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Undulator-Based Positron Production in the Final Focus Test Beam (E-166)

The International Polarized Positron Production Collaboration. 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. Overview.

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Undulator-Based Positron Production in the Final Focus Test Beam (E-166)

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  1. The International Polarized Positron Production Collaboration Undulator-Based Positron Production in the Final Focus Test Beam (E-166) K.T. McDonald, J.C. Sheppard, Co-SpokespersonsSLAC Experimental Program Advisory Committee November 20, 2002

  2. Overview • Positron production via e- showers in a thick target is marginal at a Linear Collider due to target heating. • Alternative: use 150 GeV e- in an undulator to produce 10 MeV g’s, which are then converted to positrons(with less heating of target) [TESLA baseline]. • With helical undulator, get polarized g‘s and polarized e+ (Mikhailichenko, 1979). • Physics is clear, but this scheme has never been tested. • Can produce 10 MeV g’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 ofundulator-based polarized positron productionin the FFTB.

  3. NLC Baseline Positron Production Scheme • 20 MeV e+ collected from shower max of 6 GeV e- in a 4 radiation length target; pre-accelerated to 250 MeV • Energy deposition at shower max exceeds single target material limit • Would need multiple targets to meet goal of 1 e+/e- • Positrons from shower max have no memory of incident e- polarization.

  4. Polarized Positrons from Polarized g’s • High energy E&M interactions are helicity conserving, Forward positrons from g -> e+e- remember the g 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. - spair(10 MeV) ~ 1/5 spair (1 GeV). • Need ~ 100 g’s per useful e+. (Olsen & Maximon, 1959)

  5. Polarized g’s from a Helical Undulator • - g’s per period (with K = eBl / 2pmc2 = 1) a = 1/137. • (g intensity ~ K2, but spectrum ragged for K > 1.) • 100 g’s/positron => ~ 10,000 periods. • Period l ~ undulator diameter ~ 1 cm  ~ 100 m long. • Eg Ee2/l 10 MeV g’s for 150 GeV e-. • Helical undulator simpler to fabricate than planar.

  6. Linear Collider Polarized Positron System Layout 2 Target assemblies for redundancy Polarized e- source for system checkout (and e-e-, ggrunning).

  7. TESLA, NLC, and FFTB Positron Production

  8. 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 produced 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+). • Must have both e+ and e- polarization for Giga-Z project.

  9. The E-166 Collaboration http://www-project.slac.stanford.edu/lc/local/PolarizedPositrons/pdfs/E-166TLD.pdf • The E-166 Collaboration includes: • Participation fromall major Linear Collider Labs(CERN, DESY, KEK, SLAC) and JLAB. • Participation from several universities with past FFTB experience. • e- polarization experts fromSLD and HERMES. • e+ polarization experience via the Japanese groups.

  10. Scope of E-166 • Make polarized photons (Stage 1): • Use a 1-m-long, short-period, pulsed helical undulator (lu = 2.4 mm, K = 0.17) in the 50-GeV Final Focus Test Beam  Egmax ~ 10 MeV. • Characterize the g polarization with a transmission polarimeter. • Then make polarized positrons (Stage 2): • g’s are converted to polarized positrons in a < 0.5 radiation length target (Ti and/or W). • Characterize the positron polarization by converting e+ back into g’s and using a transmission polarimeter.

  11. Undulator Design PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION SCHEME. BASIC DESCRIPTION. Alexander A. Mikhailichenko CBN 02-10, LCC-106

  12. Electron Beam Requirements • 50 GeV desired (48 GeV OK, but Eg Ee2, so not lower). • 30 Hz (parasitic operation OK). • 1010 electrons per pulse. • Electrons need not be polarized (the undulator provides the needed g beam polarization). • Spot size of 40 x 40 microns (12 x 3 microns achieved in E-150 at same location). • Corresponds to ex,y = 2•10-5 m-rad in both planes (up from 1.65•10-5 m-rad calculated from SLC) , with b*x,y 7.5 m. • Very low current in E-166 (only 1/60 of E-158).

