Beam dump and positron production studies for CEPC and past CLIC studies
Armen ApyanNorthwestern University
Evanston, IL, US
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Outline
Polarized Positron Production Methods
CLIC Main Beam Dump
Summary
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Polarized Positron Production
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The Concept
+ decay of: naturally existing radioactive isotopes, short – life isotopes produced by an accelerator
Concerning polarization, positrons emitted from beta decays are longitudinally polarized but are subject to a large energy spread, a wide angular distribution, low intensity, etc.
e+e- pair production of photons Positron beam is longitudinally polarized at the
upper limit of the e+ energy.
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The Methods of Polarized Positrons Production
• Circularly polarized g by bremsstrahlung of electrons (longitudinally polarized) in amorphous or crystalline target and e+e- production in converter target.
• Circularly polarized g from high energy e- (could be unpolarized) beam passing through an helical undulator and e+e- production in converter target.
• Circularly polarized g from Compton backscattering of circularly polarized laser beam on e- (could be unpolarized) beam and e+e- production in converter target.
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Conventional Scheme based on oriented crystal
Separate crystalline target for production of circularly polarized ’s by coherent bremsstrahlung of longitudinally polarized electrons
Step-1: Produce Circularly polarized γ
Step 2: Convert γ’s to e+
Separate amorphous target for e+ production:
Capture system
High intensity
BendingMagnet To beam dump
Pol. e- beam
Crystaltarget
Pol. ’s
e+
Amorphous converter
e-
A. Apyan, H. Braun, M. Velascofor CLIC, 2005
Crystal increases the yield of the photons, not the polarization
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Choice of Crystal Radiator and Converter
Circularly Polarized Photon Sources:
High Z amorphous target
Tungsten 0.2 mm thick
Bremsstrahlung – stable, proven method.
Crystal
Diamond single crystal 1cm:•Tight lattice (small lattice constant 3.567Å
•Low Z=6
Coherent Bremsstrahlung – stable, proven method.
Polarized Positron Converter:
High Z materialAmorphous Tungsten 0.3 mm thick
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Number of Photons and Positrons per Incident Electron
Proposed configurations give the following yield of photonsTungsten radiator 0.007
g/e-
Diamond single crystal 0.03 g/e-
The crystal scheme provides ~3 times larger photon yield than amorphous configuration with the same beam parameters.
Tungsten was used as a positron converter for both configuratons
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Polarized Positron Beam Production Based on Helical Undulator
Circularly polarized g from high energy e- beam passing through an helical undulator and e+e- production in converter target.
high energy
e- beam
Helical undulator
BendingMagnet
Pol. ’s
To beam dump
Converter target Capture system
e+
e-
The electron beam is coaxial with the undulator. The highest energy photons take on the polarization of the undulator field, so that a helical undulator leads to circularly polarized photons. The intensity of undulator photons depends on the intensity of the virtual photons of the undulator, and hence on the square of its magnetic field strength.
The photons are produced by scattering of virtual photons of a helical undulator with period λu off an electron beam.
References:V.E. Balakin, A.A. Mikhailchenko,
“The Conversion System for
Obtaining High Energy Electrons and Positrons”, Preprinit BINP-79-85, 1979.
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Observation of Polarized Positrons from an Undulator-Based Source: E166 Experiment
Electron beam energy 46.6±0.1 GeV
Repetition rate of 10 Hz with 1–4x109 e/pulse
Normalized beam emittances 2.2(0.5)X105 mrad
Transverse spot size σx σy 35 μm
Helical undulator length 1m
Undulator aperture 0.9mm
References:G. Alexander et al., Observation of Polarized Positrons from an Undulator-Based Source ,Phys. Rev. Let., 100, 210801 (2008)
The photon beam impinged upon a 0.2-radiation-length tungsten target T1 to produce positrons and electrons which were separated in spectrometer, and the polarization and rate of the positrons were measured in transmission polarimeter TP1. The unconverted photons were monitored in a second transmission polarimeter, TP2.
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Laser based Polarized Positrons Source
Circularly polarized g from Compton backscattering of laser beam on e- beam and e+e- production in converter target.
CO 2 la
ser
Converter target
Capturesystem
High intensity
e- beam
BendingMagnet
Pol. photo
ns
To beam dump
Pol. ’s e-
e+
References:T. Omori, “A Polarized Positron Beam for Linear Collider”, KEK Preprints 98-237 and 99-188.
