| Paper | Title | Page |
|---|---|---|
| TUPLS016 | Characterization of Crystals for Steering of Protons through Channelling in Hadronic Accelerators | 1523 |
|
||
| Channeling of relativistic particles through a crystal may be useful for many applications in accelerators, and particularly for collimation in hadronic colliders. Efficiency proved to be dependent on the state of the crystal surface and hence on the method used for preparation. We investigated the morphology and structure of the surface of the samples that have been used in accelerators with high efficiency. We found that crystal fabrication by only mechanical methods (dicing, lapping, and others) leads to a superficial damaged layer, which is correlated to performance limitation in accelerators. A planar chemical etching was studied and applied in order to remove the superficial damaged layer. RBS analysis with low-energy protons highlighted better crystal perfection at surface, as a result of the etching. Finally, measurement with 70-GeV protons at IHEP demonstrated a superior performance of the chemically cleaned crystals with respect to conventional samples. A protocol for preparation and characterization of crystal for channelling has been developed, which may be of interest for reliable operation with crystals in accelerators. | ||
| TUPLS017 | Optics Study for a Possible Crystal-based Collimation System for the LHC | 1526 |
|
||
| The use of bent crystals as primary collimators has been long proposed as an option to improve the cleaning efficiency of the LHC betatron and momentum collimation systems. These systems are presently based on two-stage collimation with amorphous scatterers and absorbers. Crystals are expected to help by channeling and extracting the halo particles with large angles, resulting in higher cleaning efficiency. Independent of ongoing studies for crystal qualifications (not reported here), it is important to understand the required deflection angles and the possible locations of absorbers for the LHC layout. Optics studies have been performed in order to specify the required angles for various LHC beam energies and possible locations of absorbers for the deflected halo beam. A possible layout for crystal-assisted collimation at the LHC is discussed, aiming for a solution which would not change the LHC layout but would make use of the existing collimator location. | ||
| TUPLS018 | Collimation Efficiency during Commissioning | 1529 |
|
||
| The design of the LHC collimation system naturally focused on understanding and maximizing the ultimate performance with all collimators in place. However, for the commissioning of the LHC it is important to analyze the collimation efficiency with certain subsets of collimators, with increased collimation gaps and relaxed set-up tolerances. Special studies on halo tracking and energy deposition have been performed in order to address this question. The expected cleaning performance and intensity limits are discussed for various collimation scenarios as they might be used during commissioning and initial operation of the LHC. | ||
| TUPLS019 | Critical Halo Loss Locations in the LHC | 1532 |
|
||
| The requirements on cleaning efficiency in the LHC are two to three orders of magnitude beyond the needs at existing super-conducting colliders. The LHC will therefore operate in unknown territory, which can only be assessed by powerful simulation tools. Such tools have been developed at CERN over the last years, making it possible to perform detailed simulations of the LHC cleaning processes and multi-turn loss patterns around the LHC ring. The simulation includes all collimators, diluters and absorbers in the LHC. Proton loss maps are generated with a 10 cm resolution, which allows performing advanced studies for quenches of super-conducting magnets along with the analysis of the deposited energy in the machine elements. The critical locations of beam halo losses are discussed, both for the ideal machine and for various scenarios of closed-orbit distortion and beta-beating. From these results it can be shown that it is sufficient to use a limited number of BLM's for the setup and optimization of the LHC collimation system. | ||
| TUPLS021 | First Observation of Proton Reflection from Bent Crystals | 1535 |
|
||
|
We recently suggested using short bent crystals as primary collimators in a two-stage cleaning system for hadron colliders, with the aim of providing larger impact parameters in the secondary bulk absorber, through coherent beam-halo deflection*. Tests with crystals a few mm long, performed with 70 GeV proton beams at IEHP in Protvino, showed a channeling efficiency exceeding 85%. We also observed disturbing phenomena such as dechanneling at large impact angle, insufficient bending induced by volume capture inside the crystal, multiple scattering of non-channeled protons and, for the first time, a proton flux reflected by the crystalline planes. Indeed, protons with a tangent path to the curved planes somewhere inside the crystal itself are deflected in the opposite direction with respect to the channeled particles, with an angle almost twice as large as the critical angle. This effect, up to now only predicted by computer simulations**, produces a flux of particles in the wrong direction with respect to the absorber, which may hamper the collimation efficiency if neglected.
*A. Afonin et al. PhysRevLett.87.094802(2001).**A. M. Taratin and S.A.Vorobiev, Phys.Lett. A119(1987)425. |
||
| TUPLS022 | Experimental Study of Crystal Channeling at CERN-SPS for Beam-halo Cleaning | 1538 |
|
||
| An efficient and robust collimation system is mandatory for any superconducting hadron collider, in particular for the LHC, which will store a beam of unprecedented high intensity and energy. The usage of highly efficient and short primary bent-crystal collimators might be a possibility for reaching nominal and ultimate LHC intensity. Over the last years, groups in Russia (St. Petersburg) and Italy (Ferrara) have developed crystal production methods, which considerably improve the crystal quality. In view of the crystal-collimation experiments at the Tevatron and of the potential improvement compared with the phas·10-1 LHC collimation system, considering the recent progress in crystal technology, we proposed experiments for crystal characterization in the SPS beam lines. Major objectives will be: 1) qualification of the new crystals to be used in the Tevatron; 2) measuring the channeling efficiency of long crystals with 1 mrad and/or 8 mrad bending angle; and 3) comparison of loss patterns around the ring for a crystal with one for amorphous material. In this paper we will report the progress towards the SPS experiment. |