Accuracy of the transverse emittance measurements of the CERN large hadron collider
High energy accelerators and storage rings are designed to collide charged particle beams and study their collision products. The production rate of the collision products has to be maximized in order to reduce the statistical uncertainty of the produced events. Monitoring the transverse distribution of the accelerated species allows to measure and optimize the beam transverse emittance, which directly affects the secondary particles production rate. The beam transverse emittance is measured by a class of diagnostics, the transverse profile monitors, designed to observe the particles transverse distributions. This thesis work aims at determining the accuracy of two classes of profile monitors presently installed in the CERN accelerators and foreseen for the Large Hadron Collider (LHC): wire scanners and residual gas monitors. The explanation of the linear dynamics that characterize the particles transverse motion in an accelerator is followed by the description of the principles of operation of the studied monitors. In addition, numerical simulations predict the effect of limited resolution and excessive noise. The particles transverse distributions are parameterized with a Gaussian density distribution and the approximation procedures are studied. Two types of wire scanners are used in the Super Proton Synchrotron (SPS), based on a linear and rotational displacement of the wire respectively. The systematic differences between the wire scanner monitors have been measured with several beam conditions and vary from below 1% to about 35% of the absolute beam normalized emittance. The reasons of such discrepancies are discussed. The statistical fluctuations of the wire scanner monitors, expressing their repeatability, resulted to be below 3% of the absolute emittance. In terms of beam size, the repeatability is about 11 μm for the linear wire scanners and varies from 39 to 58 μm for the rotational. Comparative measurements between the SPS wire scanners and the Ionization Profile Monitor (IPM) underline the IPM limitations when used to measure beam transverse distributions with Gaussian widths smaller than 500 μm. In terms of beam size, the maximum discrepancy between the two classes of instruments decreased from 250 μm to 150 μm after the IPM hardware optimization. The wire scanner monitors, installed in all the LHC pre-accelerators, have been utilized to characterize the beam transverse emittance during acceleration from 1.4 GeV to 450 GeV . For the first time, this kind of measurements were carried out systematically for the different beam types that will be employed first for the LHC commissioning phase and later for its operation as a collider.
Section de physique
Faculté des sciences de base
Institut de physique de l'énergie et des particules
Jury: Bernd Dehning, François Meot, Minh Quang Tran, Albin Friedrich Wrulich
Public defense: 2006-1-12
Record created on 2005-10-27, modified on 2016-08-08