The main advantages of the Compton scheme are that the positron source is imposed independently with respect to the main linac and the required drive electron beam energy is much lower as compared to the undulator scheme
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Compton Source R&D at ATF
References:T. Omori et al., “Efficient Propagation of Polarization from Laser Photons to Positrons through Compton Scattering and Electron-Positron Pair Creation“. Phys. Rev. Let., 96, 114801, 2006.
A fundamental scheme of polarized positron production. Right-handed polarized laser photons are backscattered off relativistic electrons resulting in production of left handed polarized rays in the forward direction (in the high-energy part of the spectrum). Pair creation of the rays through a tungsten plate generates left-handed positrons in the high-energy part.
The magnitude of the positron polarization was calculated as 73 ± 15 ±19%, where the first error is a statistical one and the second error is systematic one which comes from the uncertainty in a Monte Carlo simulation.
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Polarized Positron Source for CEPC
Which scheme of polarized positron production is good for CEPC ?
The three concepts have their own problems connected with the cost and technical complexity.
Many investigations were done towards the polarized positron production in the last decade.
Several existing Monte Carlo codes can help in simulation of the positron production with high accuracy. For example:Laser – electron interaction ------- CAIN, Guinea-PigEnergy deposition, particle interaction ---- GEANT4, FLUKAMagnetization of Iron ----------- POISSONAnd many other.
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Beam Dump Consideration
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Design Consideration: CLIC post-collision line
• Transport particles of all energies and intensities from IP to dump
• Separation of the outgoing beams for diagnostics (luminosity monitoring)
• Control beam losses in the magnets• Minimize background in the experiments• Stay clear of the incoming beam
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Baseline Design
1. Separation of disrupted beam, beamstrahlung photons and coherent pairs
2. Back-bending region to direct the beam onto the final dumpAllowing non-colliding beam to grow to acceptable size
intermediate dump
side view
27.5m
67m
1.5m C-shape magnets
window-frame magnets
carbon based absorbers
ILC stylewater dump
4m 315m6m
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Some Numbers
e+e- collision creates disrupted beamHuge energy spread, large x,y div in outgoing beam
total power of ~10MW
High power divergent beamstrahlung
photons2.2 photons/incoming e+e-
2.5 E12 photons/bunch train
total power of ~4MW
Coherent e+e- pairs5E8 e+e- pairs/bunchX
170kW opposite charge
Incoherent e+e- pairs
4.4E5 e+e- pairs/bunchX 78 W
Right sign coherent beamBeamstrahlung photons
Disrupted beam
Collided 1.5TeV Beam at water dump 315m from IP
• Uncollided beam: sx = 1.56mm, sy =2.73mm 5.6mm2
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Particles distribution on the CLIC Main Beam Dump
Photons
Disrupted beam
Coherent beam
The CLIC post collision line is designed to transport the un-collided beams and the products of the collided beams with a total power of 14MW to the main beam dump.
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Main Beam Dump (History)
• 1966: SLAC beam dump
– 2.2 MW average beam power capacity
– Power absorption medium is water
• 2000: TESLA– 12 MW beam power capacity– Water dump
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Concept of the Water based Beam Dump
The basic principle of water dump is to present the incoming beam with a region of cold water. The beam dissipates its energy into water. It is essential that the volume of water exposed to the core of the beam be moved transverse to the momentum vector of the beam to prevent “volume boiling”. To renew the water volume in the central part of the shower, between successive bunch trains, a water flows transverse to the direction of the beam.
2.2 MW beam dumpD.R. Walz etal, 1965
This presents the following portion of the incoming beam with fresh cold water.
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Baseline Main Dump Design (CLIC)
• 2010: CLIC 14 MW water dump– Cylindrical vessel– Volume: 25m3, Length: 10m– Diameter of 1.8m– Water pressure at 10bar (boils at 180C)– Ti-window, 1mm thick, 60cm diameter
baseline for CLIC 2010 main dump
CLIC ILCBeam energy 1500 GeV 500 GeV
# particles per bunch 3.7 x 109 2 x 1010
# bunches per train 312 2820Duration of bunch train 156 ns 950 ms
Uncollided beam size at dump sx, sy 1.56 mm, 2.73 mm 2.42 mm, 0.27 mm
# bunch trains per second 50 5Beam power 14 MW 18 MW
Length 10.0 m
Diameter 1.8 m
60.0 cm diameter
window (Ti)1.0 mm thick
20.0 mm thick stainless steel vessel
Dump axisILC type water dump
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General Parameters of Water Dump
1.The water beam absorber is a cylindrical vessel with an entrance and exit windows in both sides.
