1. K.T. McDonald DoE Review Aug. 10, 2006 19 E-166 Experiment E-166 is a demonstration of undulator-based polarized positron production for linear colliders. – E-166 uses the 50 GeV FFTB beam in conjunction with 1 m-long, helical undulator ( = 2.4 mm, ID = 0.9 mm) to make 10-MeV polarized photons. –These photons are converted in a ~ 0.5 rad. len. thick target into e+ (and e - ) with ~ 50% polarization. –The polarization of the positrons and photons will be measured. – E-166 was approved in June 2003, for a main data run in 2005.

2. K.T. McDonald DoE Review Aug. 10, 2006 20 (47 Collaborators) E-166 Collaboration (16 Institutions)

3. K.T. McDonald DoE Review Aug. 10, 2006 21 E-166 Beam Measurements Photon flux and polarization as a function of undulator K (P  ~ 75% for E  > 5 MeV). Positron flux and polarization vs. energy for K = 0.17, 0.5 r.l. of Ti (P e + ~ 50%). Positron flux and polarization for 0.1 r.l. and 0.25 r.l. Ti and 0.1, 0.25, and 0.5 r.l. W targets. Each measurement is expected to take about 20 minutes. A relative polarization measurement of 5% is sufficient to validate the polarized positron production processes. The undulator will be pulsed in a random pattern of beam pulses to permit continuous monitoring of undulator-off background levels.

4. K.T. McDonald DoE Review Aug. 10, 2006 22 E-166 Undulator (built at Cornell U. by Mikhailichenko) Two helices wound on a 0.9-mm ID, 1-m-long hypodermic needle. 1.4 kA pulses, ~ 5 μs long provide field of ~ 1 T on axis,  k ~ 0.15.

5. K.T. McDonald DoE Review Aug. 10, 2006 23 Positron Spectrometer (magnets built at Princeton)

6. K.T. McDonald DoE Review Aug. 10, 2006 24 Aerogel Cerenkov Counters (built at Princeton) Two Cerenkov counters with aerogel of index n = 1.0095 are now in the FFTB tunnel. Index measured with a Michaelson interferometer.

7. K.T. McDonald DoE Review Aug. 10, 2006 25 Linac ran at 46.6 GeV, rather than 50 GeV,  max. positron energy only 8 MeV. Positron asymmetries observed of order 1%. Preliminary calculation of analyzing power implies that positron polarizations of up to 80-90% were observed. Preliminary Results from Runs of E-166 in June and Sept 2005

8. K.T. McDonald DoE Review Aug. 10, 2006 26 Implications The use of undulator-based production of positrons was accepted as the ILC baseline at SNOWMASS’05. The use of a helical undulator for production of polarized positrons is the recommended option. This option must now be established as cost effective.