Beam dump and positron production studies for CEPC and past CLIC studies

Beam dump and positron production studies for CEPC and past CLIC studies

Armen ApyanNorthwestern University

Evanston, IL, US

26/05/2014 FCC-ee Accelerator meeting #6 1

Outline

Polarized Positron Production Methods

CLIC Main Beam Dump

Summary


Polarized Positron Production


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.


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.


Conventional Scheme based on oriented crystal

Separate crystalline target for production of circularly polarized g’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. g’s

e+

Amorphous converter

e-

A. Apyan, H. Braun, M. Velascofor CLIC, 2005

Crystal increases the yield of the photons, not the polarization


Choice of Crystal Radiator and Converter

Circularly Polarized Photon Sources:High Z amorphous target

Tungsten 0.2 mm thick

Bremsstrahlung – stable, proven method.

CrystalDiamond single crystal 1cm:•Tight lattice (small lattice constant 3.567Å

•Low Z=6Coherent Bremsstrahlung – stable, proven method.

Polarized Positron Converter:

High Z materialAmorphous Tungsten 0.3 mm thick


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


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. g’s

To beam dump

Converter target Capture systeme+

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.


Observation of Polarized Positrons from an Undulator-Based Source: E166 Experiment

Electron beam energy 46.6±0.1 GeVRepetition rate of 10 Hz with 1–4x109 e/pulseNormalized beam emittances 2.2(0.5)X105 mradTransverse spot size σx σy 35 μm Helical undulator length 1mUndulator 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.


Laser based Polarized Positrons SourceCircularly polarized g from Compton backscattering of laser beam on e- beam and e+e- production in converter target.

CO 2 laser

Converter target

Capturesystem

High intensity

e- beam

BendingMagnet

Pol. photons

To beam dump

Pol. g’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


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.


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.


Beam Dump Consideration


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


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


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 photons

2.2 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- pairs4.4E5 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


Particles distribution on the CLIC Main Beam Dump

PhotonsDisrupted beamCoherent 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.


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


Concept of the Water based Beam DumpThe 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.


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


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 X0)

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.


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


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


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.


Beam-beam simulation by Guinea-PIG

TLEPArmen’s simulation 12M (multiplied by 30)CERN simulation 360M


HF2012 workshop report, p. 42, Table 8.2

CAIN and Guinea-PIG simulations for CEPC and TLEP


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.


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.


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.


Beam Dump


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


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.

Beam dump and positron production studies for CEPC and past CLIC studies

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