Advancing Positron Production in FFTB of Undulator-Based E-166 Machine

1. Positron Production and Test in the FFTB of Undulator-Based Concepts(E-166)Machine Advisory CommitteeThursday, November 7, 2002R. Pitthan andJ. C. Sheppard

2. Statement at MAC in May, 2002 Reason was: May-2002-Design was possible, but cumbersome at best. New directions desired even without polarization.

3. Polarized Positron Collaboration Personnel http://www-project.slac.stanford.edu/lc/local/PolarizedPositrons/pdfs/E-166TLD.pdf • Since May, we have been able to form an InternationalCollaboration – E-166 - to explore Polarized Positron Production, which includes: • Past experts from SLD and HERMES • Participation from all major LC Labs (CERN, DESY, KEK, SLAC) and JLAB • The Japanese Collab. which has done the only experiments sofar (at the ATF).

4. Why 2 Stages? Because of the uncertainty in the level of detector backgrounds, propose two stages. The goals for the stages are: • Stage I flux, spectra and polarization of undulator gammas - background measurements for positron polarimetry - preliminary positron polarimetry if backgrounds permit • Stage II background improvements and positron polarimetry

5. What Does This Collaboration Want to Do? • Make polarized photons: • Using a meter-long, short-period, pulsed helical undulator (lu = 2.4 mm, K=0.17), • And using the SLAC low emittance electron beam at 50 GeV, • To produce polarized photons in the energy range of a few MeV up to a cutoff energy of about 10 MeV. • Then make polarized positrons: • Photons are converted to polarized positrons in a target which is ~ 0.5 radiation lengths in thickness (targets of both Ti and W will be studied). • Yield, spectrum, and polarization of the photons and positrons will be measured and compared to the results of simulations.

6. Primary Questions I • I. We all know we need positrons for e+e-, but what are polarized positrons good for? The first question can be partially answered with this plot: Even a modest 50% e+ polarization raises the effective polarization from, e.g., 80% to above 92%.

7. From: American Linear Collider Working Group, SLAC-R-570, “The Whitebook” Long List I (Whitebook): In general, a polarized positron beam at a LC would be: • enhancing signal-to-background • increasing the effective luminosity • improving asymmetry measurements with increased statistical precision and reduced systematic errors • improving sensitivity to non-standard couplings In particular, • Suppression of W-pair backgrounds can be improved by a factor 3 with 60% positron polarization. • By limiting the running time allotted for LL and RR modes to 10%, the effective luminosity for annihilation processes can be enhanced by 50%.

8. Long List II (Whitebook, continued) • With 60% e+ polarization, for asymmetry measurements, the effective polarization is substantially increased (e.g., from 80% to 95%) and the systematic precision is improved by a factor 3. • With these features, a polarized positron beam may provide critical information for clarifying the interpretation of new physics signals. • Polarized positrons are needed to realize the full potential for precision measurements, especially those anticipated for Giga-Z running at the Z0-pole. The unusualeffect that adding two errors results in a lower error for the combined error comes from the algebraic form for

9. Primary Question II II. Is this FFTB Test any good for NLC needs? We believe yes:This test is a 1% length scale demonstration of undulator-based production of polarized positrons : • Photons are produced in the same energy range (10 MeV) and polarization characteristics (<100%) as for the collider; • The same target geometry and material(s) (slab geometry of Ti and W) are used as in the linear collider; • The polarization of the produced positrons is in the same range as in the linear collider; • The simulation tools being used (and developed) to model the experiment are the same that are being used to design the polarized positron system for the NLC.

10. Primary Question II - Continued • This demonstration directly tests the present design approach to the issues of polarized positron production. • The test will validate the methodologies and benchmark the design codes: undulator radiation models for photon characterization, undulator design codes for undulator fabrication, POL-EGS4 and POL-GEANT for polarized e+ production, and BEAMPATH (Batygin-RIKEN) for collection and transport. • The test will provide confidence that the NLC design is based on solid, demonstrated principles all working together at the same time. • The test develops the photon diagnostics required for a collider.

11. Low Energy Polarimetry Not Needed for LC Operation, but … • Polarimetry at the few 1/10% level at the present time is only possible atthe GeV level with Compton backscattering. This is where Linear Colliders plan to do the precision measurements. • However, from SLAC's experience with positron production (and polarized cathodes) the lesson is that it is important to be able to track the yield (and the polarization) back to the inception. • When at SLC the e+ yield was not as large as expected, each stage of the positron system was suspect (and guilty). • Consequently for NLC, to determine where yield and polarization are being lost, one must be able to measure these quantities close to a complicated source (undulator, target and capture in the polarized positron case), at low energy. • For this reason, the low energy polarimetry experience gained at E-166 in the MeV region could be invaluable for starting up and tuning a future collider.

