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The E166 Experiment K. Peter Schüler

The E166 Experiment: Undulator-Based Production of Polarized Positrons. K. Peter Schüler (DESY) - on behalf of the E166 Collaboration. e+ source options for the ILC undulator source scheme for ILC E166 – proof-of-principle demonstration of the undulator method undulator basics

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The E166 Experiment K. Peter Schüler

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  1. The E166 Experiment: Undulator-Based Production of Polarized Positrons K. Peter Schüler (DESY) - on behalf of the E166 Collaboration • e+ source options for the ILC • undulator source scheme for ILC • E166 – proof-of-principle demonstration • of the undulator method • undulator basics • transmission polarimetry • results & conclusions PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  2. e+ source options for the ILC existing/proposed positron sources: ← ILC large amount of charge req‘d ! 3 Concepts: • ‚conventional‘ • laser Compton based (see M. Kuriki‘s talk) • undulator-based (this talk) PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 2

  3. conventional positron source (as used with SLC at SLAC) thick W-Re target: strong multiple scattering, less efficient e+ capture PRO: established technology (although not at req‘d level) CON: pushing technical limits of target materials; req‘s multiple targets and beamlines; very high activation levels no polarization option PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 3

  4. undulator source scheme for ILC • PRO: • photoprod. in thin target 0.4 X0 Ti-alloy •  lower e+ beam emittance • less energy deposition in target (1/5) • and AMD (1/10) • less neutron induced activation (1/16) • polarized positrons • CON: • need high-energy electron drive beam • (coupled e+/e- operation) • long undulator (150-300 m) req‘d positron beam profile PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 4

  5. undulator source scheme for ILC auxiliary keep-alive source PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 5

  6. E166 – proof of principle demonstration of the undulator method PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 6

  7. PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  8. undulator basics E166 ILC (RDR) electron beam energy (GeV) 46.6 150 field (T) 0.71 0.86 period (mm) 2.54 11.5 K value 0.17 0.92 photon energy w0max (MeV) 7.9 10.0 beam aperture (mm) 0.89 5.85 active length (m) 1 147 M (no. of periods) 394 12800 PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 8

  9. undulator basics E166 Photon Spectrum E166 Photon Polarization first harmonic (dominating) expressions: Spectrum: Angular Distribution: Polarization: w0max = 7.9 MeV E166 Photon Yield: = no. of photons per high-energy beam electron PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 9

  10. The E166 Experiment 2004/2005  setup and checkout Oct. 2005  4 weeks of data taking PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 10

  11. E166 experimental setup < 8 MeV 46.6 GeV 4 – 8 MeV DM: electron beam dump magnets T1:g e+ prod. target (0.2 X0 W) T2: e+ g reconv. target (0.5 X0 W) PosSi: e+ flux monitor (Silicon) CsI: Cesium Iodide calorimeter SL: solenoid lens J: movable jaws C1 – C4: photon collimation AG1, AG2: aerogel detectors AG1Si, AG2Si: silicon detectors GCAL: Si/W-calorimeter PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 11

  12. E166 photo gallery PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 12

  13. transmission polarimetry • Compton Transmission Polarimetry for Low-Energy Photons • relies on spin dependence of Compton effect in magnetized iron: • Positron Polarimetry: (a) transfer e+ polarization to photon • via brems/annihilation process • (b) then infer e+ polarization from • measured photon pol. as in method 1. PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 13

  14. analyzer magnets: overview electron polarization of the iron: M = (B–B0)/m0 = magnetization n = electron density μB = Bohr magneton g‘ = magneto-mechanical factor active volume Photon Analyzer: 50 mm dia. x 150 mm long Positron Analyzer: 50 mm dia. x 75 mm long Pe≈ 0.07 ΔPe/Pe < 0.05 (aim of experiment) PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  15. spin and magnetization g‘ = magneto-mechanical factor: obtained from Einstein - de Haas type experiments, related to gyromagnetic ratio: γ = (g‘/2) ∙ (e/m) Note: g‘ = 2  Ms / M = 1 (pure spin magnetization) γ = e/m g‘ = 1  Ms / M = 0 (pure orbit magnetization) γ = ½ (e/m) … and its ultimate implementation (Scott 1962) g‘ = 1.919 ± 0.002 for pure iron i.e. orbital effects contribute about 4% the principle PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  16. analyzer magnets  Analyzer e+ Analyzer Pickup Coils CsI-Detector e+Analyzer PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  17. field distribution modeling(Vector Fields OPERA-2d) end Bz (T) 22.5 20 15 10 5 R = 0 mm Z (mm) center longitudinal field distribution: Bz (R,Z) field drops gradually towards the ends: Leff / L < 1 PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  18. field distribution in 2d(Vector Fields OPERA-2d) Z (mm) R (mm) longitudinal field distribution: Bz (R,Z) (shown for one quadrant) PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  19. flux measurements: measure voltage transients in pickup coils upon current reversals Positron Analyzer (-60  +60 amps) voltage transient PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  20. flux and field measurements: results Z = 0 (center) Note: polarimetry was always done at full saturation over the central region (±60A) PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  21. electron polarization of the iron DPe/Pe ~ 2% PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  22. photon asymmetries detector asymmetry d (%) Dd (%) AG2Si (silicon) 3.883 0.062 AG2 (aerogel) 3.307 0.123 GCAL (Si/W-calo) 3.665 0.071 AG2Si AG2 measured photon asymmetries are in reasonable agreement with simulation results (3.2-3.5%) based on the theoretical undulator polarization spectrum and detector response functions, but no detailed spectral shape analysis is possible. GCAL PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

  23. e+ analysis: energy deposition in CsI crystals undulator on: signal + background undulator off: background • good signal/background separation • in central crystal • background comes from beam halo • hitting the undulator • undulator on/off measurements were • taken on alternating machine pulses • for effective background separation PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 23

  24. positron asymmetries central crystal asymmetry dvs. run cycle number for e+ spectrometer setting at 140 A data samples and spectrometer settings PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 24

  25. positron asymmetries & beam polarizations results for the central CsI crystal e+ /e- e+ /e- = analyzing power from simulations = electron polarization of the iron PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler 25

  26. conclusions • successful demonstration of the undulator method • undulator functioned as predicted • successful polarimetry of low-energy  and e+ • confirmed expected γ e+ spin-transfer mechanism • measured high positron polarization with ~ 80% max. PSTP 2007 at BNL 10-14 Sep. 2007 The E166 Experiment K. Peter Schüler

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