Undulator-Based Positron Production in the Final Focus Test Beam (E-166) K.T. McDonald, J.C. Sheppard, Co-Spokespersons SLAC Experimental Program Advisory

Undulator-Based Positron Production in the Final Focus Test Beam

(E-166)

K.T. McDonald, J.C. Sheppard, Co-Spokespersons SLAC Experimental Program Advisory Committee

November 20, 2002

The International Polarized Positron Production The International Polarized Positron Production CollaborationCollaboration

K.T. McDonaldNovember 20, 2002 2

E-166 Undulator-Based Production of Polarized Positrons

Overview

– Classical positron production schemes are marginal due to target heating and radiation damage.

– Alternative: use 150 GeV e- in an undulator to produce 10 MeV ’s, which are then converted to positrons (with less heating of target) [TESLA baseline].

– With helical undulator, get polarized ‘s and polarized e+ (Balakin&Mikhailichenko, 1979).

– Physics is clear, but this scheme has never been tested.

– Can produce 10 MeV ’s via 50 GeV e- in SLAC FFTB using a 1-mm bore undulator.

– FFTB available only thru 2005 (then LCLS).– E-166 collaboration proposes to demonstrate the

physics and technology of undulator-based polarized positron production in the FFTB.



– 25 MeV e+ from shower max of 6 GeV e- in a 4-6 radiation length target.

– Heating at shower max many times that from useful positrons alone.

Need multiple targets to meet goal of 1 e+/e-.

– Positrons from shower max have no memory of incident e- polarization.

NLC Baseline Positron Production Scheme



Polarized Positrons from Polarized ’s

– High energy E&M interactions are helicity conserving Forward positrons from -> e+e- remember the polarization.

– Only 2 charged particles in the target per positron Less heating of target.

– e+ polarization is degraded by Bremsstrahlung in the target Use target < 0.5 rad. Len.

– Only upper half of positron spectrum has good polarization.

-pair(10 MeV) ~ 1/5 pair (1 GeV). Need ~ 100 ’s per useful e+.

(Olsen & Maximon, 1959)



Polarized ’s from a Helical Undulator

- ’s per period (with K = eB / 2mc2 = 1) (1/137)

- ( intensity ~ K2, but spectrum ragged for K > 1.)- 100 ’s/positron => ~ 10,000 periods.- Period ~ undulator diameter ~ 1 cm ~ 100 m long. First-order cutoff at ~ 10 MeV ’s for 150 GeV e-.- Helical undulator simpler to fabricate than planar.



Table 1: TESLA, NLC, FFTB Polarized Positron Parameters

Parameter Units TESLA NLC FFTB Beam Energy, Ee GeV 150-250 150 50 Ne/bunch - 3x1010 8x109 1x1010

Nbunch/pulse - 2820 190 1 Pulses/s Hz 5 120 30 Undulator Type - planar helical helical Undulator Parameter, K - 1 1 0.17 Undulator Period

u cm 1.4 1.0 0.24 1st Harmonic Cutoff, Ec10 MeV 9-25 11 9.6 dN/dL photons/m/e- 1 2.6 0.37 Undulator Length, L m 135 132 1 Target Material - Ti-alloy Ti-alloy Ti-alloy, W Target Thickness r.l. 0.4 0.5 0.5 Yield % 1-5 1.8† 0.5 Capture Efficiency % 25 20 - N+/pulse - 8.5x1012 1.5x1012 2x107

N+/bunch - 3x1010 8x109 2x107

Positron Polarization % - 40-70 40-70 † Including the effect of photon collimation at = 1.414.

TESLA, NLC, and FFTB Positron Production



Sample Collider Polarized Positron System Layout

2 Target assemblies for redundancy

Polarized e- source for system checkout (and ee, running).



Physics Motivation for Polarized Positrons– Electroweak processes e+e- -> WW, Z, ZH couple

only to e-Le+

R or e-Re+

L (and not e-Le+

L or e-Re+

R).

– Slepton and squark production dominantly via e-Re+

L. Can double rate using polarized positrons

(or suppress rate if both e- and e+ are polarized).– Effective polarization enhanced,

and error decreased, in electroweak asymmetry measurements, (NL – NR) / (NL + NR) = Peff ALR, Peff = (P- - P+) / (1 – P-P+).



The E-166 Collaboration

The E-166 Collaboration includes:

– Participation from all major LC Labs (CERN, DESY,

KEK, SLAC) and JLAB.

– Participation from several universities.

– e- polarization experts from SLD and HERMES.

– e+ polarization experience via the Japanese groups.



Scope of E-166

Make polarized photons (Stage 1):

Use a 1-m-long, short-period, pulsed helical undulator (u = 2.4 mm, K = 0.17) in the 50-GeV Final Focus Test Beam Emax ~ 10 MeV.

Characterize the polarization with a transmission polarimeter.

Then make polarized positrons (Stage 2): ’s are converted to polarized positrons in a < 0.5 radiation length target (of both Ti and W).