12. What is Not Done Experimentally in the FFTB • E-166 does not test target thermal hydrodynamics and radiation damage in the target (done in separate engineering studieswith Livermore), nor capture efficiency (done as separate accelerator physics design) and high precision (0.3%) positron polarization diagnostics. The latter is an improvement on what was achieved at SLD by only a factor of X2, and is planned for the actual collider at higher energies. • The only physics explicitly omitted from the simulations at the present time is depolarization of positrons in the target due to multiple scatter and ionization loss. These effects are estimated to be small in comparison with the dominant depolarization of about 10%, from bremsstrahlung, which is included in the simulations.

13. Always Good to ActuallyDO Something One big problem for SLC was the combined mechanical (J/g) and radiation (neutron induced atomic dislocations, Mrad) damage to the target. While working on the Proposal we realized that with the externally produced gs we could stay below the onset of production of the evil Giant Dipole Resonance (GDR) neutrons. • Guidance from shell model: while the GDR maximum is at 2 shell spacings (~80 A-1/3 MeV), neutrons start to come out of the nucleus at one shell spacing = ~40 A-1/3 MeV. • When using titanium (Ti), this translates into ~22 MeV, for the GDR, and ~11 MeV for the neutron onset. Ti can only be used with externally produced gs, it cannot withstand the bremsstahlung cascades from high energy electrons. • This conceptual insight may be the first step toward fighting the bane of high intensity positron production, target damage, but more work is needed.

14. NLC Polarized Positron System Layout 2 Target assembles for redundancy Polarized e- Source for system checkout, and possibly e-e- , ggrunning.

15. Required NLC Helical Undulator Length The length for the cut photon spectrum is Lc: Ng = 2.6 photons/m/e-, Tc = 50%, Yc= 2.92%, xc=20% S3 = 59% Lc is the length for a unity gain system, i.e. no overhead factors, or enhancements. Optimization on target material, thickness, undulator characteristics, photon cut, beam energy, etc. will be made in the future.

16. Back to the FFTB Experiment (E-166) The experiment we are proposing uses a meter-long, short-period pulsed helical undulator, and the SLAC low emittance electron beam at 50 GeV, to produce circularly polarized photons in the energy range of a few MeV up to a cutoff energy of about 10 MeV. Those photons are converted to polarized positrons in targets of varying radiation lengths. We plan to study targets of titanium (Ti) and tungsten (W), which are both candidates for collider positron targets. The goal of the experiment is to measure the yield, spectrum, and polarization of the photons and positrons, and to compare the results to simulations.

17. Back to the FFTB Experiment (E-166) This test is a 1% length scale demonstration of undulator-based production of polarized positrons for linear colliders: • Photons are produced in the same energy range and polarization characteristics as for the collider; • The same target thickness and material are used as in the linear collider; • The polarization of the produced positrons is expected to be in the same range as in the linear collider; • The simulation tools being used to model the experiment are the same as those being used to design the polarized positron system for a next linear collider.

18. FFTB Schematic 50 GeV, Low Emittance Electron Beam 2.4 mm period, K=0.17, helical undulator 10 MeV, polarized photons 0.5 r.l. converter target 51%-54% positron polarization Moffeit/Woods

19. g Detector, Transmission Polarimetry Moffeit/Woods

20. TESLA, NLC, and FFTB Positron Production

21. Double Undulator Scheme, Mikhailichenko One of the traditional concepts in polarimetry is to have a random change in the sign of the polarization. In lieu of a better scheme, which we should develop for the collider, we will randomly pulse one or the other of 2 otherwise identical undulators with the opposite screw sense. For the collider we could think of either a pulsed spin rotator at the 2-GeV scale, or a pulsed beam splitter magnet with two spin rotators, or a variety of other ideas, all not well developed as of yet.

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

23. Photon Intensity, Angular Dist., Number,Polarization

24. Polarized Positron Production in the FFTB Convert polarized photons (from the undulator) to polarized positrons in a 0.5 r.l. (1.79 cm) thick target of Ti-alloy (yield is about 0.5% ) Longitudinal polarization of the positrons is 54%, averaged over the full spectrum Note: for 0.5 r.l. W converter, the yield is about 1% and the average polarization is 51%.

25. Photon Transmission Polarimetry: Iron Block Use polarization dependent Compton cross section in magnetized iron (an old idea, Goldhaber 1957, currently being used by Fukuda et al. at KEK-ATF; also at Bates, MIT). Measure transmission asymmetry of polarized photons through a 15 cm thick, iron block for parallel and anti-parallel magnetization.Advantage:due to the tight collimation, “one-scatter-and-you-are-out”, reduces background).

26. Photon Transmission Polarimetry II Clearly the flux goes down with increased length, but the asymmetry increases with length. For 10 MeV gammas an optimum would be about 10 cm.

27. Preliminary Positron Polarimetry: Iron Block Use the photon transmission concept: convert the positrons back to gammas. 54% Positron Polarization converts to 8.4% Photon Polarization. This results in a ~2.5% measurement asymmetry. High flux (relatively, compared to ATF) in FFTB important for statistics.

