GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS

R.Chehab/ R&D on positron sources for ILC/ Beijing, 2007

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GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS

R.CHEHAB (IPNL &LAL/IN2P3-CNRS), B.MOUTON, R.ROUX, A.VARIOLA, A.VIVOLI (LAL/IN2P3-CNRS), France


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GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS

INTRODUCTION Polarized positrons are, now, considered for the positron source

dedicated to the future linear collider (ILC). The first idea presented for VLEPP was based on photon generation in a long helicoidal undulator providing circularly polarized photons which created longitudinally polarized pairs in a thin target. The alternative method considered here consists in the generation of circularly polarized photons by Compton interaction between a circularly polarized Nd:Yag laser beam and an electron beam in a so-called Compton ring or in an ERL (Energy Recovery Linac). Some results obtained with this scheme are presented here.


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GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS…

PLAN Photon generation: the two schemes => Compton ring & ERL Positron converter Positron beam capture: Matching lenses (AMD or QWT) +

L-Band linac Simulation results

- Influence of the electron beam energy in Compton process

- Comparison of the Compton ring and ERL

- Comparison of two kinds of matching lenses: AMD & QWT

- Optimisation in the linac

Summary and conclusions


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GENERATION AND TRANSPORT OF A POSITRON BEAM…

PHOTON GENERATION: THE COMPTON RING The installation is made of: * an electron ring in which energetic electrons (E= 1 to 2 GeV) collide

with a circularly polarized laser beam in Fabry-Perot cavities of high finesse. The laser considered, here, is a Nd:YaG ( 1.064 m).

* a thin converter target (amorphous W; L=0.4 Xo). * a capture section with a matching device; the preferred one is the

Adiabatic Matching Device (AMD) with a magnetic field tapering slowly from a maximum value (here, 6 Tesla) to a minimum value (0.5 Tesla). This device is followed by a solenoid coil with this field value imbedding some accelerating sections. These sections are L-Band to manage a large aperture.

* a linac with standard quadrupole system brings the e+ to the DR


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GENERATION AND TRANSPORT OF A POSITRON BEAM…:COMPTON RING


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GENERATION AND TRANSPORT OF A POSITRON BEAM…

ENERGY RECOVERY LINAC (ERL) The ERL is made of a CW superconducting linac with f = 1.3 GHz,

having a maximum energy of 2 GeV and one return arc. ERL injector, using DC photocathode, works at a frequency of 20

MHz and delivers 20 ps RMS bunches. Bunch charge is 1.5 nC. Mean current is 30 mA. A bunch compression, at the end of the injector, shortens the bunch to 1 ps RMS

A Nd:YaG laser (1.064 m) associated to 10 Fabry-Perot cavities (500 mJ/cavity) provides the photon beam which crosses the electron beam at 8 degrees.

The distance between the Compton interaction point and the conversion target is of 10 meters.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: ERL


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: THE TARGET

POSITRON CONVERTER The positron converter is a

piece of amorphous tungsten 0.4 Xo thick (1.4 mm)

The target is inside the magnetic lens: the pairs are submitted to the maximum magnetic field at the target exit.

L= 0.4 Xo

L


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GENERATION AND TRANSPORT OF A POSITRON BEAM:CAPTURE&ACCELERATION

After the target an Adiabatic Matching Device (AMD) captures the positrons (and electrons) before acceleration in L-Band sections. The simulated pre-accelerator is comprising 5 cavities, each one providing an acceleration of 8-9 MeV.

For one application we shall consider a Quarter Wave Transformer (QWT)


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: MATCHING DEVICE

ADIABATIC MATCHING DEVICE

The magnetic field is tapering from a maximum value (6 Tesla) to a minimum value (0.5 Tesla) which corresponds also to the value of the magnetic field on the accelerating sections. The tapering length is 0.5 m

The iris aperture radius is of 23 mm.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: MATCHING DEVICE

QUARTER WAVE TRANSFORMER

The magnetic field has a (quasi) step-like shape. The maximum field (6 Tesla) extends on 10 cm. The transition to the lower field is on 5 cm. The lower field is of 0.5 Tesla as for the solenoid on the accelerating sections.

Maximum and minimum field are similar for the AMD and QWT for easier comparison.


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GENERATION AND TRANSPORT OF A POSITRON BEAM..:SIMULATION RESULTS

The results are concerning:

* the comparison of the two schemes: Compton Ring & ERL * the influence of the electron beam energy in the Compton ring * the comparison between two matching systems: AMD and QWT

The simulation programmes used are: - CAIN for the photon generation - EGS for the pair creation - PARMELA for the beam transport


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GENERATION AND TRANSPORT OF A POSITRON BEAM..: Compton Ring vs ERL

COMPTON RING The energy (top) and phase

(bottom) distributions at the exit of the target are presented.

