The Crystal Collimation System of the Relativistic Heavy Ion Collider

Abstract

of the DissertationThe Crystal Collimation System of the Relativistic Heavy Ion Collider Crystal Channeling occurs when an ion enters a crystal with a small angle with respect to the crystal planes. The electrostatic interaction between the incoming ion and the lattice causes the ion to follow the crystal planes. By mechanically bending a crystal, it is possible to use a crystal to deflect ions. One novel use of a bent crystal is to use it to channel beam halo particles into a downstream collimator. By deflecting the halo particles into a collimator with a crystal it may be possible to improve the collimation efficiency as compared to a conventional two stage collimation system.A bent crystal was installed in the counterclockwise ring of the Relativistic Heavy Ion Collider (RHIC) prior to the FY2001 run to be used as the first stage of a two stage collimation system. We present a model and simulations to the predict crystal channeling efficiency. The simulations and model predict a channeling efficiency between 59% and 74% depending on the choice of optics.Attempts to reduce backgrounds in RHIC experiments using the crystal were unsuccessful because of the low channeling efficiency. Channeling results show a channeling efficiency 75% lower than originally predicted. Errors in the horizontal Twiss parameters at the crystal and inaccurate assumptions of the beam halo distribution are the cause of the large discrepancy. The strong scattering from the crystal made it impossible to intercept most of the scattered ions from the crystal using a single iii downstream absorber. Background at the STAR detector downstream of the crystal collimator was increased by as much as a factor of two and was rarely reduced below the uncollimated background rate. As a result, the crystal collimator was removed and the collimation system upgraded to a standard two stage system for the FY2004 gold -gold run.We also discuss the uses and effectiveness of the copper scrapers used during all RHIC runs. Measurements of diffusion rates and frequency analysis of beam losses were performed. These measurements aid in the understanding of beam growth rates.We performed beam profile measurements using the scrapers.The design and use of the upgraded collimation system is discussed. Future plans for collimation at RHIC are examined.

Volume reflection and volume refraction of relativistic particles in a uniformly bent crystal

“…The experiment [1,8] of 70GeV protons scattered by a short bent crystal is the first explicit demonstration of the volume "reflection". It should also be mentioned that preliminary results of crystal collimation at RHIC [9] for the 2001 run demonstrated some interesting results which were recently explained [10,11] by the same effect.…”

“…When the particles enter the bent crystal from outer side, the influence of the pedestal might be significant because the outer potential ring has bigger magnitude. Most deflection experiments [1,8,9] are done using a different geometry when the particles enter the piece of crystal from the front end and the influence of the pedestal in this case is quite small. The value of pedestal can be subtracted and 'effective' potential and its rectangular model are shown in Figs.…”

RHIC Loss Limitations and Collimation

“…7. The data is taken during the d-Au run and compared with simulations [7]. Though there is some agreement between the two, some more work needs to be done on the simulation.…”

Design study of a transverse-to-longitudinal emittance exchange proof-of-principle experiment

“…The transverse-to-longitudinal emittance exchange is a challenging scheme to implement and a thorough proof-of-principle experiment is required before confidently incorporating this phase-space manipulation into the design of next-generation accelerators. Such an experiment is planned at the Argonne Wakefield Accelerator (AWA) and at the Fermilab A0 facility [7].…”