The E166 Experiment K. Peter Schüler 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 The E166 Experiment: Undulator-Based Production of Polarized Positrons K. Peter Schüler (DESY) - on behalf of the E166 Collaboration PSTP 2007 at BNL Sep. 2007

The E166 Experiment K. Peter Schüler e+ source options for the ILC 2 existing/proposed positron sources: ILC 3 Concepts: large amount of charge reqd ! conventional laser Compton based (see M. Kurikis talk) undulator-based (this talk) PSTP 2007 at BNL Sep. 2007

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

The E166 Experiment K. Peter Schüler undulator source scheme for ILC 4 PRO: photoprod. in thin target 0.4 X 0 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 ( m) reqd positron beam profile PSTP 2007 at BNL Sep. 2007

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

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

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

The E166 Experiment K. Peter Schüler undulator basics 8 PSTP 2007 at BNL Sep E166 ILC (RDR) electron beam energy (GeV) field (T) period (mm) K value photon energy 0 max (MeV) beam aperture (mm) active length (m) M (no. of periods)

The E166 Experiment K. Peter Schüler undulator basics 9 PSTP 2007 at BNL Sep E166 Photon SpectrumE166 Photon Polarization Spectrum:Angular Distribution:Polarization: first harmonic (dominating) expressions: E166 Photon Yield: = no. of photons per high-energy beam electron 0 max = 7.9 MeV

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

The E166 Experiment K. Peter Schüler E166 experimental setup 11 PSTP 2007 at BNL Sep GeV 4 – 8 MeV C1 – C4: photon collimation AG1, AG2: aerogel detectors AG1Si, AG2Si: silicon detectors GCAL: Si/W-calorimeter DM: electron beam dump magnets T1: g e+ prod. target (0.2 X 0 W) T2: e+ g reconv. target (0.5 X 0 W) PosSi: e+ flux monitor (Silicon) CsI: Cesium Iodide calorimeter SL: solenoid lens J: movable jaws < 8 MeV

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

The E166 Experiment K. Peter Schüler transmission polarimetry 13 1.Compton Transmission Polarimetry for Low-Energy Photons relies on spin dependence of Compton effect in magnetized iron: 2.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 Sep. 2007

14 analyzer magnets: overview active volume Photon Analyzer: 50 mm dia. x 150 mm long Positron Analyzer: 50 mm dia. x 75 mm long The E166 Experiment K. Peter Schüler P e 0.07 ΔP e /P e < 0.05 (aim of experiment) electron polarization of the iron: M = (B–B 0 )/ 0 magnetization n = electron density μ B = Bohr magneton g = magneto-mechanical factor PSTP 2007 at BNL Sep. 2007

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

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

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

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

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

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

21 electron polarization of the iron The E166 Experiment K. Peter Schüler PSTP 2007 at BNL Sep P e /P e ~ 2%

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

The E166 Experiment K. Peter Schüler e+ analysis: energy deposition in CsI crystals 23 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 Sep undulator on: signal + background undulator off: background

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

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

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. The E166 Experiment K. Peter Schüler PSTP 2007 at BNL Sep. 2007