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ILC Positrons. Positron Production for the ILC. J. C. Sheppard SLAC July 26, 2007. Positron Production for the ILC. What is the ILC e+ System How to make e+ Something about polarization Who works on this stuff What are the design issues What next. Parameter Reference Upgrade
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ILC Positrons Positron Production for the ILC J. C. Sheppard SLAC July 26, 2007
Positron Production for the ILC What is the ILC e+ System How to make e+ Something about polarization Who works on this stuff What are the design issues What next
Parameter Reference Upgrade Beam Energy (GeV) 250 500 RF gradient (MV/m) 28 35 Two-Linac length (km)27.00 42.54 Bunches/pulse 2625 2625 Particles/bunch (1010) 2 2 Beam pulse length (µs ) 968 968 Pulse/s (Hz) 5 5 x(IP) (nm) 543 489 y(IP) (nm) 5.7 4.0 z(IP) (mm) 0.3 0.3 δE (%) 3.0 5.9 Luminosity (1033cm−2s−1) 25.6 38.1 Average beam power (MW) 22.6 45.2 Total number of klystrons 603 1211 Total number of cavities 18096 29064 AC to beam efficiency (%) 20.8 17.5 WORLD Collaboration Multi-billion dollar project Proposed e+e– linear collider 0.5-1.0 TeV center-of-mass energies Major elements Electron injector Electron damping ring Main electron linac Electron beam delivery to IR Positron Source Positron damping ring(s) Main positron linac Positron beam delivery to IR IR Detectors at IR THE INTERNATIONAL LINEAR COLLIDER (ILC)
POSITRON SOURCE DESIGN ISSUES • Drive beam • Electrons or photons • Photons allow for the possibility of polarized positrons • How are the photons made • Multi-hundred GeV electron beam through an undulator • Compton back-scattering laser beam on a multi-GeV electron beam • Drive beam phase space • Target • Choice of material • Target heat/shock/stress • Positron capture • Beam heating • Capture RF • Capture magnetic field • Damping ring acceptance • Target vault
ILC Layout e+ & e- Damping Rings centrally located positron source uses 150 GeV electron beam L-band superconducting RF for acceleration
EM Shower Conventional e+ to damping rings POSITRON PRODUCTION SCHEMES – DRIVE BEAMS 6 GeV e- W-Re Target Undulator-based (from USLCTOS)
COMPTON-BASED POSITRON SOURCE – LASERS GLC laser power : 9.8 MW peak power per laser bunch ~ 400 kW average power (40kW with use of mirrors) ~ 73 GW peak power ILC bunch structure 2820 * 5 ~= 95 * 150 but 2820 bunch pulse trains may be able to use mirrors to relax laser parameters
ERL based Compton scheme & requirements to lasers Tsunehiko OMORI (KEK) my talk is inspired by Variola-san's talk at KEK Nov/2006 and Rainer-san's suggestion at SLAC Apr/2004 PosiPol2007@LAL 23/May/2007
UNDULATOR BASED POSITRON SOURCE Need to use ILC electron beam – possible reliability, machine development and commissioning issues Can use electron source for commissioning Long helical undulator, small aperture permanent magnet warm pulsed superconducting
Polarized Positrons from Polarized g’s Circular polarization of photon transfers to the longitudinal polarization of the positron. Positron polarization varies with the energy transferred to the positron. (Olsen & Maximon, 1959)
Polarized Positron Production in the FFTB Polarized photons pair produce polarized positrons in a 0.5 r.l. thick target of Ti-alloy with a yield of about 0.5%. Longitudinal polarization of the positrons is 54%, averaged over the full spectrum Note: for 0.5 r.l. W converter, the yield is about 1% and the average polarization is 51%.
Photon Number Spectrum Number of photons per e- per 1m undulator: Old BCD: 2.578 UK1: 1.946; UK2: 1.556; UK3: 1.107 Cornell1: 0.521; Cornell2: 1.2; Cornell3: 0.386 Gai and Liu, ANL
Photon Spectrum and Polarization of ILC baseline undulator Results of photon number spectrum and polarization characteristic of ILC undulator are given here as examples. The parameter of ILC undulator is K=1, lu=1cm and the energy of electron beam is 150GeV. Figure1. Photon Number spectrum and polarization characteristics of ILC undulator up to the 9th harmonic. Only those have energy closed to critical energy of its corresponding harmonics have higher polarization Gai and Liu, ANL
Initial Polarization of Positron beam at Target exit(K=0.92 lu=1.15) Gai and Liu, ANL
ILC Positron Polarization,captured~ 30% Pol Gai and Liu, ANL
ILC Positron Polarization In the case of the ILC baseline, the composite polarization of the captured positrons is about 30%. Spin rotation to preserve the polarization in the damping ring(s) is included To upgrade to higher polarization, the incident photon beam is collimated to remove the low energy, reversely polarized component of the spectrum (gq = 1.414). The length of the undulator needs to be increased to compensate for the loss in absolute flux.
