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

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-BasedProduction of Polarized Positrons

K. Peter Schüler (DESY) - on behalf of the E166 Collaboration

PSTP 2007 at BNL 10-14 Sep. 2007


e+ source options for the ILC

2

existing/proposed positron sources:

← ILC

3 Concepts:

large amount ofcharge req‘d !

• ‚conventional‘• laser Compton based (see M. Kuriki‘s talk)• undulator-based (this talk)

PSTP 2007 at BNL 10-14 Sep. 2007


3

conventional positron source (as used with SLC at SLAC)

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

thick W-Re target:strong multiple scattering,less efficient e+ capture


undulator source scheme for ILC

4

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


undulator source scheme for ILC

5PSTP 2007 at BNL 10-14 Sep. 2007

auxiliary keep-alive source


E166 – proof of principle demonstrationof the undulator method

6PSTP 2007 at BNL 10-14 Sep. 2007

7The E166 Experiment K. Peter Schüler

PSTP 2007 at BNL 10-14 Sep. 2007


undulator basics

8PSTP 2007 at BNL 10-14 Sep. 2007

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 0

max (MeV) 7.9 10.0 beam aperture (mm) 0.89 5.85 active length (m) 1 147 M (no. of periods) 394 12800


undulator basics

9PSTP 2007 at BNL 10-14 Sep. 2007

E166 Photon Spectrum E166 Photon Polarization

Spectrum: Angular Distribution: Polarization:

first harmonic (dominating) expressions:

E166 Photon Yield: = no. of photons per high-energy beam electron

0max = 7.9 MeV


The E166 Experiment

10PSTP 2007 at BNL 10-14 Sep. 2007

2004/2005 setup and checkout

Oct. 2005 4 weeks of data taking


E166 experimental setup

11PSTP 2007 at BNL 10-14 Sep. 2007

46.6 GeV

4 – 8 MeV

C1 – C4: photon collimationAG1, AG2: aerogel detectorsAG1Si, AG2Si: silicon detectorsGCAL: Si/W-calorimeter

DM: electron beam dump magnetsT1: 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 calorimeterSL: solenoid lensJ: movable jaws

< 8 MeV


E166 photo gallery

12PSTP 2007 at BNL 10-14 Sep. 2007


transmission polarimetry

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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 10-14 Sep. 2007

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analyzer magnets: overview

active volumePhoton Analyzer: 50 mm dia. x 150 mm longPositron Analyzer: 50 mm dia. x 75 mm long

The E166 ExperimentK. Peter Schüler

Pe ≈ 0.07

ΔPe/Pe < 0.05 (aim of experiment)

electron polarization of the iron:

M = (B–B0)/0 magnetization n = electron densityμB = Bohr magneton g‘ = magneto-mechanical factor

PSTP 2007 at BNL 10-14 Sep. 2007

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spin and magnetization


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‘ = 1.919 ± 0.002 for pure iron i.e. orbital effects contribute about 4%

Note: g‘ = 2 Ms / M = 1 (pure spin magnetization) γ = e/m g‘ = 1 Ms / M = 0 (pure orbit magnetization) γ = ½ (e/m)

PSTP 2007 at BNL 10-14 Sep. 2007

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analyzer magnets


CsI-Detector

e+ Analyzer

Pickup Coils

e+ Analyzer Analyzer

PSTP 2007 at BNL 10-14 Sep. 2007

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field distribution modeling(Vector Fields OPERA-2d)


R = 0 mm

1015

22.5

Bz (T)

Z (mm)

longitudinal field distribution: Bz (R,Z)field drops gradually towards the ends: Leff / L < 1

center

end

PSTP 2007 at BNL 10-14 Sep. 2007

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5

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field distribution in 2d(Vector Fields OPERA-2d)


longitudinal field distribution: Bz (R,Z)(shown for one quadrant)

R (mm)

Z (mm)

PSTP 2007 at BNL 10-14 Sep. 2007

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flux measurements:


measure voltage transients in pickup coils upon current reversals

Positron Analyzer (-60 +60 amps)

PSTP 2007 at BNL 10-14 Sep. 2007

voltage transient

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flux and field measurements: results


Note: polarimetry was always done at full saturation over the central region (±60A)

Z = 0 (center)

PSTP 2007 at BNL 10-14 Sep. 2007

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electron polarization of the iron


PSTP 2007 at BNL 10-14 Sep. 2007

Pe/Pe ~ 2%

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photon asymmetries


PSTP 2007 at BNL 10-14 Sep. 2007

detector asymmetry (%) (%) AG2Si (silicon) 3.883 0.062 AG2 (aerogel) 3.307 0.123 GCAL (Si/W-calo) 3.665 0.071

AG2Si

AG2

GCAL

measured photon asymmetries are in reasonable agreementwith simulation results (3.2-3.5%) based on the theoreticalundulator polarization spectrum and detector response functions, but no detailed spectral shape analysis is possible.


e+ analysis: energy deposition in CsI crystals

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• 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

undulator on: signal + backgroundundulator off: background


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central crystal asymmetry vs. run cycle numberfor e+ spectrometer setting at 140 A

positron asymmetries

PSTP 2007 at BNL 10-14 Sep. 2007

data samples and spectrometer settings


positron asymmetries & beam polarizations

25PSTP 2007 at BNL 10-14 Sep. 2007

e+ / e-e+ / e-

results for the central CsI crystal

= analyzing power from simulations = electron polarization of the iron

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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

Documents

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