Intense source of positron using channeling effect in crystals

Abstract : The research work carried out for this PhD is concerning a special kind of positron source using channeling radiation and other crystal effects in an axially oriented crystal to generate a high number of photons which create, subsequently, a large number of pairs in an amorphous target. Photon generation and pair creation are developing in two targets separated by some distance allowing a sweeping magnet to get off the charged particles away from the amorphous converter. Such a scheme is called a hybrid positron source and has been adopted for the CLIC baseline. This kind of sources present big advantages with respect to the conventional sources where large emittance and important heat deposition are met. After some recall on the physical phenomena of interest for our study, detailed simulations are worked out using a special program dealing with crystal effects from Prof. Strakhovenko and the GEANT4 code; these tools led to a complete description of the positron source concerning the photons from one side and the positrons, from the other side, for which the main characteristics have been determined: transverse and longitudinal phase space, energy spectrum, time distribution,...Emphasis has been put on two points: first the matching devices capturing the positrons after the target which are essential for good accepted yields and the energy deposition density which is an important question for the reliability of the targets. Concerning the former point, three matching devices have been studied and their features compared: the Adiabatic Matching Device (AMD) largely used or considered for the positron sources, the Quarter Wave Transformer (QWT) and also the Lithium lens. For the latter point, related to the energy deposition and heating of the targets, we have tried to optimize the energy deposition density lowering its maximum value (PEDD); the average heat deposition has also been optimized using special converter material in granular shape, as considered for the neutrino factories. Very promising results allowed us to consider the hybrid positron source as an alternative to the difficult case of ILC; a special scheme for the transformation of the ILC beam pulses has been used, after KEK proposition. The positron beam transport has also been studied in the first part of the positron pre-accelerator including the solenoid and the first part of the quadrupole channel.
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Accelerator Physics [physics.acc-ph]. Université Paris Sud - Paris XI, 2012. English


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Submitted on : Wednesday, June 20, 2012 - 4:36:41 PM
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Chenghai Xu. Intense source of positron using channeling effect in crystals. Accelerator Physics [physics.acc-ph]. Université Paris Sud - Paris XI, 2012. English. <tel-00710353>

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