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

This project aims to demonstrate the physics and technology of undulator-based polarized positron production in the Final Focus Test Beam (FFTB). The scheme involves using a helical undulator to produce polarized photons, which are then converted to polarized positrons in a target. The collaboration includes participation from major linear collider labs and universities.

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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 • 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 ~ undulator diameter ~ 1 cm  ~ 100 m long. • First-order cutoff at ~ 10 MeV g’s for 150 GeV e-. • Helical undulator simpler to fabricate than planar.

  6. TESLA, NLC, and FFTB Positron Production

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

  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 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+).

  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. • e- polarization experts fromSLD and HERMES. • e+ polarization experience via the Japanese groups.

  10. Technical 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 (of both Ti and W). • Characterize the positron polarization by converting them back into g’s and using a transmission polarimeter.

  11. Financial and Social Scope • Recent FFTB experiments (E-162 and E-164) have benefited from the original investment in E-157 (~ 1.3 M$). E-166 is in the same range. • Typical beam time requirements were from a few times several a week to several times 4-6 weeks. • The collaboration is a strong one. Members range from experts in polarimetry at DESY, KEK, JLAB, and SLAC (SLD) to the accelerator physicists responsible for E-158.

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

  13. Electron Beam Properties Required • 50 GeV desired (48 GeV OK). • 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 corresponds to 1-2 10-5 m-rad in both planes, with bx,y 10 m. The only recent experience with 50 GeV beam is from E-158. With the low current required in E-166 (only 1/60 of E-158) experts state “easy”.

  14. Double Undulator Scheme • Sign of e- polarization can be chosen 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 + pulsed spin rotator at a few GeV, or some better idea.

  15. 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) = 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%.)

  16. 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

  17. 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 1-Compton polarimetry.

  18. 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

  19. e+ Polarimeter Using Transmission Polarimetry • Convert the e+ back to g’s. • 54% e+ pol.  8.4% 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?

  20. Compton/Annihilation Polarimeter for g’s and e+ • V. Gharibyan, K.P. Schuler: For g’s, can useCompton scatteringin the polarized foil (+ sweep magnet SM): • Asymmetry d = Pg • Pfoil • AC, AC(q)up to 0.7  max ~ 0.05 Pg. • Scattering polarimeter is “noninvasive”. • Annihilation polarimetry for e+: • e+e-gg in a thin magnetized iron foil. • Asymmetry d = 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.)

  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. • 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. Positron Production &Test in FFTB of Undulator-Based Concept • Recommendation to proceed at the condition to avoid impacting the critical path effort towards demonstrating the NLC RF power source technology • Method critical for TESLA but possibly generic for all LC • FFTB ideal and only possible place for test • Resources from International Linear Collider laboratories and from University (ideal subject of collaboration) • DESY support encouraged and US support maximized (NLC funding and man-power minimized) • Challenging: Undulator, Polarization measurements…. • Delay resources commitments until early 2004 not to interfere with NLC priority NLC • (J.P.Delahaye, L.Klaisner, J.Rosenzweig)

  23. Remarks by S. Ozaki (MAC Chair) 11/8/02 • The Committee notes the potential importance of the undulator based polarized positron production technology for both TESLA and NLC/JLC Projects. The Committee also notes potential value of this technology for other applications. The Committee therefore recommends that this proposal be given a favorable consideration by the Laboratory. However, the committee notes that the priority of the experiment proposed should be lower than the rf power demonstration project at this point. It also notes that much clearer and sharper definition of the proposal must be made. The Laboratory management is encouraged to seek further external support for this experiment. • With the outstanding effort of Tom Himel and company, grassroots efforts of organizing university groups for linear collider R&D have taken off with amazing enthusiasm. This grassroots movement is of significant value for the linear collider program. The Committee recommends that the NLC collaboration be supportive of this movement, though these R&D activities may result in some cost to the mainline NLC R&D.

  24. 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, followed by Stage 1 running with the g polarimeter(s), and Stage 2 running with the e+ polarimeter(s) – all before the FFTB is dedicated to the LCLS (~ 2005).

  25. 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 of FY04 and in FY05. • Desire 1-2 weeks of beam checkout in FY03 for background studies in the FFTB – parasitic T-experiment OK. • Stage 1 and 2 production running in FY04/05, 2-3 weeks each.

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