1. Research on a new crystal-assisted positron source for the ILC
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A_R&D_06 proposal
T.Suwada
Accelerator Laboratory, KEK
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

2. Collaboration Japanese Group
K.Furukawa, T.Kamitani, T.Omori, M.Satoh, T.Sugimura, T.Suwada, J.Urakawa, Accelerator Laboratory, KEK
K.Umemori, IMSS, KEK
T.Takahashi, S.Kawada, Hiroshima University
French Group
A.Variola, O. Dadoun, F. Poirier, I. Chaikovska, LAL-Orsay
R.Chehab, X. Artru, M. Chevallier, IPNL-Lyon
with collaboration of V. Strakhovenko , BINP-Novosibirsk
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

3. Introduction
In order to achieve high luminosities, positron sources must be boosted towards the next generation of B-factories and e+e- linear colliders.
The present RDR-based ILC e+ (positron) source uses photo-production from a long helical undulator to generate positrons.
The crystal-assisted positron source would be one of the alternative options for the ILC.
This new scheme has been already adopted as the CLIC positron source.
We propose it is very efficient that feasibility studies based on the new scheme could be conducted under powerful collaboration between France and Japan.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

4. Reference Design Report (RDR) Design
Undulator
Located at 150GeV point in electron linac
Helical
Pitch 1.15cm, B=0.86T (K=0.92)
Beam aperture 5.85mm
Target
Ti Alloy (0.4X0)
Wheel with diameter 1m, thickness 1.4cm
Rotating speed 100m/s (2000rpm)
Capture
Flux concentrator
KAS (Keep Alive Source)
Independent, conventional
10% intensity
K. Yokoya, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

5. Difficult feasibility on the RDR-based e+ source
Rotating speed of the target is required to be more than 100 m/s (2000 rpm) in vacuum in order to reduce heat load on the target.
 This may cause Mechanical difficulty.
Dedicated machine protection systems are needed before and after undulator.
 This may require complicated protection systems.
Electron linac up to 150 GeV point must always be operated near full gradient.
 It is difficult to separate the e+ source itself from the primary electron delivering system.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

6. Alternative options of the proposed e+ source
Polarized e+ source
Laser-Compton scheme with circularly polarized laser and 1.2 GeV electron beam (A collaboration between France and Japan has been also started to conduct experiments at ATF for polarized e+ source).
Unpolarized e+ source
Liquid lead target
 Liquid lead itself can withstand heavy heat load, and however, it is difficult to fabricate a robust vacuum window (BN disk, etc.) against the heavy heat load.
Crystal-assisted target
 This is one of the bright schemes for providing high-intensity e+ production and also for reducing heavy heat load on the target.
 This new scheme has been also adopted in the baseline configuration of the CLIC e+ source.
 However, this new scheme has no total reliable answer yet for the ILC, and thus, we need much more intensive investigations.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

7. Radiation processes in single-crystal - so-called atomic undulator -
Simulations by French group
(1994), Photon Energy Spectrum
2 GeV e-
Crystal W
Amorphous W
20 GeV e-
Crystal W
Amorphous W
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

8. AN ATOMIC UNDULATOR FOR THE GENERATION OF UNPOLARIZED POSITRONS FOR CLIC AND ILC
An hybrid source using an atomic undulator (first proposition for a crystal source, Proceedings of PAC, Chicago, 1989)
R.Chehab, A.Artru, F.Couchot, A.R.Nyaiesh, F.Richard
Simulations of an hybrid source made of Ge or Si crystals as radiators and amorphous W as converters:
For Ge and Si crystals oriented on <110> axis and 1 cm thick, we had e+ accepted yields of 0.5 to 0.6 e+/e- for 0.5 Xo thick converter and 1.2 to 1.3 e+/e- for 1 Xo thick; the incident e- energy is 20 GeV
R. Chehab, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

9. Brief History of Theoretical Studies
Theoretical investigations were intensively conducted mainly in France (LAL, Lyon) and Novosibirsk
The characteristics of the electron-photon showers developed in axially aligned crystal target were studied in an energy range (<20 GeV) of an incident electron beam.
X.　Artru, et al., NIMB 48 (1990) 278.
X.　Artru, et al., NIMA, 344 (1994) 443.
V.N. Baier, et al., NIMB 103 (1995) 147.
The crystal effect is much larger at the higher energies.
V.N.Baier and V.M.Strakhovenko, PRSTAB 5, (2002).
The amount of the energy deposition in tungsten crystal is considerably reduced by 20-50% in comparison with that in a conventional tungsten.
X.　Artru, et al., PRSTAB 6, (2003).
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

