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G.PEI, J.GAO, X.Li, Z.ZHOU (IHEP/Beijing/China)X.ARTRU, R.CHEHAB, M.CHEVALLIER (IPNL/IN2P3/Lyon/France), V.M.STRAKHOVENKO (Visitor from BINP at IPNL)

A.VARIOLA, A.VIVOLI, F.ZOMER (LAL/IN2P3/Orsay/France) presented by R.Chehab

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INTRODUCTION The positron source is one of the crucial elements of

the ILC. There is a strong interest on polarized positrons and, among the different possibilities, we have chosen to study the so-called Compton Source using the collision of a circularly polarized laser with an electron beam in the GeV energy range. For unpolarized positrons the recent promising results, at CERN and KEK, on crystal positron sources, using the strong enhancement in photon production in aligned crystals due to channeling, made us study this kind of source as the starting solution for which the Compton source is taken as an upgrade. For the two kinds of sources, the capture, acceleration and emittance preservation are considered.

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PLAN *The Compton Source * Capture and Transport of the positrons * Emittance (Longitudinal & Transverse) preservation * The crystal positron source * Collaboration status and perspectives

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ILC POSITRON SOURCE-IHEP-IN2P3 COLLABORATION

THE “COMPTON” SOURCE The lay-out is represented: an Energy Recovery Linac (ERL) provides the energetic e- (1

to 2 GeV) colliding with circularly polarized photons from Nd:Yag laser. Capture is made with AMD (Adiabatic Matching Device) and pre-acceleration (150 MeV) with warm L-Band sections. The main e+ linac is a superconducting L-Band

Compton cavities

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ILC POSITRON SOURCE- IHEP-IN2P3 COLLABORATION

PHOTONS AND POSITRONS (“Compton “source ) Simulations made with CAIN (Compton process) and EGS4 (pair

production) provide 6-D emittances and polarization.

Photons:Polarization/Energy Positrons:Polarization/Energy

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ILC POSITRON SOURCE- IHEP-IN2P3 COLLABORATION

CAPTURE AND PRE-ACCELERATION (“Compton”source) An AMD is put after a W target (~0.4Xo) with a magnetic field tapering

from 6 to 0.5 Tesla on 50 cm. Adiabatic transformation of the phase space is represented below:

The e+ transverse momentum is reduced to 3 MeV/c and Transverse size expanded to accelerator aperture ~20 mm. Simulations made with PARMELA confirmed that

aperture

aperture

pT

pT

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EMITTANCE PRESERVATION ( “Compton” Source) The geometrical transverse emittance is reduced with acceleration. The

longitudinal emittance must be reduced before injection in the Damping Ring which has limited acceptance. Bunch lengthening after the target is the main contribution (spiralization in magnetic fields, differences in velocities); a bunch compression must be provided. A magnetic chicane provided the compression shown (simulations with PARMELA) :

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CRYSTAL POSITRON SOURCE Enhancement of photon production by channeled electrons in

crystals provides a powerful source of photons, which materialized, give an intense e+ source. Pair production may occur in the same crystal or, preferably, in amorphous converters put after. Such solution is represented below (a distance of 2 meters is chosen corresponding to precise simulations): this set of 2 targets substitutes the Compton cavities in the lay-out.

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ADVANTAGES OF A CRYSTAL SOURCE YIELD: the yield obtained with 8 mm thick W crystals (or 4 Xtal+4 Am.)

fit with ILC requirements (cf CERN experiment) DEPOSITED POWER: due to faster development of the showers in

crystals w.r.t. amorphous, there is less deposited power in equivalent targets (giving same total yield) => see X.Artru et al: PRST-AB 6 (2003) 091003

PEDD: the Peak Energy Deposited Density, responsible of important thermal gradients leading to target destruction (see SLC target) can be advantageously handled in an hybrid scheme (see figure). As a consequence, the number of targets can be drastically reduced.

RADIATION RESISTANCE: a test made at SLAC with a W crystal showed no damages up to a fluence of 2.1018e-/mm2

RELIABILITY: a W crystal 10.5 mm thick is installed since july 2006 on KEKB linac and serves as the positron source without diminution in the yield.

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COLLABORATION STATUS 1)The works made separately concerned: # Generation of polarized positrons in thin amorphous targets by

polarized rays. {use of CAIN +EGS4 (and GEANT4) by LAL-IPNL and GEANT4 by IHEP)

# Capture and transport of the positrons (PARMELA by LAL) # Generation of positrons due to intense production in crystals

with channeling 2) Visits to IHEP by 2 french collaborators during CCAST ILC in Nov.

2007; discussions prepared the incoming programme

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PERSPECTIVES # Collaborations with crossed simulations made on the

generation of polarized positrons # Optimization of the capture and transport parameters for the

emitted positrons; work sharing is considered concerning the longitudinal and transverse phase spaces.

# Welcome of a PhD student from IHEP in LAL/IPNL; the student would be situated in Orsay with frequent visits to IPN-Lyon. A co-direction for the thesis is wished.