9 Summary
Positrons in the energy range of 4-8 MeV with longitudinal polarization of ~75%
have been produced and measured in E-166. The relative errors on the
polarization measurements are about 5% and are dominated by the systematic
uncertainties. This experiment has successfully demonstrated a method for
polarized positron production suitable for the next generation of linear colliders.
The method uses a high energy electron beam in conjunction with a short period,
helical undulator to produce circularly polarized photons with energies of several
MeV. The photons pair are converted in a thin target to generate longitudinally
polarized positrons. The tools, techniques, and methodologies developed for the
experiment are directly applicable to the efforts required for the design of
polarized positron sources for the ILC and for CLIC.
A one-meter long, 2.54 mm period helical undulator was built at Cornell. The
device is made by wrapping a bifilar winding of copper wire on a stainless steel
vacuum chamber. It was pulsed at 2000 A for ~12 microseconds at a repetition
rate of up to 10 Hz and has strength parameter K=0.19. The undulator was
installed in the FFTB at SLAC. A low emittance electron beam with energy 46.6
GeV was passed through the undulator to produce circular polarized photons.
The first harmonic cutoff of the photon spectrum was about 7.8 MeV. Photons
incident onto a 0.7 mm thick tungsten target produced positrons (and electrons)
with energies in the range of a few MeV. A focusing solenoid in combination
with a pair of dipole magnets were installed for the purpose of collecting the
positrons and separating the positrons from the incident photon beam. A total
photon-positron offset of about 46 cm was made to permit the installation of
shielding around the CsI array used for the positron polarization measurements.
The dipole pair served as a spectrometer.
Positron polarization was measured far several positron energies. In addition, the
polarization of the incident photon beam was analyzed. Photon transmission
polarimetry was used to measure the polarization of positrons and electrons as
well as the photon beam. The relative transmission of photons through
magnetized iron was measured for magnetization parallel and anti-parallel to the
direction of photon propagation. For the direct undulator photon beam, the length
of the iron cylinder was 15 cm and a combination of Si-W and Aerogel-Cerenkov
detectors were used to measure the incident and transmitted photon flux and
energy. For positron polarimetry, positrons were “reconverted” to photons in a
thin tungsten radiator; and the relative transmission through the magnetized iron
of the converted photons was measured using a 3x3 array of CsI crystals. A 7.5
cm long magnetized Fe cylinder was used for the positron polarization
measurements. Simulations were made using a variety of programs to model all
aspects of the experiment. The measured results agree well with the simulations.
Undulator performance was modeled using MERMAID. Its performance was
characterized by measuring the total photon flux as a function of excitation
current and by measurement of the flux and energy transmitted through
magnetized iron. The measured photon characteristics agreed well with
expectations. At the end of the run, the transformer oil in the undulator container
was replaced with ferro-fluid; and an enhancement of the flux was observed. The
undulator itself was tested at currents up to 2300 A. During the data runs, it was
pulsed at 10 Hz and 2000 A. Over 4x107 undulator pulses were made over the
course of the run with out a single failure.
Undulator photon polarization was explored by comparing the relative
transmission asymmetry of the undulator spectrum through a 15 cm long,
magnetized iron cylinder. The analysis relies upon the spectral transmission
properties of magnetized iron and the polarization dependent term in the Compton
scattering within the iron. The processes were modeled using a version of
GEANT3 modified to include spin dependent scattering effects. Si-W and
Aerogel detectors used for the measurements have different spectral responses
and hence give somewhat different values for the asymmetry. The measured
asymmetries agreed well with the expectations. In addition to photon polarization
measurements, a Si-flux counter was used to measure the total photon flux
produced from the undulator. Again, the measured flux as a function of undulator
strength agreed well with predictions.
Positron polarization was measured at 6 different energy setting of the analyzing
spectrometer. In addition, an electron polarization measurement was measured at
a single energy setting by reversing the polarity of the spectrometer. Over the
measured energy range of 4-8 MeV, the positron (and electron) polarization was
about 75% and a relative measurement error of about 5%. The measured results
agree well with the model predictions. To do the modeling, GEANT4 has been
updated to include a number of spin dependent effects. The spin updates have
been incorporated in the standard version of GEANT4 and are available to all
users.
The modeling tools developed for E166 are being used to design the undulatorbase polarized positron sources for the next generation of linear colliders. The
agreement of undulator performance, collection, and analyzing systems with
expectation serve to benchmark the validity of the design methodologies.