10. Brief History of Experimental Studies
Experimental investigations
Proof-of-principle experiments
At LAL-Orsay experiment (Ee- from 1.1 to 2 GeV), only the photon yield and energy radiated were measured. The confirmation of enhancement in soft γ–ray production was obtained. (X.Artru, et al., NIMB 119 (1996) )
At INS-Tokyo experiment, Yoshida, et al., first demonstrated a clear enhancement of the e+ yield in a tungsten crystal target using a 1.2-GeV electron beam. (K. Yoshida, et al., Phys. Rev. Lett. 80, 1437, 1998)
Systematic experimental studies
Chehab, et al. studied positron-yield enhancements from tungsten crystal targets with 6 and 10 GeV electrons at CERN-SPS (WA103). (R.Chehab, et al., Phys. Lett B 525 (2002) 41, X.Artru, et al., NIM B 240 (2005) 762.)
Suwada, et al. systematically studied the positron-production efficiency as a function of the tungsten crystal thickness with 4 and 8 GeV electrons at the KEKB injector linac. (T.Suwada, et al., Phys. Rev. E 67 (2003) , T.Suwada, et al., PRSTB 10, (2007)). In this activity, a French participant (R.Chehab, LAL-Orsay) joined.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

11. Positron-production experiment at CERN(WA103)
Primary beam: tertiary e- beams of the SPS, E=6, 10 GeV
Fig.1. The setup scheme.
Drift chamber in two parts. Drift chamber in two parts.
DC1 is outside the magnetic field. DC2 is in the magnetic
field of MBPS magnet.
MBPL is the sweeping magnet.
Fig.11. E=10 GeV, Target thickness (tungsten crystal, Wc)=4 mm,
Wc crystal axis <111>,
Left: e+ momentum distributions,
Right: e+ angular distributions
On-axis
On-axis
Off-axis
Off-axis
X.Artru, et al., NIMB 240 (2005) 762.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

12. Application of tungsten crystal target to the KEKB positron source
CERN Courier, Vol. 46 No.10 Dec. 2006, p.7. 
E=4 GeV,
Wc thickness
=10.5 mm,
<111> Wc axis
Ne+/Ne- conversion
efficiencies
Optimization of the crystal target thickness
25% increase
Crystal W
Amorphous W
This work was accomplished in collaboration with a French colleague
(R. Chehab, IPNL-Lyon)
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

13. AN ATOMIC UNDULATOR FOR THE GENERATION OF UNPOLARIZED POSITRONS FOR CLIC AND ILC
THE HYBRID SCHEME FOR FUTURE LINEAR COLLIDERS
Assuming thin crystal target and, hence, moderate heating in it and in order to lower the amount of energy deposited in the amorphous target and also the PEDD, the following scheme has been proposed by
[R.Chehab, V.M.Strakhovenko, A.Variola]
This new scheme is the basis of the present common research between France and Japan.
Magnet
Putting a drift between the 2 targets allows sweeping off the charged particles coming from the crystal; only the ϒ impinge on the amorphous target
R. Chehab, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

14. on channeling with GEANT4. The new program will be used for extended
+ X. Artru simulation
The present work is consisting in the combination of X. Artru’s program
on channeling with GEANT4. The new program will be used for extended
simulations and for the KEKB experiment (data analysis).
O. Dadoun, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

15. Site Set up Ready at the KEK Linac Looking up from Down stream
Max. Ee-=8GeV
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010 15

16. Conclusions
It has a lot of experimental and theoretical investigations on a new crystal-assisted positron source since Chehab-san’s proposal in 1989.
After intensive experimental studies at CERN-WA103 and at KEKB-Linac, this new scheme has been successfully applied to the KEKB positron source. This application could be completed under the powerful collaboration between French (LAL-Orsay, IPNL-Lyon) and KEK.
This collaboration is much efficient with the tests of the crystal-assisted positron source which are carried out at KEKB. The French group is providing the needed simulations on crystal.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