Characterize the positron polarization by converting them back into ’s and using a transmission polarimeter.



Table 3: FFTB Helical Undulator System Parameters

Parameter Units Value Number of Undulators - 2 Length m 0.5 Inner Diameter mm 0.889 Period mm 2.4 Field kG 7.6 Undulator Parameter, K - 0.17 Current Amps 1800 Pulse Width s 30 Inductance H 1.8x10-6 Wire Type - Cu Wire Diameter mm 0.6 Resistance ohms 0.125 Repetition Rate Hz 30 Power Dissipation W 225 T/pulse

0C 4

Undulator Design

PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION

SCHEME. BASIC DESCRIPTION.

Alexander A. Mikhailichenko

CBN 02-10, LCC-106



Double Undulator Scheme

• Price is factor of 2 in rate (0.2 /e- in 50 cm).

• At the Collider, need better scheme: e.g., single undulator + pulsed spin rotator at ~ GeV. Or other.

• Sign of e- polarization can be chosen randomly on a pulse-by-pulse basis at the GaAs photoemission source.

• In lieu of better scheme, use 2 undulators of opposite helicity, pulsing only one.



Polarized Positron Yield at the FFTB

• e+/ = 0.005 in 0.5 r.l. Ti target.

• N+ = N- (/e-) (e+/) = (2e10)

(0.2)(0.005) = 2e7 / pulse.

• Longitudinal polarization, Pave, of the positrons is 54%, averaged over the full spectrum

• (For 0.5 r.l. W converter, yield is double and Pave is 51%.)



Layout of E-166 in the FFTB

50 GeV, low emittance e- beam.

2.4 mm period, K = 0.17, helical undulator.

10 MeV polarized ’s.

0.5 r.l. converter target.

51%-54% e+ polarization.

Moffeit/Woods



Polarimetry via Transmission Thru an Iron Block

- Measure only ’s transmitted thru a block of magnetized iron.

- Compton depends on both P and Pe.

- = (++ - +-) / (++ + +-) = 0.05 for 10 MeV ’s in 15 cm Fe.

- Transmission polarimetry is a “1-Compton” polarimetry.



Transmission Polarimeter + Detector

Moffeit/Woods

To avoid soft backgrounds, convert ’s to e+e- and detect Cerenkov light.

Deconvolve spectrumvia use ofseveralCerenkovradiatorswith gradedenergythresholds.



e+ Polarimeter Using Transmission Polarimetry

Convert the e+ back to ’s.

54% e+ pol. 8.4% pol.

Compton 2.5% asymmetry.

Will be statistics limited.

Can it work close to beam?



Compton/Annihilation Polarimeter for ’s and e+

Annihilation polarimetry for e+:

• e+e- in a thin magnetized iron foil.

• Asymmetry = Pe+• Pfoil • AA, Pfoi ~ 1/13, AA ~ 1 at 90 in CM frame, max ~ 0.08 Pe+.

• Problem is low rate (high background?).

• (If use e+e- scattering, A ~ 7/9.)

For ’s, can use Compton scattering in the polarized foil (+ sweep magnet SM):

•Asymmetry = P• Pfoil • AC, AC() up to 0.7 max ~ 0.05 P

•Scattering polarimeter is “noninvasive”.



E-166 as Linear Collider R&D– E-166 is a proof-of-principle demonstration of undulator

based production of polarized positrons for a linear collider.• This technique is much less demanding on target performance than

conventional positron production with e- on a thick target.• A helical undulator is simpler than a planar one -- and provides

polarized positrons.

– The pulsed helical undulator is a scale model, 1% in length, ~ 20% in diameter, of that appropriate for a collider.

– The and e+ polarimeters are prototypes of those appropriate for low-energy diagnostics at a collider. (High energy polarimetry is also needed at a collider, but is well demonstrated at present e- and e+e- facilities.)

– The hardware and software expertise developed for E-166 will be the basis for implementation of polarized positrons at a linear collider.



Endorsement by the NLC Machine Advisory Committee

• Well, I hope that we have an endorsement to mention here!



E-166 Beam Request

– E-166 is to be performed in the FFTB, with the undulator just upstream of the e- dump magnets, and polarimeters downstream of these.

– E-166 needs initial background studies (parasitic T-experiment)

– followed by Stage 1 running with the polarimeter(s),

– and Stage 2 running with the e+ polarimeter(s) – all before the FFTB is dedicated to the LCLS (~ 2005).



Time Line

– We plan to start construction of equipment in FY03, finish construction and installation in the first 6 month of FY04, and do the experiments in the second half.

– In case background problems require additional measures, or we find unexplained fact, we will request additional time in FY05.



E-166 Budget Estimate (1)



E-166 Budget Estimate (2)

Documents

Undulator-Based Positron Production in the Final Focus Test Beam (E-166) K.T. McDonald, J.C. Sheppard, Co-Spokespersons SLAC Experimental Program Advisory