Mean energy value is 19 MeV RMS Energy value is 10.7 MeV

The large phase distribution is due to the wide bunches (6 mm) in the Compton ring. RMS value is 8.2 degree


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GENERATION AND TRANSPORT OF A POSITRON BEAM..: Compton Ring vs ERL

ERL The energy (top) and phase

(bottom) distributions are given at the exit of the target.

The narrow phase distribution is due to the narrow bunches delivered by the ERL.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: Compton Ring vs ERL

COMPTON RING The beam radius variation (cm)

along the propagation axis z is presented (top) in the case of:

- an e- beam in CR with E=1.8 1.8 GeV and AMD lens

* The beam length variation (ps) along z is presented (bottom), for the same hypotheses.

Transverse emittance at the end of solenoid:

x= 69 mm mrad

y= 73 mm mrad


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COMPTON RING The variation of the relative

energy spread (top) along the axis is given for the case of CR with AMD lens

The losses along the axis z (bottom) are given for the same hypotheses


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GENERATION AND TRANSPORT OF A POSITRON BEAM…:Compton ring vs ERL

ERL The beam radius (top) and the

beam length (bottom) variations along the propagation axis z are given. The case concerns:

* an electron beam energy of 1.8 GeV in the ERL

* an AMD matching lens Transverse emittance at solenoid exit :

x= 68 mm mrad

y= 70 mm mrad


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ERL The energy dispersion

(top) and the losses (bottom) are given for the ERL case with E-=1.8 GeV and the AMD as matching system.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…:E-=1.3 GeV in CR

Positron energy spread The positron energy

distribution is given at the target exit for the case E-=1.3 GeV in the Compton ring.

Mean value is: 11.9 MeV RMS value is: 5.5 MeV


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GENERATION AND TRANSPORT OF A POSITRON BEAM…:E-=1.3 GeV in CR

POSITRON BEAM EMITTANCE AT TARGET

We represent the positron beam emittances in the two planes.

Emittance value is: x=830 mm mrad

y=720 mm mrad

The transverse beam distribution is represented also.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: CR (1.8 GeV) & QWT

BEAM EMITTANCE AT THE END OF THE FIRST PART OF PREACCELERATOR ~50 MeV

The emittance figures are given here: the emittance values are:

x=74 mm mrad

y=72 mm mrad

The transverse beam distribution is also given.


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GENERATION AND TRANSPORT OF A POSITRON BEAM…:CR(1.8 GeV) & QWT

COMPTON RING & QWT The beam radius evolution

along the z axis (top) as the beam length (bottom) along the same axis are represented.

Bunch lengthening is occuring rapidly in the first cm, where the magnetic field remains strong (6 Teslas) on 10 cm


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: CR(1.8 GeV) & QWT

COMPTON RING & QWT The relative energy spread

variation along the z axis is represented (top). The losses (bottom) are also presented.

Much part of the losses are occuring in the very first part of the matching system


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GENERATION AND TRANSPORT OF A POSITRON BEAM…: SUMMARY & CONCLUSIONS

SUMMARY AND CONCLUSIONS Simulations have been carried out on the Compton scheme with no

polarized positrons. The main results have shown that: * Concerning the two possible schemes: Compton Ring and ERL, we got

shorter positron bunches at the target, for the latter (1 ps vs 20 ps); the difference holds along the preaccelerator, however the bunch lengthening due mainly to the spiralization in the magnetic fields makes imperative in both case use of bunch compression. Lateral dimensions and emittances do not show differences, when compared.

* Concerning the electron beam energies in CR (1.3 and 1.8 GeV) , the positron spectra at the target are different, as expected; narrower spectrum is for the 1.3 GeV case. Transport in the preaccelerator does not show any significant difference.


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GENERATION AND TRANSPORT OF A POSITRON BEAM..: SUMMARY & CONCLUSIONS

SUMMARY & CONCLUSIONS * Concerning the comparison of two matching systems (AMD & QWT),

the differences are mainly in the accepted yields: the positron yield at the end of the solenoid, for the QWT case, represents less than half of the yield for the AMD. That was expected due to the larger momentum acceptance of the AMD. The emittances are quite close in both cases (around 70 mm mrad, at ~ 50 MeV). Maximum emittance at the end of the solenoid depends essentially on the low magnetic field which is the same in both cases.

Improvements(?) * Use simulations with polarized particles * Optimize the matching systems in order to transmit maximum of

polarized particles * Optimize the Compton scheme (bunch compression,…)

GENERATION AND TRANSPORT OF A POSITRON BEAM CREATED BY PHOTONS FROM COMPTON PROCESS

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