Beam dump and positron production studies for CEPC

Beam dump and positron production studies for CEPC and past CLIC studies Armen Apyan Northwestern University Evanston, IL, US 26/05/2014 FCC-ee Accelerator meeting #6 1

Outline u Polarized Positron Production Methods u CLIC Main Beam Dump u Summary 26/05/2014 FCC-ee Accelerator meeting #6 2

Polarized Positron Production 26/05/2014 FCC-ee Accelerator meeting #6 3

The Concept + decay of: Ø naturally existing radioactive isotopes, Ø 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 Ø Positron beam is longitudinally polarized at the upper limit of the e+ energy. 26/05/2014 FCC-ee Accelerator meeting #6 4

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. 26/05/2014 FCC-ee Accelerator meeting #6 5

Conventional Scheme based on oriented crystal Separate crystalline target for production of circularly polarized ’s by coherent bremsstrahlung of longitudinally polarized electrons Separate amorphous target for e+ production: Step-1: Produce Circularly polarized γ Step 2: Convert γ’s to e+ Crystal target Amorphous converter Capture system e+ High intensity Pol. e- beam Pol. ’s Bending Magnet 26/05/2014 Crystal increases the yield of the photons, not the polarization e- To beam dump FCC-ee Accelerator meeting #6 A. Apyan, H. Braun, M. Velasco for CLIC, 2005 6

Choice of Crystal Radiator and Converter Circularly Polarized Photon Sources: Crystal High Z amorphous target Diamond single crystal 1 cm: Tungsten 0. 2 mm thick • Tight lattice (small lattice constant 3. 567Å • Low Z=6 Bremsstrahlung – proven method. stable, Coherent Bremsstrahlung – stable, proven method. Polarized Positron Converter: High Z material Amorphous Tungsten 0. 3 mm thick 26/05/2014 FCC-ee Accelerator meeting #6 7

Number of Photons and Positrons per Incident Electron Tungsten was used as a positron converter for both configuratons Proposed configurations give the following yield of photons Tungsten radiator 0. 007 /e. Diamond single crystal 0. 03 /e 26/05/2014 FCC-ee Accelerator meeting #6 The crystal scheme provides ~3 times larger photon yield than amorphous configuration with the same beam parameters. 8

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. The photons are produced by scattering of virtual photons of a helical undulator with period λu off an electron beam. Helical undulator Converter target e+ high energy e- beam Capture system Pol. ’s e- Bending Magnet To beam dump References: V. E. Balakin, A. A. Mikhailchenko, “The Conversion System for Obtaining High Energy Electrons and Positrons”, Preprinit BINP-79 -85, 1979. 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. 26/05/2014 FCC-ee Accelerator meeting #6 9

Observation of Polarized Positrons from an Undulator-Based Source: E 166 Experiment Electron beam energy 46. 6± 0. 1 Ge. V Repetition rate of 10 Hz with 1– 4 x 109 e/pulse Normalized beam emittances 2. 2(0. 5)X 105 mrad Transverse spot size σx σy 35 μm Helical undulator length 1 m Undulator aperture 0. 9 mm The photon beam impinged upon a 0. 2 radiation-length tungsten target T 1 to produce positrons and electrons which were separated in spectrometer, and the polarization and rate of the positrons were measured in transmission polarimeter TP 1. The unconverted photons were monitored in a second transmission polarimeter, TP 2. References: G. Alexander et al. , Observation of Polarized Positrons from an Undulator-Based Source , Phys. Rev. Let. , 100, 210801 (2008) 26/05/2014 FCC-ee Accelerator meeting #6 10

Laser based Polarized Positrons Source Circularly polarized from Compton backscattering of laser beam on e- beam and e+e- production in converter target. r s n to l. o P o h p C O 2 e as l Converter target Capture system e+ High intensity Pol. ’s e- beam Bending Magnet e- 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 To beam dump References: T. Omori, “A Polarized Positron Beam for Linear Collider”, KEK Preprints 98 -237 and 99 -188. 26/05/2014 FCC-ee Accelerator meeting #6 11

Compton Source R&D at ATF 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. 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. 26/05/2014 FCC-ee Accelerator meeting #6 A fundamental scheme of polarized positron production. Righthanded polarized laser photons are backscattered off relativistic electrons resulting in production of left handed polarized rays in the forward direction (in the highenergy part of the spectrum). Pair creation of the rays through a tungsten plate generates lefthanded positrons in the high-energy part. 12

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 ---- GEANT 4, FLUKA Magnetization of Iron -----POISSON And many other. 26/05/2014 FCC-ee Accelerator meeting #6 13

Beam Dump Consideration 26/05/2014 FCC-ee Accelerator meeting #6 14

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 26/05/2014 FCC-ee Accelerator meeting #6 15

Baseline Design intermediate dump carbon based absorbers 1. 5 m side view ILC style water dump C-shape magnets 27. 5 m window-frame magnets 67 m 6 m 4 m 315 m 1. Separation of disrupted beam, beamstrahlung photons and coherent pairs 2. Back-bending region to direct the beam onto the final dump àAllowing non-colliding beam to grow to acceptable size 26/05/2014 FCC-ee Accelerator meeting #6 16