17. Conclusions (cont’d)
This new scheme has been already adopted as the CLIC positron source.
The crystal-assisted positron source should be intensively investigated as one of alternative options although the RDR-based positron source for the ILC is based on undulator-based scheme.
Up to now, many articles and contributions to the international accelerator conferences and workshops have been elaborated in common by Japanese and French physicists.
The powerful collaboration between France and Japan has been continued even now, and however, officially approved collaboration encourages us to consider varieties of applications with crystal targets in not only the high-energy applications (the next generation of the B-factories and e+e- linear colliders) but also materials sciences.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

18. Funding requests in this year
From French group
Euro/unit
Nb of units
Total (Euro)
PhD Student visit in France
1750/month
6 months
10500
Visit to Japan (2 visits)
200/day
2x10 days
4000
Travels (2)
1000
2 travels
2000
Total
16500
From Japan group
kYen/unit
Nb of units
Total (kYen)
Travel
220
10 travels
2200
Visit to France
20/day
40 days
800
Total
3000
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

19. Backup Slides KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

20. Scheme of the prototype of liquid lead positron production target.
courtesy of Prof. Urakawa
Driving motor
Rotating vacuum feedthrough
Vacuum pump
Vacuum tank of the system
Cog-wheel pump
Target head
Window thickness 4mm, BN disks for windows
Diameter 12mm
Liquid lead transport tubes
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

21. Experimental Studies at KEK
e+ targets tested
Tungsten crystal (Wc),
Diamond, Si, etc.
4, 8 GeV e-
Target configuration for Wc
e+ momentum acceptance
ΔP/P~2.4% (FWHM)
Geometrical acceptance
ΔΩ~1msr at Pe+=20MeV/c.
8 GeV e-
Target configuration for Diamond, Si
Beam charge
~0.1/bunch
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

22. Previous Experimental Results: Rocking Curves (tungsten crystal targets, crystal axis <111>) at Ee-= 4 GeV and Pe+= 20MeV/c
2.2mm-thick Wc
5.3mm-thick Wc
9mm-thick Wc
· Variations of the positron yield as a function of the rotational angle (rocking curve) of the crystal target. The broad peak width is mainly due to multiple scattering in the crystal target. The solid lines indicate the simulation results based on coherent bremsstrahlung process (by Tomsk group).
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

23. Previous Experimental Results: Positron production enhancement at Ee-= 4 GeV
Enhancements depending on the
electron energy
· Variations of the positron-yield enhancement as a function of the electron energy at Pe+=20MeV/c for the 2.2-mm-thick tungsten crystal target. The enhancements increase monotonically with the increase of the electron energy. The results of the simulation1 and 2 were obtained by Novosibirsk and Tomsk groups, respectively.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

24. Positron Source of the KEKB Injector Linac
Pulse Coil
W Target
e+ W Target
& Pulse Coil
DC solenoids &
Accelerating sections
e+ beam
e+ beam
Primary e- beam
Primary e- beam
e- beam
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

25. Tungsten Crystal Target: Crystal preparation
Tungsten crystal target assembly after
HIP technique
· Tungsten crystals with several thicknesses were fabricated at collaborating Tomsk Polytechnic University.
· They were developed based on the solid-state re-crystallisation process of large-size crystal ingots.
· Also, a technique for sample cutting along crystallographic planes without any crystal quality damage was developed.
· The surface mosaic spreads of the crystal structure on both the beam-entrance and -exit sides were measured by an X-ray scattering with 0.5 mrad.
Fabricated tungsten crystals with dimensions 5mm×5mm×12mm and 5mm×5mm×14mm
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

26. Tungsten Crystal Target: Crystal target fabrication after post-machining
(Left) Mechanical drawing of the target assembly. The tungsten crystal is fixed to the cylindrical copper body with a diameter of 50 mm for water cooling. The heat deposited on the crystal target is conducted through a cooling water channel (water flow1.5l/min) composed of a copper pipe (4mm in diameter). Electrons (blue arrow) impinge the tungsten crystal target and they are converted to the electrons (blue arrow) and positrons (red arrow).
(Right) Target assembly after the post-machining process installed in a vacuum chamber seen from downstream. Two thermocouples are mounted 7.5 mm away from the center of the tungsten crystal, and a small hole with a 3 mm diameter is penetrated through the copper body for the transport of the electron beam.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

