Transcript generation and transport of a positron beam created by photons

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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sources for ILC/ Beijing, 2007
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GENERATION AND TRANSPORT OF A
POSITRON BEAM CREATED BY PHOTONS
FROM COMPTON PROCESS
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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…
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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…
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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 ( l=1.064 mm).
* 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…
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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 (l=1.064 mm) 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
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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.
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L= 0.4 Xo
g
L
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GENERATION AND TRANSPORT OF A
POSITRON BEAM:CAPTURE&ACCELERATION
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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
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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
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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
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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
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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
(s=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
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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
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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:
ex= 69 mm mrad
ey= 73 mm mrad
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GENERATION AND TRANSPORT OF A
POSITRON BEAM…: Compton Ring vs ERL
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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
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ERL
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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
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Transverse emittance at solenoid exit :
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ex= 68 mm mrad
ey= 70 mm mrad
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GENERATION AND TRANSPORT OF A
POSITRON BEAM…: Compton Ring vs ERL
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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
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Positron energy spread
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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
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POSITRON BEAM
EMITTANCE AT TARGET
We represent the positron beam
emittances in the two planes.
Emittance value is:
ex=830 mm mrad
ey=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
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BEAM EMITTANCE AT THE
END OF THE FIRST PART OF
PREACCELERATOR ~50
MeV
The emittance figures are given
here: the emittance values are:
ex=74 mm mrad
ey=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
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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
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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
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SUMMARY AND CONCLUSIONS
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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
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SUMMARY & CONCLUSIONS
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* 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,…)
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