Beam dump and positron production studies for CEPC and past CLIC studies Armen Apyan Northwestern University Evanston, IL, US 26/05/2014FCC-ee Accelerator meeting #61

Outline  Polarized Positron Production Methods  CLIC Main Beam Dump  Summary 26/05/2014FCC-ee Accelerator meeting #62

Polarized Positron Production 26/05/2014FCC-ee Accelerator meeting #63

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. 26/05/2014FCC-ee Accelerator meeting #64

The Methods of Polarized Positrons Production Circularly polarized  by bremsstrahlung of electrons (longitudinally polarized) in amorphous or crystalline target and e + e - production in converter target. Circularly polarized  from high energy e- (could be unpolarized) beam passing through an helical undulator and e + e - production in converter target. Circularly polarized  from Compton backscattering of circularly polarized laser beam on e- (could be unpolarized) beam and e + e - production in converter target. 26/05/2014FCC-ee Accelerator meeting #65

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 Bending Magnet To beam dump Pol. e - beam Crystal target Pol.  ’s e+e+ Amorphous converter e-e- A.Apyan, H. Braun, M. Velasco for CLIC, 2005 Crystal increases the yield of the photons, not the polarization 26/05/2014FCC-ee Accelerator meeting #66

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 material Amorphous Tungsten 0.3 mm thick 26/05/2014FCC-ee Accelerator meeting #67

Number of Photons and Positrons per Incident Electron Proposed configurations give the following yield of photons Tungsten radiator  /e - Diamond single crystal 0.03   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 26/05/2014FCC-ee Accelerator meeting #68

Polarized Positron Beam Production Based on Helical Undulator Circularly polarized  from high energy e- beam passing through an helical undulator and e + e - production in converter target. high energy e - beam Helical undulator Bending Magnet Pol.  ’s To beam dump Converter targetCapture system e+e+ 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 λ u 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, /05/2014FCC-ee Accelerator meeting #69

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 1–4x10 9 e/pulse Normalized beam emittances 2.2(0.5)X10 5 mrad Transverse spot size σ x σ y 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, (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. 26/05/2014FCC-ee Accelerator meeting #610

Laser based Polarized Positrons Source Circularly polarized  from Compton backscattering of laser beam on e- beam and e + e - production in converter target. CO 2 laser Converter target Capture system High intensity e - beam Bending Magnet Pol. photons To beam dump Pol.  ’s e-e- e+e+ References: T. Omori, “A Polarized Positron Beam for Linear Collider”, KEK Preprints and 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 26/05/2014FCC-ee Accelerator meeting #611

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, , 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. 26/05/2014FCC-ee Accelerator meeting #612

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-Pig Energy deposition, particle interaction ---- GEANT4, FLUKA Magnetization of Iron POISSON And many other. 26/05/2014FCC-ee Accelerator meeting #613

Beam Dump Consideration 26/05/2014FCC-ee Accelerator meeting #614

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 26/05/2014FCC-ee Accelerator meeting #615

Baseline Design 1.Separation of disrupted beam, beamstrahlung photons and coherent pairs 2.Back-bending region to direct the beam onto the final dump  Allowing 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 style water dump 4m 315m 6m 26/05/2014FCC-ee Accelerator meeting #616

Some Numbers e + e - collision creates disrupted beam Huge 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- pairs 5E8 e+e- pairs/bunchX  170kW opposite charge Incoherent e+e- pairs 4.4E5 e+e- pairs/bunchX  78 W Right sign coherent beam Beamstrahlung photons Disrupted beam Collided 1.5TeV Beam at water dump 315m from IP Uncollided beam: s x = 1.56mm, s y =2.73mm  5.6mm 2 26/05/2014FCC-ee Accelerator meeting #617

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. 26/05/2014FCC-ee Accelerator meeting #618

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 26/05/2014FCC-ee Accelerator meeting #619

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 dump D.R. Walz etal, 1965 This presents the following portion of the incoming beam with fresh cold water. 26/05/2014FCC-ee Accelerator meeting #620

Baseline Main Dump Design (CLIC) 2010: CLIC 14 MW water dump –Cylindrical vessel –Volume: 25m 3, 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 CLICILC Beam energy1500 GeV500 GeV # particles per bunch3.7 x x # bunches per train Duration of bunch train156 ns 950  s Uncollided beam size at dump  x,  y 1.56 mm, 2.73 mm2.42 mm, 0.27 mm # bunch trains per second505 Beam power14 MW18 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 axis ILC type water dump 26/05/2014FCC-ee Accelerator meeting #621

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 m 3 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 C. 6. Water flow rate around m/s 7. Window made of Ti or other material, 1mm thick and 60 cm diameter. 26/05/2014FCC-ee Accelerator meeting #622

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, s x = 1.56mm, s y =2.73mm  5.6mm 2 26/05/2014FCC-ee Accelerator meeting #623

Main Beam Dump Issues Maximum energy deposition per bunch train: 270 J/cm 3 Remove heat deposited in the dump –Minimum water flow of 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 7 Be, 3 H 26/05/2014FCC-ee Accelerator meeting #624

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. 26/05/2014FCC-ee Accelerator meeting #625

Beam-beam simulation by Guinea-PIG TLEP Armen’s simulation 12M (multiplied by 30) CERN simulation 360M 26/05/2014FCC-ee Accelerator meeting #626 HF2012 workshop report, p. 42, Table 8.2

CAIN and Guinea-PIG simulations for CEPC and TLEP 26/05/2014FCC-ee Accelerator meeting #627

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. 26/05/2014FCC-ee Accelerator meeting #628

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. 26/05/2014FCC-ee Accelerator meeting #629

Conclusion (3) Use electron main linac ( GeV). Long helical undulator m. Needs to be aligned. Problem on design, construction, commissioning, maintenance. Undulator based SchemeLaser 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. 26/05/2014FCC-ee Accelerator meeting #630

Beam Dump 26/05/2014FCC-ee Accelerator meeting #631 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 H 2 O 2 and H 2. 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 dump  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 26/05/2014FCC-ee Accelerator meeting #632 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 H 2 O 2 and H 2. 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.