27. Previous Experimental Results: Positron production enhancement at Ee-= 4 GeV
Positron production efficiencies
depending on the target thickness
·Variations of the positron-production efficiency as a function of the target thickness at Pe+=20MeV/c. The results shows that the optimum thickness of ~10mm gives the maximum positron yield for the tungsten crystal target.
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

28. Performance of the crystal target: Positron-production efficiency measurements
· (Left) Positron-production efficiencies (PPE) of the 1st bunch measured for each beam pulse. The data were obtained after adjusting the incident angles of the primary electron beam with upstream steering magnets. For the sake of comparison, the data of the previous tungsten plate (June 2006) were plotted. The solid lines are gaussian-function fits of the data. The PPE is defined by Ne+ / Ne-, where Ne+ is the number of positrons captured in the positron capture section and Ne- is the number of the primary electrons.
· The results show that the increase of the positrons is 25 ±2% (28 ±2%) for the 1st (2nd) bunch on average. The results are consistent with those obtained in the previous experiments.
for 1st e+ bunch
Positron-Production Efficiency
Results of Tungsten Crystal Target
PPE(1st)=0.25±0.01,
PPE(2nd)=0.26±0.01
Results of Previous Tungsten Target
PPE(1st)=0.2±0.01,
PPE(2nd)=0.2±0.01
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

29. Performance of the crystal target: Linearity of the positron intensity to the primary-electron intensity
· (Left) Positron beam intensity plotted versus the primary electron beam intensity in a bunch. The solid line through the data indicates a linear-function fit of the data.
· The present results show that the positron yields increase linearly with the increase of the primary electron intensity without any abnormal behaviors within the experimental errors.
one bunch operation
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

30. Performance of the crystal target: Steady-state heat load measurements
· The results show that the temperature rise of the crystal target changes linearly as a function of the beam repetition rate, and that the steady-state heat load normalized by the primary electron charges is clearly reduced by ~20% in comparison with the previously-used tungsten target.
· The measured steady-state heat loads are consistent with simulation results performed by Artru, et al. (X. Artru, et al., Phys. Rev. ST Accel. Beams 6, (2003)).
T. Suwada, et al., PRSTB 10, (2007).
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

31. AN ATOMIC UNDULATOR FOR THE GENERATION OF UNPOLARIZED POSITRONS FOR CLIC AND ILC
THE PEDD (Peak Energy Deposition Density) IN AMORPHOUS TARGETS/HYBRID
ϒ , e+, e-
ϒ , e+, e-
Only ϒ
Only ϒ
The bottom curve corresponds to impinging photons, exclusively: upper curves correspond to {ϒ, e+, e-} contributions from the crystal
From V.M.Strakhovenko
R. Chehab, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010

32. AN ATOMIC UNDULATOR FOR THE GENERATION OF UNPOLARIZED POSITRONS FOR CLIC AND ILC
1-CLIC: incident beam: 5 Gev; t(Xtal)=1.4 mm; t(am)=10mm
ACCEPTED POSITRON YIELD
* For an incident e- beam with σ = 1 mm => h = 1 e+/e-
* For an incident e- beam with σ = 2.5 mm => h = 0.9 e+/e-
PEDD
Assuming an incident e- pulse of e-, we have :
CRYSTAL AMORPHOUS
PEDD/e PEDD/total PEDD/e PEDD/total
(GeV/cm3/e-) J/g (GeV/cm3/e-) J/g
σ=1mm
σ=2.5mm
An entirely amorphous target, 9 mm thick, with the same incident e- beam would have provided the same accepted yield and a PEDD of 150 J/g (s=1mm) or 40 J/g (s=2.5 mm). This shows the advantages of an hybrid scheme leading to a unique target with a PEDD < 35 J/g using an e- beam with s= 2.5 mm.
R. Chehab, POSIPOL2010, KEK, June2010
KEK Linac, Tsuyoshi Suwada
FJPPL Workshop 2010 (FJPPL'10), LAPP, Annecy-Le-Vieux, France, June15-16, 2010