  13. Double Undulator Scheme • Sign of e- polarization can be chosen randomly on a pulse-by-pulse basis at the GaAs photoemission source. • To control e+ polarization on a pulse-by-pulse basis, use 2 undulators of opposite helicity, pulsing only one. • Price is factor of 2 in g rate (0.2 g/e- in 50 cm). • [At the Collider, better to use a single undulator (still can be pulsed) + pulsed spin rotator at a few GeV.]

  14. Polarized Positron Yield at the FFTB • e+/g = 0.005 in 0.5 r.l. Ti target. • N+ = N-(g/e-) (e+/g) = (2e10)(0.2)(0.005) = 2•107 / 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%.]

  15. 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 g’s. 0.5 r.l. converter target. 51%-54% e+ polarization. Moffeit/Woods

  16. g Polarimetry via Transmission Thru an Iron Block • Measure only g’s transmitted thru a block of magnetized iron. • sCompton depends on both Pg and Pe. • 3% transmission thru 15 cm iron for Eg > 5 MeV. • d = (s++ - s+-) / (s++ + s+-) = 0.05 for 10 MeV g’s in 15 cm Fe. • sd/d ~ 1/(2 dN) ~ 0.02 per pulse. • Transmission polarimetry is a kind of Compton polarimetry.

  17. Transmission Polarimeter +g Detector To avoid soft backgrounds, convert g’s to e+e- and detect Cerenkov light. Deconvolve g spectrum via use of several Cerenkov radiators with graded energy thresholds. Moffeit/Woods

  18. e+ Polarimeter Using Transmission Polarimetry • Convert the e+ back to g’s. • 54% e+ pol.  44% g pol. • dCompton~ 2.5% asymmetry. • ~ 10 e-e+e- per pulse. • Need 1 hour for sd/d = 0.1 • Can it work close to g beam?

  19. Option to Bring e+ Outside the FFTB Use a 20-90º bend + solenoid channel to transport positrons thru FFTB wall to the polarimeter. Moffeit/Woods

  20. Compton/Annihilation Polarimeter for g’s and e+ • V. Gharibyan, K.P. Schuler: Use variants on Møller scattering in a magnetized iron foil. • For g’s, can useCompton scattering(+ sweep magnet SM): • Asymmetry d = Pg • Pfoil • AC, AC(q)up to 0.7  max ~ 0.05 Pg. • Scattering polarimeter is “noninvasive”. • For e+, can use annihilation polarimetry • e+e-gg in the foil. • Asymmetry d = Pe+• Pfoil • AA, Pfoi ~ 1/13, AA ~ 1,  max ~ 0.08 Pe+. • Problem is low rate (high background?). • (If use e+e- scattering, A ~ 7/9.)

  21. 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. • [LCLS explores large-scale, long-term implementation of an undulator.] • The pulsed helical undulator is a scale model, 1% in length, ~ 20% in diameter, of that appropriate for a collider. • The g 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.

  22. The MAC was supportive of the E-166 scientific goals and experimental plan.

  23. 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 [1-2 weeks in FY03] • followed by Stage 1 running with the g polarimeter(s) [2-3 weeks in FY04] • Stage 2 running with the e+ polarimeter(s) [2-3 weeks in FY05] • All before the FFTB is dedicated to the LCLS (~ 2005).

  24. Summary • Undulator-based production of positrons offers relief on target stress issues at a linear collider, and improved physics opportunities via positron polarization. • This test will provide confidence that the design proposed for the next generation of linear colliders is based on solid, experimentally demonstrated principles, all working together at the same time. • [Parallel efforts should continue to study collider target operational issues.] • With 50 GeV e-, the FFTB at SLAC is the only existing facility suitable to demonstrate this experimentally untested concept. • The E-166 collaboration is strong, with experts in polarimetry at DESY, KEK, JLAB, and SLAC (SLD), and accelerator physicists responsible for E-158. • E-166 can be run interleaved with PEP-II with 2-3 weeks of beam time over each of the next 3 years. • The E-166 budget of ~ $1M is comparable to that of other recent efforts in FFTB.

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