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

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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  ’s, which are then converted to positrons (with less heating of target) [TESLA baseline]. –With helical undulator, get polarized  ‘s and polarized e + (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.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons –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. NLC Baseline Positron Production Scheme

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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)

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Polarized  ’s from a Helical Undulator -  ’s per period (with K = eB / 2  mc 2 = 1)   = 1/137. -(  intensity ~ K 2, but spectrum ragged for K > 1.) -100  ’s/positron => ~ 10,000 periods. -Period ~ undulator diameter ~ 1 cm  ~ 100 m long.  E   E e 2 /  10 MeV  ’s for 150 GeV e -. -Helical undulator simpler to fabricate than planar.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Linear Collider Polarized Positron System Layout 2 Target assemblies for redundancy Polarized e - source for system checkout (and e  e ,  running).

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons TESLA, NLC, and FFTB Positron Production

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Physics Motivation for Polarized Positrons –Electroweak processes e + e - -> WW, Z, ZH couple only to e - L e + R or e - R e + L (and not e - L e + L or e - R e + R ). –Slepton and squark produced dominantly via e - R e + 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, (N L – N R ) / (N L + N R ) = P eff A LR, P eff = (P - - P + ) / (1 – P - P + ). –Must have both e + and e - polarization for Giga-Z project.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons The E-166 Collaboration The E-166 Collaboration includes: – Participation from all major Linear Collider Labs (CERN, DESY, KEK, SLAC) and JLAB. – Participation from several universities with past FFTB experience. – e - polarization experts from SLD and HERMES. – e+ polarization experience via the Japanese groups.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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  E  max ~ 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 (Ti and/or W).  Characterize the positron polarization by converting e + back into  ’s and using a transmission polarimeter.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Undulator Design PULSED HELICAL UNDULATOR FOR TEST AT SLAC THE POLARIZED POSITRON PRODUCTION SCHEME. BASIC DESCRIPTION. Alexander A. Mikhailichenko CBN 02-10, LCC-106

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Electron Beam Requirements –50 GeV desired (48 GeV OK, but E   E e 2, so not lower). –30 Hz (parasitic operation OK). –10 10 electrons per pulse. –Electrons need not be polarized (the undulator provides the needed  beam polarization). –Spot size of 40 x 40 microns (12 x 3 microns achieved in E-150 at same location). Corresponds to  x,y = m-rad in both planes (up from m-rad calculated from SLC), with  * x,y  7.5 m. Very low current in E-166 (only 1/60 of E-158).

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Double Undulator Scheme Price is factor of 2 in  rate (0.2  /e - in 50 cm). [At the Collider, better to use a single undulator (still can be pulsed) + pulsed spin rotator at a few GeV.] 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.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Polarized Positron Yield at the FFTB e + /  = in 0.5 r.l. Ti target. N + = N - (  /e - ) (e + /  ) = (2e10)(0.2)(0.005) = / pulse. Longitudinal polarization, P ave, of the positrons is 54%, averaged over the full spectrum [For 0.5 r.l. W converter, yield is double and P ave is 51%.]

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons  Polarimetry via Transmission Thru an Iron Block -Measure only  ’s transmitted thru a block of magnetized iron.  Compton depends on both P  and P e. -3% transmission thru 15 cm iron for E  > 5 MeV.  = (  ++ -  +- ) / (  ++ +  +- ) = 0.05 for 10 MeV  ’s in 15 cm Fe.   /  ~ 1/(2   N) ~ 0.02 per pulse. -Transmission polarimetry is a kind of Compton polarimetry.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Transmission Polarimeter +  Detector Moffeit/Woods To avoid soft backgrounds, convert  ’s to e + e - and detect Cerenkov light. Deconvolve  spectrum via use of several Cerenkov radiators with graded energy thresholds.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons e + Polarimeter Using Transmission Polarimetry  Convert the e + back to  ’s.  54% e + pol.  44%  pol.   Compton ~ 2.5% asymmetry.  ~ 10 e -  e +  e - per pulse.  Need 1 hour for   /  = 0.1  Can it work close to  beam?

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Option to Bring e + Outside the FFTB Moffeit/Woods Use a 20-90º bend + solenoid channel to transport positrons thru FFTB wall to the polarimeter.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons Compton/Annihilation Polarimeter for  ’s and e + V. Gharibyan, K.P. Schuler: Use variants on Møller scattering in a magnetized iron foil. For  ’s, can use Compton scattering (+ sweep magnet SM): Asymmetry  = P  P foil A C, A C (  ) up to 0.7   max ~ 0.05 P  Scattering polarimeter is “noninvasive”. For e +, can use annihilation polarimetry e + e -   in the foil. Asymmetry  = P e+ P foil A A, P foi ~ 1/13, A A ~ 1,   max ~ 0.08 P e+. Problem is low rate (high background?). (If use e+e- scattering, A ~ 7/9.)

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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  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.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons The MAC was supportive of the E-166 scientific goals and experimental plan.

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons –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  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). E-166 Beam Request

K.T. McDonaldNovember 20, E-166 Undulator-Based Production of Polarized Positrons 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.