US Institutions • Institutions doing substantial work on ILC e+ development • SLAC • overall coordination & leadership for the RDR • define parameters • target hall, remote handling, activation • beamline optics and tracking • NC L-Band accelerator structures and RF systems • Experiments – E166, FLUKA validation experiment • LLNL • target simulations (thermal hydraulics and stress, rotordynamics, materials) • target design (testing and prototyping) • pulsed OMD design • ANL • optics • tracking • OMD studies • eddy current calculations • Cornell • undulator design, alternative target concepts
European Institutions • Institutions doing substantial work on ILC e+ development • Daresbury Laboratory • EDR leadership • undulator design and prototyping • beam degradation calculations • Rutherford-Appleton Laboratory • remote handling • eddy current calculations • target hall activation • Cockcroft and Liverpool University • target design and prototyping • DESY-Berlin • target hall activation • spin preservation • photon collimation • E166
ILC Polarized Positron System Technical Issues 1. Demonstrate undulator parameters 2. Demonstrate NC SW structure hi power rf performance 3. Spinning target pre-prototype demonstration 3. Eddy current measurements on spinning target 4. Selection and Technical design of Optical Matching Device 5. System engineering for e+ source remote handling 6. System engineering for photon dump 7. System design compatibility with ILC upgrade scenarios: polarization and energy
ILC Positron EDR Milestones • Sep 07: Full layout with l/4 XMFR OMD • Dec 07: EDR Scope definition: design depth and breadth, cost, schedule, staff • Jun 08: Full upgrade scenario: polarization and ILC energy • Sep 08: OMD selection (dc immersed, pulsed FC, l/4 XMFR), Und parameter selection • Dec 08: Freeze layout, full component and civil specifications (yield, overhead, remote handling, upgrades) • Jan 09: EDR detailed component inventory • May 09: First cost review • Dec 09: Deliver EDR and preconstruction work plan
ILC Positron Design Issues, Undulator Ne+ = ecYgLungNe- ec (Adr,DEdr,Ac,ee+) ~ 15%-25% Yg(Eg, X0,Z) ~ 1%-5% ng(K,lu) ~ 2 Lu ~ 100 m
ILC Positron Design Issues, Target FOM =[aE/2(1-n)/Cv/r]/UTS(fatigued) Thermoelastic stress wrt material strength Targets break rather than melt DE/mass < 100 J/g High strength Ti-alloy (Ti6%Al4%V)
ILC Positron Design Issues, Target Need to spread out the energy deposition This is done by spinning the target at 100 m/s Same problem with windows but do not know how to spin Can imagine an entrance window Exit window will not survive
ILC RDR Baseline Positron Source • RDR Parameters • Centre of undulator to target: 500m • Active (K=0.92, period=1.21mm) undulator: 147m • Photon beam power: 131kW • Beam spot: >1.7 mm rms Toward the ILC
Baseline Target Design • Wheel rim speed (100m/s) fixed by thermal load (~8% of photon beam power) • Rotation reduces pulse energy density from ~900J/g to ~24J/g • Cooled by internal water-cooling channel • Wheel diameter (~1m) fixed by radiation damage and capture optics • Materials fixed by thermal and mechanical properties and pair-production cross-section (Ti6%Al4%V) • Wheel geometry (~30mm radial width) constrained by eddy currents. • 20cm between target and rf cavity. T. Piggott, LLNL Drive motor and water union are mounted on opposite ends of through-shaft. Toward the ILC
Target Progress • Baseline target/capture • RAL, ANL and Cornell have done Eddy current simulation which produce consistent results with multiple codes. Estimates for power dissipation in the target are >100kW for a constant field and are considered excessive. • Evaluation of ceramic target material is on-going. No conclusions. • Radiation damage of the superconducting coil is still TBD but may not be worthwhile unless a solution can be found for the eddy currents. • ANL simulation of beam heating in windows shows that an upstream window is feasible but a downstream window is not. • Alternative target/capture • Capture efficiency for the lithium lens focusing and ¼ wave solenoid is still TBD • Thermal heating and stress for the lithium lens is still on-going. • Thermal stress calculation for the liquid metal target is still on-going
Capture versus Optical Matching Device Type 0.4 0.3 Positron Capture (arb. units) 0.2 0.1 0 No OMD ¼ l xfrm Pulsed FC Immersed From F. Zhou, W. Liu
Optical Matching Device (OMD) • Optical Matching Device • factor of 2 in positron yield (3 if immersed target) • DC solenoid before target or pulsed flux concentrator after target • Pulsed device is the baseline design • Target spins in the magnetic field of the OMD • Eddy currents in the target – need to calculate power • Magnetic field is modified by the eddy currents – effect on yield?? • Eddy current mitigation • Reduce amount of spinning metal • Do experiment to validate eddy current calculations • Look for low electrical / high thermal conductivity Ti-alloys • Other materials such as ceramics • No OMD • Use focusing solenoidal lens (1/4 wave) – lower fields • OMD is upgrade to polarization(??)
