
The Evolution of the ATLAS Computing Model
Despite the all too brief availability of beam-related data, much has been learned about the usage patterns and operational requirements of the ATLAS computing model since Autumn 2007. Bottom-up estimates are now more detailed, and cosmic ray running has exercised much of the model in both duration and volume. Significant revisions have been made in the resource estimates, and in the usage of those resources. In some cases, this represents an optimization while in others it attempts to counter lack of functionality in the available middleware. There are also changes reflecting the emerging roles of the different data formats. The model continues to evolve with a heightened focus on end-user performance, and the state of the art after a major review process over winter 08/09 is presented.