Some Numbers • Uncollided beam: sx = 1. 56 mm, sy =2. 73 mm 5. 6 mm 2 e+e- collision creates disrupted beam Huge energy spread, large x, y div in outgoing beam total power of ~10 MW High power divergent beamstrahlung photons 2. 2 photons/incoming e+e 2. 5 E 12 photons/bunch train Collided 1. 5 Te. V Beam at water dump 315 m from IP total power of ~4 MW Right sign coherent beam Beamstrahlung photons Coherent e+e- pairs Disrupted beam 5 E 8 e+e- pairs/bunch. X à 170 k. W opposite charge Incoherent e+e- pairs 4. 4 E 5 e+e- pairs/bunch. X 78 W 26/05/2014 FCC-ee Accelerator meeting #6 17

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 14 MW to the main beam dump. 26/05/2014 FCC-ee Accelerator meeting #6 18

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 26/05/2014 FCC-ee Accelerator meeting #6 19

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. 26/05/2014 FCC-ee Accelerator meeting #6 20

Baseline Main Dump Design (CLIC) CLIC ILC Beam energy 1500 Ge. V # particles per bunch 3. 7 x 109 2 x 1010 # bunches per train 312 2820 Duration 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 5 Beam power 14 MW 18 MW • 2010: CLIC 14 MW water dump – Cylindrical vessel 60. 0 cm diameter – Volume: 25 m 3, Length: 10 m window (Ti) 1. 0 mm thick – Diameter of 1. 8 m – Water pressure at 10 bar (boils at 180 C) – Ti-window, 1 mm thick, 60 cm diameter baseline for CLIC 2010 main dump 26/05/2014 20. 0 mm thick stainless steel vessel ILC type Dump axis water dump FCC-ee Accelerator meeting #6 Diameter 1. 8 m Length 10. 0 m 21

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 1800 C. 6. Water flow rate around 1 -1. 5 m/s 7. Window made of Ti or other material, 1 mm thick and 60 cm diameter. 26/05/2014 FCC-ee Accelerator meeting #6 22

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. 5 Te. V, sx = 1. 56 mm, sy =2. 73 mm 5. 6 mm 2 26/05/2014 FCC-ee Accelerator meeting #6 23

Main Beam Dump Issues • Maximum energy deposition per bunch train: 270 J/cm 3 • Remove heat deposited in the dump – Minimum water flow of 25 -30 litre/s with v=1. 5 m/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 10 bar hydrostatic pressure). dimensioning water tank, window, etc. . • Radiolytical/radiological effects – Hydrogen/oxygen recombiners, handling of 7 Be, 3 H 26/05/2014 FCC-ee Accelerator meeting #6 24

Simulation Tools Guinea Pig CAIN Daniel Schulte Kaoru Yokoya Ph. D Thesis 1996 ABEL 1984 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. 26/05/2014 FCC-ee Accelerator meeting #6 25

Beam-beam simulation by Guinea-PIG TLEP HF 2012 workshop report, p. 42, Table 8. 2 Armen’s simulation 12 M (multiplied by 30) CERN simulation 360 M 26/05/2014 FCC-ee Accelerator meeting #6 26

CAIN and Guinea-PIG simulations for CEPC and TLEP 26/05/2014 FCC-ee Accelerator meeting #6 27

Conclusion (1) Polarized or unpolarized e+e- colliders? All mentioned schemes can be used to produce either polarized or unpolarized positron beams. The schemes use different methods for production of circularly polarized gamma beams. The schemes use the same method for converting gamma beam into electronpositron 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. 26/05/2014 FCC-ee Accelerator meeting #6 28

Conclusion (2) Conventional Scheme with crystal • Low positron yield. • • Needs to be checked. • 26/05/2014 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. FCC-ee Accelerator meeting #6 29

Conclusion (3) Undulator based Scheme Laser based Scheme High intensity positron beam. Highly polarized positronbeam up to 60%. High Intensity positron beam. Highly polarized positron beam up to 70%. Ease to switch laser polarization state Use electron main linac (150 -250 Ge. V). Long helical undulator 150220 m. Needs to be aligned. Problem on design, construction, commissioning, maintenance. 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. 26/05/2014 FCC-ee Accelerator meeting #6 30

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 H 2 O 2 and H 2. Production of radioactive isotopes – photospallation on oxygen. Beam dump luminosity monitor based on detection of high energy muons is considered after the main beam dump u High energy muons escape the main dump nearly unaffected, except for small energy losses due to ionization. u Transverse distribution of muons depends on the offset of primary beams. Each of the two beam dumps of CEPC must be able to dissipate 360 k. J of beam energy in the 0. 18 ms circulation time, which equates to a power of 2 GW. 26/05/2014 FCC-ee Accelerator meeting #6 31

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 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 k. J 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. 26/05/2014 FCC-ee Accelerator meeting #6 32