Eddy Current Experiment Proposed experiment Layout at Cockcroft Institute/Daresbury (this summer) Eddy current calculation mesh - S. Antipov, W. Liu, W. Gai - ANL
Calculated Eddy Current Power Nominal RPMs sTiAlV = 6e5
Pulsed Flux Concentrator: 7T, 1 ms, 5 Hz Pulsed Flux Concentrator, circa 1965: Brechna et al.
OMD Progress • Plans and Actions (baseline target/capture): • ANL will simulate eddy currents in the pulsed magnet configuration. • UK will evaluate suitability of non-conducting materials for the target • Daresbury/Cockroft/RAL will spin a one meter target wheel in a constant magnetic field and will measure the forces. • Eddy simulations will be calculated and benchmarked against this configuration • Plans and Actions (alternative targets/capture): • ANL will determine the capture efficiency for ¼ wave focusing optics and lithium lens. • LLNL will evaluate the survivability of lithium lens to beam stress • Cornell will specify an initial design of a liquid metal target. LLNL will calculate the Stress-strain behavior of the outgoing beam window.
Undulator Challenges • High fields • Pushing the limits of technology • Short Periods • Shorter periods imply higher fields • Narrow apertures • Very tight tolerances - Alignment critical • Cold bore (4K surface) • Cannot tolerate more than few W of heating per module • Minimising impact on electron beam • Must not degrade electron beam properties but have to remove energy from electrons • Creating a vacuum • Impossible to use conventional pumps, need other solution • Minimising cost • Minimise total length, value engineering
UK Undulator Recent Highlights • Two 12mm period SC undulator prototypes built and tested • Period reduced to 12mm from 14mm • Better, more reproducible, fabrication technique • Full inclusion of iron for the first time • One 11.5mm period SC undulator built and tested • Period further reduced to RDR value of 11.5mm • New SC wire used (more SC and less Cu) • Field strength measured greater than expected, possibly due to increase in SC content of wire • Best ever field quality results (well within spec) • Full length prototype will use these parameters • Full length prototype construction started • 4m prototype design complete • Fabrication has commenced • Undulator impact studies ongoing • Emittance growth due to misalignments & wakefields shown to be <2% • Paper on undulator technology choice published by Phys. Rev. ST-AB • Paper on vacuum issues submitted to JVSTA
First 500mm long prototype Prototype 5 • Same parameters as RDR Baseline undulator • 11.5 mm period • 6.35 mm winding diameter • Peak on-axis field spec of 0.86T (10 MeV photons) • Winding directly onto copper tube with iron pole and yoke • New wire with more aggressive Cu:SC ratio of 0.9:1.0
Measurements for Prototype 5 1st results from prototype 5 at RAL Quench current 316A Equates to a field of 1.1 T in bore RDR value is 0.86 T 80% of critical current (proposed operating point) would be 0.95 T Measured field at 200A 0.822 T +/- 0.7 % (spec is +/- 1%) Prototype 5 details Period : 11.5 mm Magnetic bore: 6.35 mm Configuration: Iron poles and yoke
Summary of Prototype Results Fe former & yoke Fe former Prototype 5 @ 250A @ 200A Aluminium former
Specification for 4m Undulator Module No Beam Collimators or Beam Pipe Vacuum pumping ports in the magnet beam pipe
4m Prototype Module Construction has started, will be complete by Autumn 07 Stainless steel vacuum vessel with Central turret 50K Al Alloy Thermal shield. Supported from He bath U beam Support rod Stainless Steel He bath filled with liquid Helium. Magnet support provided by a stiff U Beam Beam Tube Superconducting Magnet cooled to 4.2K
Magnet Design Concept Steel Yoke. Provides 10% increase in field and mechanical support for former Winding pins 2 start helical groove machined in steel former PC board for S/C ribbon connections Steel yoke Cu beam pipe, withconductor wound on to tube OD
STATUS OF CORNELL UNDULATOR PROTOTYPING Alexander Mikhailichenko, Maury Tigner Cornell University, LEPP, Ithaca, NY 14853 A superconducting, helical undulator based source has been selected as the baseline design for the ILC. This report outlines progress towards design, modeling and testing elements of the needed undulator. A magnetic length of approximately 150 m is needed to produce the desired positron beam. This could be composed of about 50 modules of 4 m overall length each. This project is dedicated to the design and eventual fabrication of one full scale, 4 m long undulator module. The concept builds on a copper vacuum chamber of 8 mm internal bore
Fig.1;Extensible prototype concept for ILC positron undulator . Diameter of cryostat =102mm
Several 40 cm long undulator models with 10 and 12 mm period, Ø8 mm clear bore have been made and measured. See Table OFC vacuum chamber, RF smoothness *) Wire – Ø0.6 mm bare; **) Wire – Ø0.4 mm bare; ***) Wire – Ø0.3 mm bare Fig.3: Field profile – conical ends. 6 layer, 12 mm period – orthogonal hall probes. 1Tesla full scale For aperture diameter 5.75 mm we expect: for period 8mm – K~0.4 ; for period 10mm -K~0.9