2. Volume of water around 25 m3
3. Length of dump around 10 m (sufficient multiple of X
0)
4. Diameter of dump about 1.8 m.5. Pressure of water 10 bar, at which water boils
at 1800C.6. Water flow rate around 1 -1.5 m/s7. Window made of Ti or other material, 1mm
thick and 60 cm diameter.
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Longitudinal and Transverse Distributions of Paricles
The issue for non-colliding beams are the small beam spot and consequently the high power density on a small point of impact on the dump window and the dump itself.
Uncollided beam: E=1.5TeV, sx = 1.56mm, sy =2.73mm 5.6mm2
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Main Beam Dump Issues• Maximum energy deposition per bunch train: 270 J/cm3
• Remove heat deposited in the dump
– Minimum water flow of 25-30 litre/s with v=1.5m/s• Guarantee dump structural integrity
– Almost instantaneous heat deposition generate a dynamic pressure wave inside the bath!
– Cause overstress on dump wall and window (to be added to 10bar hydrostatic pressure).
dimensioning water tank, window, etc..• Radiolytical/radiological effects
– Hydrogen/oxygen recombiners, handling of 7Be, 3H
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Simulation Tools
CAIN
Kaoru Yokoya
ABEL 1984
Guinea Pig
Daniel Schulte
PhD Thesis 1996
These codes are fundamental tools for R&D on future linear colliders. These programs simulate beam-beam interactions in high-energy e+e colliders and the impact of the beam-beam effect on luminosity and background.
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Beam-beam simulation by Guinea-PIG
TLEPArmen’s simulation 12M (multiplied by 30)CERN simulation 360M
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HF2012 workshop report, p. 42, Table 8.2
CAIN and Guinea-PIG simulations for CEPC and TLEP
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Conclusion (1)
Polarized or unpolarized e+e- colliders?
The schemes use different methods for production of circularly polarized gamma beams.
The schemes use the same method for converting gamma beam into electron-positron pairs.
There are many Monte-Carlo simulation and experimental research devoted to positron polarized production. It is time to think about CEPC positron production scheme.
All mentioned schemes can be used to produce either polarized or unpolarized positron beams.
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Conclusion (2)
Low positron yield.
Needs to be checked.
Conventional Scheme with crystal
• Set of comparatively thin successive targets may increase the yield of photons, positrons and ease heating /stress problems for each target
• Small Multiple scattering angle• Well established physics of the processes
of interaction of electrons with single crystal
• Low cost and the simple technical solution make the conventional method attractive.
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Conclusion (3)
Use electron main linac (150-250 GeV).Long helical undulator 150-220m. Needs to be aligned.Problem on design, construction, commissioning, maintenance.
Undulator based Scheme Laser based Scheme
High Intensity positron beam.Highly polarized positron beam up to 70%.Ease to switch laser polarization state
High intensity positron beam.Highly polarized positronbeam up to 60%.
Required high intensity laser.The positrons accepted per one bunch crossing does not fit the high CLIC and ILC requirements.For this reason, stacking of the positron bunches was proposed.The performance of the scheme is complicated.
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Beam Dump
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It was seen that the deposited energy through the shower spreads to the whole volume of water dump. Which can cause:
Heating of the water.
Radiolysis – water molecule is broken up into H+,(OH) and other radicals. The result will be H2O2 and H2.
Production of radioactive isotopes – photospallation on oxygen.
Each of the two beam dumps of CEPC must be able to
dissipate 360 kJ of beam energy in the 0.18 ms
circulation time, which equates to a power of 2 GW.
Beam dump luminosity monitor based on detection of high energy muons is considered after the main beam dumpHigh energy muons escape the main dump nearly unaffected, except for small
energy losses due to ionization.Transverse distribution of muons depends on the offset of primary beams.
Beam Dump
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It was seen that the deposited energy through the shower spreads to the whole volume of water dump. Which can cause:
Heating of the water.
Radiolysis – water molecule is broken up into H+,(OH) and other radicals. The result will be H2O2 and H2.
Production of radioactive isotopes – photospallation on oxygen.
Each of the two beam dumps of CEPC must be able to
dissipate 360 kJ of beam energy in the 0.18 ms circulation
time, which equates to a power of 2 GW.
The luminosity monitoring system is considered at the CLIC beam dump. Luminosity monitor is based on detection of high energy muons
High energy muons escape the main dump nearly unaffected, except for small energy losses due to ionization.
Transverse distribution of muons depends on the offset of primary beams.