28. Scattering Compton Polarimeter, DESY Collabs. Concept similar to Møller polarimetry: scatter off thin magnetized iron foil, change polarization of foil. Since scattered photons are measured, charged particle back ground is suppressed. In this case asymmetry is a function of the scattering angle. Plotted both for sum of single photons and calometric signal (sum of energies).

29. Present State of Affairs Equipment to produce polarized positrons in FFTB is “standard” (including not so “standard” pulsed undulator). Ongoing discussion of photon polarimetry; this is not viewed as a problem, mainly a question of style. Preliminary positron polarimetry has been modeled. Ongoing discussion of positron polarimetry; this is seen as key to making a demonstration. Positron polarimetry is very doable in a low background environment.

30. Issues Approval Process: Seek support from MAC As E-166 before SLAC’s EPAC: Funding: NLC SLAC Research Division Cornell, DESY University Consortium LC (DOE) LC R & D (NSF)

31. Issues, cont’d Question of Detector Backgrounds that may adversely affect e+ polarization measurement, may be okay Straightforward cure for stage 2: bring positrons outside of FFTB enclosure Rest of measurements look to be very doable

32. The FFTB Experiment, E-166 This test is a 1% length scale demonstration of undulator-based production of polarized positrons for linear colliders: • Photons are produced in the same energy range and polarization characteristics as for the collider; • The same target thickness and material are used as in the linear collider; • The polarization of the produced positrons is expected to be in the same range as in the linear collider; • The simulation tools being used to model the experiment are the same as those being used to design the polarized positron system for a next linear collider.

33. Recent Positron System Notes LCC-0079, "Energy Deposition Using EGS4," J. C. Sheppard, April 2002 LCC-0082, "NLC Positron Target Heating," D. C. Schultz, Y. K. Batygin, V. K. Bharadwaj, J. C. Sheppard, June 2002. LCC-0085, "Planar Undulator Considerations," J. C. Sheppard, July 2002. LCC-0086, "Energy Loss and Enrgy Spread Growth in a Planar Undulator," J. C. Sheppard, July 2002. LCC-0087, " NLC Polarized Positron Photon Beam Target Thermal Structural Modeling," Werner Stein, John C. Sheppard, July 2002

34. Recent Positron System Notes, II LCC-0088, "Thermal Stress Analyses for the NLC Positron Target," W. Stein, A. Sunwoo, J. C. Sheppard, V. Bharadwaj, D. C. Schultz, July 2002. LCC-0089, "Structural Modeling of Tesla TDR Positron Target," Werner Stein, John C. Sheppard, July 2002. LCC-0090, "Thermal Stress Analyses for a Thermal Multislug Beam NLC Positron Target," W. Stein, A. Sunwoo, J. C. Sheppard, V. Bharadwaj, D. C. Schultz LCC-0092, "Positron Yield as a Function of Drive Beam Energy for a K=1, Planar Undulator-Based Source" J.C. Sheppard, July 2002

35. Recent Positron System Notes, III LCC-0093, "Radiation damage induced by GeV electrons in W-Re targets for next generation linear colliders", M.-J. Caturla, S. Roesler, V. K. Bharadwaj, D. C. Schultz, J. C. Sheppard, J. Marian, B. D. Wirth, W. Stein and A. Sunwoo, July 2002 LCC-0095, "Helical Undulator Radiation," J. C. Sheppard, July 2002 LCC-0098, "First Test of Short Period Helical SC Undulator Prototype," A. Mikhailichenko. T. Moore, August 2002. LCC-0102,"Thermal Stress Analyses for an NLC Positron Target with a 3 mm Spot Radius Beam," W. Stein, A. Sunwoo, J. Sheppard, V. Bharadwaj, D. Schultz, September 2002.

36. Recent Positron System Notes, IV LCC-0103,"Characterization of W-26% Re Target Material," A. J. Sunwoo, D.C. Freeman, W. Stein, V. K. Bharadwaj, D. C. Schultz, J.C. Sheppard, September 2002. LCC-0106, "Pulsed Helical Undulator for Test at SLAC Polarized Production Scheme," Alexander A. Mikhailichenko, October 2002. DESY LC-DET-2002-011, “The MeV Gamma Compton Polarimeter for the SLAC FFTB Undulator Test,” V. Gharibyan and K.-P. Schuler, October 2002. SLAC-Proposal-E166, “A Two Stage Proposal to Test Production of Polarized Positrons with the SLAC 50-GeV Beam in the FFTB,” G. Alexander et al., October 2002.

37. Positron System Notes, in prep. LCC-01xx,“Report on Radiation Damage effects in a Ti Target under Photon Irradiation," M. Caturla et al., November 2002. LCC-01xx, “Design Studies for SLAC-Proposal E-166," M. Woods, K. Moffeit, and J. C. Sheppard, November 2002.